hi everyone welcome for this class on human eye and the colorful world and i'm super excited because this class is going to be using the new digital board that we have so thanks for the team to set up this board and good evening everyone welcome so we are going to be doing this live class in this new format and i'm super excited hope you guys are too so i'm new to this digital board so you know if some things go wrong please excuse me i'm also getting used to it and hope you guys would like this format and i have the chat open here so i'm constantly looking at the chat and guys if you haven't hit the like button please hit it right now and if you haven't subscribed to our youtube channel manoj academy please hit the subscribe button right now and don't forget to click on the notification bell so that you don't miss any of our classes so good evening welcome for this class and hope all of you guys are doing great we are going to be revising this human eye and the colorful world chapter so we'll be going through all the important concepts in this chapter and i'm going to make it super easy for you so good evening how are you guys and thank you guys thanks yes this digital board is all due to your love and support and you know now we are a family of around 4.5 lakhs that's right we have around 450 000 subscribers and it's all thanks to you guys and so please do share out our channel with your friends all right thank you guys and thanks welcome i'm constantly looking at the chat here and let's go ahead and get started with this class and i want all of you to participate so i'm going to be explaining important concepts and asking you questions and let's all uh interact together in this class so are you guys ready let's get started and before we start i just want to say do check out our website manoj academy.com we've got awesome courses for you and they're on big discounts like we have the cbse class 10 full courses for physics chemistry math cbse class 9 cbse class 8 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computers and you can get more live classes from me on my website so do take a look and please do share it out all right are you guys ready and excited i'm super excited and let's go ahead and start so what are we going to be doing today we are going to be looking at the different phenomena right so we are going to be looking at the different phenomena of light light is very interesting thing because you know there is light shows reflection there's reflection of light there is refraction of light which also we're going to talk about there's dispersion of light and scattering of light right so have you guys heard of these things before reflection refraction dispersion scattering and these are all the different phenomena that light shows and we're going to be talking about these all right so let's go ahead and start with reflection of light first so what is this i'm sure all of you have seen this picture this is a mirror right here right you have a mirror here and what is this called guys what is this called this is the incident ray so you can see light is being incident on the mirror so what does a mirror do a mirror or a polished surface it's going to reflect the light right so let me draw the reflected ray here so the reflected ray is going to look something like this right right this is our incident ray this is the reflected ray all right and what is this thing called if i draw this thing here what do you call this line i'm drawing something which is perpendicular 90 degree to the mirror surface and this is a very important line to draw what is it called can you guys tell me i'm looking at the chat here what do you call it so this is incident ray this is the reflected ray the mirror reflects it and what is this green line called it is called the very good i can see the correct answer ash has the right answer psi has it correct very good it's called the normal absolutely right guys because it is perpendicular to the mirror surface now why is the normal important because the normal is used to measure the angles right so you guys know what is this angle called the angle the incident ray makes with the normal not with the mirror right that is called the angle of incidence this angle is called the angle of incidence and this angle that the reflected wave makes with the normal is called the angle of reflection and according to the law of reflection how is the angle of incidence related to the angle of reflection can you guys tell me how are these two angles related so let's say this is 40 degree right let's take an example let's say this is 40 degree what will be the value of r can you guys tell me here very good so i see chaitanya saying angle of incidence equals to angle of reflection very good awesome guys and thanks for all the likes if you haven't hit the like button please hit it right now and do share our channel with your friends absolutely right you guys are saying they're equal so this will also be 40 degrees fantastic so very important this is the law of reflection right angle of incidence equals the angle of reflection i equal to r for a mirror right and this is even followed for spherical mirrors did you guys know that right there also you can draw normal uh but law of reflection holds for plane mirrors and spherical mirrors spherical mirror means concave and convex mirror okay and of course there's the other law which states that the incident ray the normal and the reflected ray they all lie in the same plane can you guys tell me what is the plane here so the incident ray the normal and the reflected ray at the point of incidence they all lie on the same plane so can you guys tell me what is the plane involved over here who can tell me so i'll give you some options here is the plane the mirror is the plane the mirror here or what is the plane here can you guys tell me some of you are saying mirror i see italia is saying mirror chaitanya saying mirror iron gauche is saying the board he's saying the digital board this new board right so again look at the sentence carefully a lot of students make mistake here the incident ray okay the incident ray the normal and the reflected ray at the point of incidence they all lie on the same plane where are they lying they are lying on this digital board not on the mirror so that's the correct answer the plane here is the board okay please take note of that all right now let's talk about refraction of light so what is reflection it was bouncing back of light by a mirror right what is refraction of light can you guys tell me refraction of light let's take a look with a simple example so here you can see light is let's say traveling from air okay so light is traveling from air to this rectangular glass block okay so can you guys see here light is traveling from air to glass okay now what's going to happen will the light go straight like this so do you guys expect this light to go you know something like this is this correct what do you guys think yes or no have i drawn this correctly come on i want all of you to participate here is this correct no very good so