hi friends I'm sure you've all heard that light travels in a straight line it's called the rectilinear propagation of light but are you ready to try your magic trick with me where we are going to bend light for the magic trick I just need you to get a transparent glass like this filled with water so pause the video here and go and get a glass of water are you ready with your glass of water next I'll be showing in our on the screen and you pause the video when you see the arrow and hold up your glass of water in front of the screen something like this and look at the arrow again and tell me what do you see this time did you see the magic did you see the arrow changing its direction it's happening due to bending of light this is called refraction of light and it's going to be the topic of this video and then as usual we'll finish off with our top three questions on this topic I have this torch here with me when I switch on the torch it gives out a beam of light a beam is composed of many rays of light as you can see each rays traveling in a straight line now for simplicity let's consider a single ray of light now how do I bend this ray of light I can't just grab the ray of light and bend it right so what do you think I need to do to bend the light you might be thinking of using a mirror so a mirror certainly changes the angle by reflecting back the light but what if I ask you I want the light to continue downwards and still bend it then what do we need to do so you need a transparent material like glass or plastic here let's imagine there's a large glass block kept here and I'm going to shine this torch on the glass block again we'll consider a single ray of light for simplicity can you see the ray of light bending as it enters the glass block this is known as refraction of light there is also a partial reflection when the light hits the glass surface but we'll ignore that for this video and our focus will only be on refraction of light so what have we learnt till now light travels in a straight line as long as it's traveling only in one material so as we saw here as long as it's traveling only in air it's traveling in a straight line and then if you look only at the glass part light is traveling in a straight line but when light travels from one material to another so here from air to glass it bends and this bending of light is called refraction of light so here's the fancy definition of refraction of light refraction of light is defined as the change in direction of light when it's traveling obliquely from one medium to another medium so what does this all mean let's break it down here so change in direction means bending of light obliquely means the light is incident at an angle and the word medium means material for example air glass water these are all materials you may also see the term optical material because optical refers to light and for refraction remember that the material needs to be transparent let's place the definition of refraction of light on our concept board have you seen any simple everyday examples of refraction one thing that you might have noticed or you can try yourself is take a pencil and put it in a glass of water the pencil will appear bent it's due to refraction of light another example is if you go and stand near a swimming pool it actually appears shallower than it really is and of course the magic trick that we saw at the beginning of the video so do go ahead and show it to your friends but don't tell them the secret and there are many many other examples of refraction but we look at those in detail in a separate video now that we know what is refraction let's take a look why refraction happens so what is the magic this glass block is doing to bend the light the answer is it changes the speed of light so let's say the speed of light in air is approximately 3 into 10 to the power 8 meter per second and since glass is denser than air it slows down the light when it enters the glass block and let's say the speed here is 2 into 10 to the power 8 meter per second so this change in speed of light when it's traveling from air to glass causes the light to bend and greater the change in speed greater will be the bending of light we've seen what is refraction why refraction takes place now let's take a look how light bends during refraction in this case we saw light bends this way but is it possible for light to bend the other way also let's go ahead and take a closer look let's look at some terms first this ray of light is called the incident ray do you know where is the point of incidence that's right it's here the point where the incident ray hits the second medium is called the point of incidence now let's draw an imaginary line perpendicular to this boundary between the two materials this perpendicular line is called the normal the light ray in the second medium is called the refracted ray can you see the incident light instead of going straight is bending towards the normal the angle the incident ray makes with the normal is called the angle of incidence and the angle the refracted ray makes with the normal is called the angle of refraction when light is traveling from air to glass like here how are these angles related that's right angle of incidence is greater than the angle of refraction here air is the rarer medium or to be more precise optically rarer medium and glass is the optically denser medium we use the term optical density because it's different from normal density normal density is the heaviness or lightness of a material and is measured as mass by volume but optical density measures how much a material is able to slow down light and note that a material that is denser than another one need not be more optically denser than the other material so this was our case one when light travels from a rarer to a denser medium the speed of light decreases the light bends towards the normal and as you can see angle of incidence is greater than the angle of refraction now can we make the light Bend away from the normal the answers yes we just need to switch the two materials so as you can see this is our case - when light travels from a denser to a rarer medium the speed of light increases and as you can see the light bends away from the normal and the angle of incidence is less than the angle of refraction here let's spin these two cases of refraction on our concept board there is another interesting case where the light is incident normally on the second medium as you can see the incident rays along the normal here but even though the light is traveling from air to glass can you see that there is no bending taking place so we say there is no refraction happening here but does the speed of light change when it enters the glass block the answer's yes as expected the light will slow down when it goes from air to glass but in this interesting case there is no bending of light happening here what is the angle of incidence here that's right it's zero the angle between the incident ray and the normal is zero what is the angle of refraction in this case correct it's zero the angle between the refracted ray and the normal here is zero let's place this special case of normal incidence on our concept board now that we are done with the topic of refraction are you ready for the top three questions on this topic coming up for you right now friends try solving these questions and let me know your answers and doubts by putting it in the comments below I promise to reply to all your comments as soon as possible so I'm going to bend away like light and you pause the video here and get these questions a shot as we discussed in this video refraction is the bending of light when light travels from one medium to another refraction is caused by the change in speed of light and do remember to subscribe to my YouTube channel and follow my facebook page and do check out my website manoj academy.com thanks for watching you