if light was traveling only in one medium you know light travels in a straight line that is called rectilinear propagation of light but here light is not traveling in the same medium right it's going from air to glass light is going from air to glass so there is a change of medium and when there is a change of medium light bends there is going to be bending of light okay so what is happening going to happen here it's not going to go like this so i'm going to erase this part okay so can you see the fancy eraser that we have here and what's going to happen is light is going to bend and to determine which direction it bends let's again go ahead and draw the normal okay so i'm going to draw the normal here at the point of incidence and if you guys can't see this part of the normal i'll make it black color here all right so that's our normal okay so what is going to happen light is going to bend because it's traveling from a rarer medium air is a rarer medium compared to glass light is going to bend towards the normal and then it's going to travel right up to here okay so can you see light didn't go straight light is bending and this is exactly refraction of light now what is going to happen here so this is very interesting again light is going to go from glass to air do you guys agree so what do you think will there be another bending at this point is there going to be another bending what do you guys think very good a lot of you said right satya is saying towards the normal very good so what is going to happen here now it's going to go from denser to rarer glass is a denser medium then air air is a gas right glass is a solid right so it's going from and obviously optically denser right for light so it's going from denser to rarer so definitely the light is going to bend away from the normal so all these rules are given in your book so please remember rarer to denser towards the normal denser to rarer away from the normal and that's what's going to happen here right so there you can see and i've drawn the normal here for you okay so this is an example of the rectangular glass block where this is refraction of light and what is refraction of light again to remind you it is basically bending of light guys it is the bending of light when light travels from one medium to another all right is this crystal clear so let's move on all right so now let me ask you this question we had talked about refraction here the bending of light now let's take a look at what's going to happen here so what do you call this you know triangular glass piece what is this called this in physics is known as a prism okay and you have this let's say this is a red colored light okay if you have this red colored light incident on this prism can you guys tell me what's going to happen here okay so i'll say that this is a so all of you can see this rays incident here and we are going to say this is red colored light red light so what is going to happen here some of you are writing dispersion okay some of you are saying deflection what do you guys think ali is saying seven colors some of you are saying scattering see a lot of you are using a lot of science words i'm constantly looking at the chat here on the screen dispersion so this is a very interesting tricky question when you have red light this means light is of a single color this is called monochromatic monochrome chromatic mono means one chroma means color so light of a single color when you have light of a single color it will not split into the seven colors because it's only red color okay so please note this is only red light i told you that it is of a single color called monochromatic so what's going to happen again light is traveling from air so it's going from air and this is also air obviously to glass right this is the glass prism so light is going from air to glass so what is going to happen can you guys tell me yes this is not white light some of you are writing white i told you this is red light okay so what is going to happen is again light will bend okay and again because it's going from rarer to denser it's going to bend towards the normal okay so it's going to bend something like this and then again from denser to rarer so it's going to bend away from the normal and it's going to come out like this so there will be two bendings and one very important thing just like rectangular glass block we had two bendings remember one here and one here because there was a change of medium same thing happening here two times okay but one very important thing for you to remember that rather than you know thinking about the normal for prism you can remember that light will bend towards the base of the prism can all of you see this light is bending towards the base of the prism okay so light tends to bend towards the base so that makes it easy to draw this diagram okay so please remember because this was monochromatic there will only be bending of light so what is really happening here since it is only bending of light we can say only refraction is happening refraction of light so this is a tricky question right so is this crystal clear to all of you guys please tell me is this clear very good some of you asking what if it is white light wow you guys predicted it so let's take a look at the next slide exactly that's my question now instead of red light if you have white light very good so white light is it monochromatic no white light is made of many colors so we call it polychromatic poly means many i hope my spelling is correct okay polychromatic many colors now what is going to happen can you guys tell me will it just bend like this and come out as white light so is this going to happen the same thing is this correct yes or no what do you guys think very good satya singh seven colors vijay is saying seven colors absolutely right conversing dispersion fantastic guys because it is polychromatic you guys know that this will not happen it's not just gonna be bending of light there's going to be dispersion of light what does that mean splitting okay so what happens in splitting it's going to split into its different colors right so when white light reaches here remember white light is made of seven different colors seven colors of the rainbow all of you know so to make it simple rather than take seven colors we are going to take rgb right we are going to consider light to be made of the three colors red green and blue just for simplicity here okay so what's going to happen when light enters there will be some bending of red light right it will bend at a certain angle there will be some bending of green light okay and there's going to be some bending of blue light all of you agree so right over here it's going to split up okay it's going to split up into the seven colors for simplicity i've drawn it otherwise i'll have to draw seven rays i've drawn red green and blue and when they emerge out again there's going to be another bending so it's going to look something like this okay all of you can see that i just changed the color of the green slightly so that you guys can see it is it visible on this new digital board can you guys see it and can you guys hit a like if you are liking this live class please hit a like right now and so what is happening here as you guys absolutely correctly said can you see there's splitting of light light is not just bending but it is splitting into its colors right so what we call this we call this the dispersion of light dispersion means splitting off light typically we say white light into its seven colors absolutely thank you guys thank you wow we have 300 likes thank you thanks a lot it's really motivating and if you haven't hit the like button please hit it right now and do share our channel with your friends okay so all of you can see this and you guys agree here that dispersion of light is taking place okay so this is what happens right just now we discussed so again for simplicity i had shown you that you know you'll get red here and the order is very important always red is on top here right red is on top and then you have all this followed by orange yellow then green and then you have you know rain blue indigo and violet vib gear right so i'll just draw these for simplicity here i think the green is not visible let me use a different color so that is the red you have the green here and then we have the blue color here okay or the violet i've just shown it for simplicity right so what is this band of seven different colors called when the white light when this white light over here right when the white light splits up what do we call this band of seven different colors can you guys tell me what is it called so what are these band of seven colors actually you know there are millions of colors because you can have dark red light red you know dark orange light orange so it's you know not just seven but we take it to be seven colors for simplicity you guys know it's the whip gear right and you can see whip gear is in reverse so it's vib gear or roy g b uh roy g biv okay because red is on top violet is below and what is it called fantastic i can see the right answers here right so ali is saying spectrum very good very good very good don't say rainbow very good some of you are saying rainbow that's it looks like a rainbow but in science terms this is known as a spectrum please note this okay so next time you see a rainbow you can tell your friend i'm seeing a spectrum in the sky sounds fancy right so basically this is called a spectrum with seven colors or actually there are millions of colors but for simplicity we take it to be these seven colors okay the whip gears fantastic so now interesting question is why does this dispersion of light happen right because previously we saw that when you take only a single color what does a prism do to light because there's a change of medium from air to glass it bends the light towards the base of the prism okay right but now we're asking this question that right we looked at the dispersion here the phenomena the phenomena is called dispersion but we're asking the question that why does dispersion happen can you guys tell me why does it happen so why is light splitting into these seven colors so when you have white light here why does the light get split up so it's very very interesting you know that white light is composed of all the seven colors right whip gear okay and the interesting thing is so all the seven colors are entering into the glass prism now when they enter what is interesting is the seven colors they had the same speed in air okay but the moment they enter the glass when they enter medium other than air or vacuum the speed of each color is different and you guys know who's the fastest so out of all these guys who's the winner who's the fastest in glass in air they have the same speed please remember that okay so air all the seven colors have the same speed but who's fastest in glass can you guys tell me very good you're saying mirage is saying they have different velocities absolutely right red absolutely right guys so red color is the fastest and that is why it bends the least okay so red will bend the least uh and who's the slowest actually violet right or we'll consider blue or violet right so the slowest guy is violet here all right and so therefore you can clearly see that red is the fastest this is red here and this is violet here okay and then all all these guys like green is somewhere in the middle so because red is the fastest in glass it bends the least while it is slowest because of the nature it bends the most and that is why you can see the colors get split up in glass okay so this is why so what is dispersion happening due to it is happening due to the change in speed of light so change in speed of light of the different colors right so change in speed of light of different colors in glass can you guys see this right so it is due to the change in speed of light of the different colors in glass and that is why dispersion occurs so it is really different bendings or different refractions but why different refractions because they are having different speeds in glass is this concept crystal clear it's a very famous question why does it happen okay so is this clear to you guys fantastic now you guys might have seen this phenomena or you might have read this in your earlier classes have you guys heard of tyndall effect what is tinder effect you know that when you see when there is dust in a room or you know there there's dew or dust you know you can see this beam of light so the picture may not be very clear for you but you guys can see that when you look at the sunlight here can you see normally light is invisible right normally light is not visible it's invisible we see three things through light but you cannot see the beam of light but sometimes you know when this dust is there it seems like the light becomes visible so we can see this beam okay and this is called tindle effect now why is uh why does tinder effect happen is it due to reflect refraction is it due to reflection is it due to dispersion can you guys tell me so what is it due to so i was asking is the tindall effect this uh tinder effect means you can now see the beam of light it's visible right or you know that when you throw light in a dusty room you can see the beam so is it due to refraction is it due to reflection is it due to dispersion or is it due to something different what do you guys think wow you guys know your physics aishwarya is saying scattering very good dark angel is saying scattering of light web is saying scattering very good awesome guys absolutely right it is none of the options i gave you tindel effect is caused due to scattering of light due to scattering of light right due to scattering of light what is the meaning of scattering you know a child scatters his toys in the room right throws around the toys we can say right or you scatter your books or your books all scattered when you're studying for your exams you have heard of this english word right so this is what so what but what do we mean in terms of scattering of light let's understand this concept because it is none of that refraction reflection or dispersion it is a new phenomena and let's take a look at it so i'm going to explain you in a very very simple way right i'll make the concept super easy because this is a difficult concept that what is scattering of light so to make it easy first let's consider so scattering of light happens when light falls on particles right it could be dust particles or it could be air molecules right so here you can see two pictures here there's a dust particle which is much larger in size as compared to an air molecule okay so these particles cause the scattering of light so first let's consider the dust example and dust particles do you guys agree dust particles are obviously larger than the air molecules okay so what is happening here let's say white light is falling on dust so what happens in scattering it's very interesting dust actually absorbs the light okay and then it re-emits it okay so the dust particle actually absorbs the light and re-emits it so if you want to understand the detail it is actually absorption and re-emission of light okay and so the light gets so when this light comes here dust absorbs it okay so dust will absorb the light and then it will scatter it in the different directions so all of you with me right now are you with me that dust absorb the light and then it can't keep the light energy with itself right it will re-emit it but now in all directions it is scattering it randomly okay but don't think dust absorbs all the light right because scattering is only happens you know five percent right most of the light will just travel straight okay because dust particles are it's not completely like blocking it right so the light that is coming on to it only five percent of it is scattered right so because of the dust particles uh like they're not absolutely densely packed right so they're they're present they will be scattering five percent of the light and so they absorb and re-emit it here can you guys see that do you guys agree okay now comes the interesting part so you guys know that white light is composed of the seven colors right so again to make it simple we'll consider three colors right so basically the red green blue so what's happening here red color is incident on the right you have red color here you have green color incident on the dust particle and blue color so please follow me carefully so basically white light i'm just showing the three colors of white light rgb okay now what does does is it takes all these colors and it re-emits all the colors equally or scatters them equally so i'm going to replace these arrows with the same thing right so we'll replace these white arrows with the colors right so we can say red is scattered this way red is scattered here and red is scattered here similarly green is also scattered green is also scattered and green is also scattered and similarly we have blue scattered here right blue scattered here and blue scattered here all of you agree now the important thing is dust will scatter all the colors equally very very important dust scatters all the colors equally is this clear so therefore what will you see if all colors are scattered equally this is all white so dust scatters all colors equally right it's not partial all colors are scattered equally so we will see white and that is why when white light falls on dust particles what is the color of the beam that you see guys can you guys tell me so when white light falls on dust particles what will be the beam of light color look like can you guys tell me here wow we have 400 likes thanks a lot guys thank you thank you so much so can you guys tell me what will be the color of light please tell me guys when white falls on dust because it is scattering all the colors equally what do you think will be the color of light black no it won't be black see black is not a color black means absence of light it's going to be white fantastic because all colors are scattered equally so red green blue it's again scattered equally here but now it becomes more interesting when we consider the air molecule the air molecule is very small and it's comparable to the wavelength of light here the dust particles are much larger than the wavelength right the length of the light wave so air molecules are much tiny so what is going to happen here is very interesting again white light is falling on the air molecule and white obviously means again we have red green so i'm still getting used to this new board you know green blue hope all of you can see it clearly here right so white light is red green blue now very interesting air molecule does not scatter all the colors equally do you know who it prefers it scatters blue more than green than red this is due to the nature of the size of the particle right it's comparable to the wavelength of light and the wavelengths which are shorter they get scattered more right because there's a you know there is a relation that's catering is inversely proportional to wavelength to the power 4 right so blue light has the smallest wavelength of course we're not considering violet we're just comparing rgb right so blue is smaller has smaller wavelength than these guys so it will get scattered more so what happens is what do you end up seeing basically what will you see that blue color is scattered the most right so i'm going to draw it with long blue lines okay because blue is being scattered the most here can you guys see that okay and green is scattered a little less compared to blue and red is scattered the least okay so because red has the largest wavelength so red is scattered even less so i'm going to draw it with tiny lines here maybe you can't even see it right because it's scattered less so do you guys can you guys see this diagram very very important so just compare this the scattering due to dust where all colors are scattered equally here all colors are not scattered equally okay and what is this known as this is called preferential scattering so let's write that down this type of scattering is called preferential again i hope my spelling is correct otherwise you guys can correct it right preferential scattering of light so what is the meaning of preferential scattering certain colors are scattered more right so certain colors are scattered more than the other colors so blue color is scattered more so what do you think will the you know if you throw a lot of light on air molecules what will it look like what is it going to look like will it look white or what will it look like here so what is the color being scattered the most here can you guys tell me so please take a look here so as you can see the air molecule because of its smaller size right it is doing preferential scattering and who is being scattered the most blue is scattered the most right the blue color is scattered the most so obviously it's going to appear to be blue right because what's happening here is blue color is being scattered more so keep this in mind very very important and we'll discuss its applications so is this concept clear right so what are the different phenomena that we talked about today we talked about we have done reflection of light which is basically bouncing back of light right reflection of light is bouncing back of light what is refraction of light refraction of light is the bending of light when it goes from one medium to another right what is dispersion of light can you guys tell me what is dispersion dispersion is the splitting of light right splitting into its colors so splitting into the colors all right and what do we have as scattering of light what is scattering of light scattering is basically absorption and re-emission all right can you guys see this are you guys liking the class this class in the new format on this digital board are you guys liking it so please take a look at this these are the four important you know different phenomena of light i've outlined it for you and we'll be discussing some now very important examples and questions so please take a look here and i'll be taking more live classes you know using this digital board on my website so if you haven't checked out our courses do go to our website minuteacademy.com we have full courses physics chemistry maths even java coding for class 8 9 and 10 cbsc icac lot of boards international board igcse so please take a look at our website and do sign up for the courses if you haven't and please do share it with your friends all right awesome very good so yes so these are the different phenomena now let's take a look at some questions can you guys tell me what causes the rainbow so we have learned these different phenomena can you tell me i'm sure all of you have seen a rainbow or you have seen it in photographs right so or you can go near a fountain you know there you can see a rainbow i've been to niagara falls you see a rainbow all the time over there just because of you know the amount of water the mist it's creating you can see a rainbow you just have to stand there and take a photograph there's a rainbow there all the time as long as the sun is there okay so what causes the rainbow can you guys tell me so is it reflection refraction dispersion scattering come on i want all of you to participate and i'm looking at the chat here yes so you know if uh i know some of you are saying you know uh reflection refraction let me ask you like this what is the main phenomena if you have to pick one let's say it's multiple choice and you have to pick one answer which one will you go for here can you guys tell me i'm looking at the chat here very good it's going to be no it's not scattering it's going to be the correct answer is going to be dispersion because we discussed right splitting of light into its seven colors the beautiful rainbow right so the main phenomena is dispersion but when you read in your book you also read that you know the it is due to the water droplets ring so rainbows caused by light falling on the water droplets and i'm sure you've read that you know when light enters right so you will not only have dispersion you also have refraction and total internal reflection so please take a look at the ray diagram given in your book if they ask you what are all the phenomena responsible then you need to write dispersion of light refraction there will be bending inside the water droplet and then there's going to be total internal reflection here okay so the light is going to reflect off at this surface so there's going to be total internal reflection here as well so you guys should make sure that you take a look at the diagram given in your book all right so those are all the different phenomena but if we ask you what is the main thing definitely it's going to be dispersion of light because that is the splitting of white light into its different colors all of you agree fantastic wow we have more than 500 likes thanks a lot guys if you haven't hit the like button please hit it right now i'm super excited uh for this live class and hope you guys are too now guys can you tell me you have heard the nursery rhyme right twinkle twinkle little star so why do the stars twinkle we have all heard the rhyme is it due to reflection refraction dispersion or scattering if you have some little cousin singing twinkle twinkle little star please don't ask him this question he will learn it in due time but now guys can you tell me what is the cause of refraction and i'll explain it to you so these are very famous questions if they say what phenomena is responsible you have to write the name right and please write the full word refraction of light or reflection of light or dispersion of light don't just write dispersion dispersion of light okay so what is going to be the answer here all right i can see a lot of different answers but clearly refraction is winning and you guys are absolutely correct the correct answer is it is the bending of light right because it's not twinkle twinkle rainbow you're not seeing different colors right so it's not dispersion there is no reflection involved in the stars right so it is refraction so why do stars twinkle let's take a look okay so let's imagine you're standing on the earth and looking at the star okay so what is uh the cause of twinkling let's understand that so star is you know emitting its light it's like a tiny source all of you agree right so light is falling from the star okay and light initially travels through vacuum so here i've shown this is the atmosphere okay so what we have here is the layers of the atmosphere very good and that's why some of you are saying atmospheric refraction okay let me write that a little more clearly all right atmosphere so you have light falling like this on the layers of the atmosphere okay so what is going to happen it's traveling from vacuum to the atmosphere so is it going to bend towards the normal or away what do you guys think this is all vacuum right because vacuum is rarer than the layers of air so definitely it's going from rarer to denser so it's going to bend towards the normal right and so then they will so there are different layers of the atmosphere so here you can see the atmosphere layers are shown here and obviously these are imaginary layers they're no real layers right but you know that the different layers have different densities so obviously light will show bending so i'm just randomly you know bending the light here and bending it here just showing it randomly okay so constantly light is bending and typically it's bending towards the normal right okay all of you can see that because the layers on top are rarer the layers below are denser right and so finally what happens is if you trace back this ray and let's say you're standing here on the earth so let's say this is you okay i'm not a great artist so i'll just draw you guys here like this so this is you and this is the ray of light that is coming to your eyes and if you trace that back the star actually appears to be here right this is the real position of the star and what is this this is called the apparent position right it appears to be here that is why it is called apparent position and is the apparent position higher or lower than the real position do you guys know can you guys tell me is the apparent position of the star higher or lower than the real position come on i want all of you to participate please write here and tell me is the apparent position of the star higher or lower oh great question somebody's asking why is not the light perpendicular you're not you know you're looking at the star like this it's not perfectly you know yes if it's perfectly in a perpendicular line then there will be no bending you're absolutely right okay very good apparent position is typically higher right so it is higher because the light is bending towards the normal so if you trace it back it is the apparent position is higher again due to atmospheric refraction but why is this start twinkling so okay the position is not correct right but why is it having a twinkling effect so now this is interesting are the layers of the atmosphere stationary will they never change will the temperature density of the layers are they completely like static layers will they never change the answer is no you know the layers of the atmosphere are constantly changing they're constantly shifting changing the temperature is changing so what is going to happen to this light the bendings will keep changing right so right now it's bending like this again it could bend a little differently so the bending is constantly changing and what happens as a result of that the apparent position of the star keeps shifting really because the rays of light coming to your eyes they are constantly changing do you guys agree with me yes or no wow we almost to 600 likes please hit the like button if you haven't right so again hear me out that since the layers of the atmosphere are constantly changing the rays of the light that are reaching your eyes are constantly shifting and changing so the apparent position of the star keeps changing and this constant shifting of the apparent position of the star gives appears like a twinkling effect right because it is a small tiny point source very far away so it's constant shift right it is constantly shifting the apparent position is changing because of the changing in the light path and so this leads to the twinkling effect so why do stars twinkle you guys were absolutely right the final answer it is due to atmospheric refraction right so due to atmospheric refraction of light so now all of you know what is the secret behind twinkle twinkle little star it's atmospheric refraction and you guys can remember this with this simple diagram so now when you read the book it should be crystal clear very good why the moon doesn't twinkle can you guys tell me because moon is not a tiny point source it is much larger so you're right even for the moon there is refraction this same thing is going on but because the moon is such a large it's an extended source of light so the shift in its position is not visible same for the planets right so since they they appear even though star is much larger than these guys because they're closer they appear to be larger and the shift in their position is not noticeable by the human eye so therefore they don't appear to twinkle because twinkling is an optical illusion really it's not twinkling right it is just the shivering right the change in the movement of the star great so now let's move on to another question the famous question why is the sky blue in color right why does the sky appear blue because you know that if you're an astronaut and you go to outer space what is the color of the sky can you guys tell me so if you're an astronaut or if you were an astronaut and you took a trip to space or you have seen the movies right what is the color of the sky for an astronaut can you guys tell me so come on i want all of you to participate here what is the color of this sky to the astronaut very good rohit is saying black absolutely right dark pitch black right because absence of light but we know that so the true color of the sky is black but why does it appear blue so what is the answer the answer lies in what we had discussed here right this is the secret it is due to scattering of light remember we talked about preferential scattering you know that our atmosphere is made up of billions and billions and billions of air molecules right so there's so many air molecules in the atmosphere and what are these air molecules doing with the sunlight that is falling on them so when sunlight is falling on the air molecules these air molecules are scattering the blue color more so we've just had the festival of holy right so what are these air molecules doing they're playing holy with blue color more with blue color and less with these colors right so holy if you take the powder and you throw it you're scattering it right do you guys agree with me so when you throw the powder you're scattering it so these air molecules are like playing the festival of holi and they are playing it with different colors so when white light falls on them they're playing it with all the seven colors but which color are they throwing the most blue color do you guys agree so which color are they throwing the most they're scattering the blue color most so when you look up at the sky right which color will come to your eyes the most it's going to be blue color so please understand remember it like this the air molecules are playing holy right so they are doing what is it they are doing preferential scattering so when white light falls on them they are scattering blue color that is their favorite color more than the other colors so when we look up at the sky it appears blue to us now many students ask why not violet while it is scattered more but our human eye is not so sensitive to violet color so yes if our human eye was sensitive to it we would see a beautiful violet sky but we don't see that we see a beautiful blue sky and you guys are absolutely right it is perfectly the right answer as you guys said it is due to scattering of light right and preferential scattering so please remember that preferential scattering preferential means certain colors are scattered more than the other colors is this crystal clear wow 640 likes thank you guys thanks a lot awesome you guys are awesome next question why is the sun red at sunrise and sunset again is it due to reflection refraction dispersion scattering these are the phenomena we talked about today what do you think is the answer so you have seen the sun is a beautiful red color during sunset right so why does it appear red due to which phenomena is it can you guys tell me so what is the answer going to be obviously it's not reflection could it be refraction can it be dispersion scattering what is the answer here so let's understand and we'll come back to the answer so let's understand the concept here so let's say you're looking at the sun at sunrise or sunset i have rarely seen many sunrises in my life you know that's always been my new year resolution i should get up early okay so hopefully this year i will see more sunrises than sunsets but for me let's take the sunset example okay i've seen the beautiful sunset at kanye kumari have you guys been there it's beautiful right it's called sunset point right the tip right so where you can watch this beautiful sunset so let's say you're standing at kanyakumari right watching the beautiful sunset so let's say this is you so you're here now you're watching the sun setting right so this is the horizon sun is setting so sun obviously you know gives white light okay don't think that the sun gives yellow or orange light or red light it gives white light it's a source of white light all the seven colors so what's happening is sun is giving white light right so white light is coming from the sun and as we discussed we can uh the white light is made of seven colors so let's simplify it into red green and blue okay so i'm going to draw that here so all the colors that are coming from the sun are red green and blue so let's draw it out guys red color green color and blue color okay all of you can see that so now based on our discussion all these colors are traveling nicely in vacuum happily they're traveling in vacuum but when they enter the atmosphere can you guys tell me which color will be scattered the most we just learned that right so out of these three colors can you guys tell me which color will be scattered the most yes come on awesome guys please try here which color is going to be scattered the most out of these three colors so this is the sun here right so what i have here this guy that you see is the sun here giving out white light so clearly blue color is scattered the most right blue color is scattered most by the atmosphere the air molecules we know that so most of the blue color we can say gets scattered away right so let's draw that here most of the blue color gets scattered away all of you agree so blue color has been scattered away because it has the smallest wavelength very good right excellent what about the green color so green color also has it has slighter larger wavelength than blue color but not as much as red so even green color is scattered away quite a bit okay so we can say even green is scattered away quite a bit i'm just drawing it roughly here okay right what about the red color so red color you guys know because of its larger wavelength it is scattered the least and so we can say most of the red color reaches our eyes of course green and blue you know other colors will reach little bit but more of the red and the orange color right so more of the red color reaches because red is scattered the least all of you guys agree so therefore if you're looking at the sunset so if you're standing here you know and looking at the sun the sunset or the sunrise if you wake up early that's great you know i should be like you so if you wake up early you're looking at the sunrise or the sunset what will be the color of the sun appear to you right because most of the red light is reaching your eyes so therefore we can say the color of the sun is going to appear to be reddish to us do you guys agree so i'm painting this sun with red color so it appears to be a whoops that's too large it appears to be a red colored sun and this is why it explains the sun is red at sunrise or sunset do you guys agree because most of the red color and who's scattered it it is the atmosphere right the atmosphere and one important point is that you can see that during sunrise or sunset the light is going through a large part of the atmosphere can you guys see that it is going through a large part it is traveling through a large part of the atmosphere because it's along the horizon and that is why most of these colors are scattered but when the sun is on top at noon time when it is overhead then it is light travels only through a small part of the atmosphere and so the colors are scattered less and therefore at noon time when you look up at the sun you will see a white colored sun right but during sunrise or sunset sun appears red does this make sense to you guys are you guys crystal clear about this concept so now when you read the book it will seem super easy right and it is all due to scattering of light awesome so let's go back and take the correct answer right as you guys correctly said the answer is due to scattering by the atmosphere so green and blue got scattered and red came to our eyes right let's take a look at the next question why is there an advanced sunrise or a delayed sunset okay sun wants us to get up early right there's an advanced sunrise right so what does this mean that even before or there's a delayed sunset right so even before the sun comes out you can actually see the sunlight i think around two minutes before or even when the sun set right so when i'm watching the sunset that kanye kumari sun is set right even after the sunset we can see it so there's a delay in sunset we can see it even two minutes later so guys can you tell me which phenomena is responsible for the advanced sunrise or delayed sunset can you guys tell me here wow you guys are physics experts absolutely right you guys are giving the correct answer so let's take a look why the correct answer is refraction of light the bending of light so let's take a look let's really understand this so why is there an advanced sunrise or a delayed sunset whichever way you look at it right there is a two minute advance or a two minute delay at sunset so again i'll go for the sunset case right so let's go for the sunset case so let's say this is sunset time this is the horizon right so this white line that you see here is the horizon okay and so horizon is the line where you know you can best see it at a beach where the land meets the water okay so this straight line along the land and let's say you're standing here looking at the sunset so what can you see here interesting the sun is actually gone below the horizon can you guys see in this diagram so this is during sunset time so let's say it's during sunset so the sun is actually set because it has gone below the horizon so will you be able to see the sun you should not be able to see the sun right because the sun has gone below the horizon below your the level that you can last see okay now the interesting thing is again remember light is traveling from the sun light is standing from the sun to you okay so first it travels through vacuum and then it goes and hits the atmosphere so obviously light is going to bend right because it's going from a different medium it's changing medium and in atmosphere there are different layers so light keeps bending and i'm just going to show it in a simple way so light will keep bending here you know and finally it's going to reach your eyes so what happens is there is bending of light because the medium is changing here from vacuum to atmosphere and within the atmosphere different layers so if you trace it back right if you trace it back so let me draw it clearly here so if you trace this light back the sun seems to be above the horizon so if you draw the sun this seems to be the apparent this is the real position right so the sun now appears to be here so please understand this was the real position of the sun but this is the apparent apparent means what appears to be this is the apparent position of the sun so can you see even though the sun is gone below the horizon even though the sunset has happened you can still see the sun right so there is a delay in sunset and it's about a two minute delay all of you guys can see that right so i'm just drawing it roughly here you know my diagram is a bit untidy i'm still you getting used to this digital board but i'm super excited to let you know that i'm going to be using this digital board for the live classes and even the live classes on our website and the youtube channel so i take more live classes on our website where if you take the full courses i'll be taking more classes there so do take a look i've put the links below and please do share it with your friends all right guys so take a look here so there is a delay in the sunset because it is actually still there so it is an optical illusion sun seems to be there and that is why we can still see the sun even after the sunset or we can still see the sun before the actual sunrise before it comes up the horizon we can see it so what is the answer it is due to again atmospheric refraction so if you want to write the full answer it is due to atmospheric refraction of light it is the atmosphere who is refracting it right all right and don't forget to write offline don't just write atmospheric refraction is it of sound are you talking about sound no obviously you're talking about light here so please mention please remember to mention that all right and one more interesting question here can you guys tell me what will happen here very famous question if you see two prisms which are arranged like this and then you have white light incident on the two prisms can you tell me what will you see at the output so what will you see at the end right output i'm using you know uh coding language like java right so what what are you gonna see here tell me wow 730 likes awesome guys you guys are rocking here fantastic and please try this question so will you uh see a spectrum or what will you see here because okay again this is white light right oops let me write it with a white color so this is white light polychromatic light so what is going to happen here guys so obviously you know let's analyze at this prism it's going to split into the rainbow color so i'll just draw red green and blue so red green and blue are okay so obviously it's going to split here but what's going to happen here right and so it comes out like this red green and blue all of you can see that okay what's going to happen at this second prism will it further split or what's going to happen come on very good yashwanth is saying white light gokulnath is saying recombination okay samyukta saying recombination wow fantastic guys very good so this is very interesting because again you know that this inverted prism will have an opposite action right if this guy was splitting this what is the first prism doing here first prism was doing dispersion it was splitting the white light into the seven colors the second prism is going to be combining the seven colors back into white light right it's going to be the opposite action so what is going to happen here these light colors will combine right very very interesting so these light colors will combine again i'm drawing it roughly here you guys will i'm sure draw a better diagram than me so these rays are gonna can you see the recombination is happening so this is very interesting that you can split light or if you recombine the seven colors what are you going to get you're going to get white light at the end so if this was dispersion this is recombination this guy is doing recombination of white line recombination and this fellow this guy the first prism here is doing dispersion so all of you can see that dispersion and recombination very very famous question and just take a look so this will get your concept clear you know it can disperse or split up and again it can recombine and guess what it recombines back to white light very good and yes newton also did this famous experiment absolutely right absolutely right wow you guys rock very good keep revising you know keep studying and i think this was the final question so hope you enjoyed this class i was i'm super excited because this is my first youtube live class uh with the digital board i'm taking more live classes on using this digital board on our website so if you haven't checked out the courses on our website manoj academy.com do check it out now i know a lot of you have taken the courses and so please do share it with your friends we have you know uh physics chemistry maths for class eight nine and ten for the cbsc board so full courses are available where i'll be taking more live classes and we also have that for icsc for class 8 9 10 physics chemistry maths so our team has put up all these full courses and they're on big discounts for a limited time so guys do check them out and we are super excited to let you know that we have the cambridge board right the international igcse course for physics and chemistry out there and we also have you know these awesome coding classes right so if you want to learn java coding java is a great language to learn computer programming and i'm going to be taking live classes in this uh advanced level and in the basic level also you have my classes there uh you guys can take this it's offered as a combo pack as well so do take a look at our courses and please do share it out with your friends and if you haven't subscribed to our youtube channel please hit the subscribe button right now and hit on the notification bell so that you don't miss out on any of our videos and do share our courses and our channel with your friends and i'm super excited to let you know that guys are technical team is working on the notes feature so soon all the stuff that i'm writing here so all the explanations i'm writing we'll be putting them as notes on our website as well so you can access the notes on manoj academy.com and we will also be having quizzes and questions out there you can ask doubts you can send us doubts or ask doubts during the live classes and uh we'll also be conducting mock tests right so very useful these courses are for you guys so do check them out and we have them for the icse and cbse board as well so hope you guys enjoyed this class wow we have 770 likes if you haven't hit the like button please hit it right now so this is sandeep noches signing off hope you guys enjoyed it and i really had fun with the digital board and with all of you you know interacting so it was great see you in the next live class so make sure you have hit the subscribe button and the notification bell so that you don't miss out on any of our classes and if you like our live classes please go to our website manoj academy.com we have these full courses where i'll be taking more live classes all right guys thanks a lot i'll sign off here see you next time bye bye take care and keep studying and i'm sure you guys will do well keep the momentum going and all the very best for your exams if you haven't had them already take care of your health and keep studying all right all the best bye