hi friends have you wondered why the sky is blue but the clouds are white why the sun appears red at sunrise or sunset why we can see a beam of light in a room filled with dust why danger signal lights are always red in color is it due to reflection of light refraction of light or dispersion of light the answers none of these it's due to a completely different phenomena known as scattering of light and that's going to be the topic of this video i'm going to make the concept of scattering of light really easy for you first let's understand what is scattering of light if i switch on this torch can you see the beam of light no because light is invisible but when i place my hand or if you place a screen then we can see the light because we are seeing the reflection of the light by the hand now let's imagine this room is filled with dust or like you see in the movies when the detective enters the dark and dusty room with his torch yes then you can clearly see the beam of light why is that because the large number of dust particles are scattering the light so we can see the beam of light similarly we can see the beam of light when light is passed through a colloid but the beam is not visible when light is passed through a solution or a suspension now do you remember what is this known as that's right it's called tinder effect tyndall effect is due to scattering of light colloids show tinder effect but not solutions and suspensions so it can be used as a test for colloids as we discussed light is scattered by dust but light can be scattered by other particles as well such as air molecules and water droplets now let's take a closer look how these particles are scattering the light well it depends on which theory of light you're considering if we consider light as a wave then we can say that these particles are reflecting the light waves in different directions so it's like scattered reflections and we say that the particles are scattering the light in random directions now if we consider light as a particle made up of photons then we say that these particles are absorbing the photons and re-emitting them in different directions in different random directions and so we say the light is scattered now do all these particles scatter the light in the same way the answer is no let's go ahead and compare their scattering before we analyze scattering let me ask you what is the color of white light is it really white that's right white light is made up of millions and millions of colors but for simplicity we take it as the seven rainbow colors or the seven whip gear colors in this video we'll simplify it even further we'll just take the three primary colors red green and blue now are the different colors scattered in the same way the answer is it depends on the size of the scattering particles so let's take a look how the different size particles scatter light the atmosphere is a mixture of many different particles such as air molecules water droplets dust particles and so on to study the scattering of light by these different size particles we are going to divide them based on the wavelength of visible light wavelength means the length of one wave so do you know what is the wavelength of visible light the correct answer is the range is 400 nanometers to 700 so we are going to divide these particles into two categories are they smaller than the wavelength of visible light or larger than that the air molecules have a very tiny size they are smaller than 0.4 nanometers or even less because the air molecules are made up of nitrogen and oxygen molecules and they are really really tiny so these particles are smaller than the wavelength of visible light and the water droplets and dust particles these are much larger than the wavelength of visible light to give you some rough numbers the size of these particles is larger than 1000 nanometer or 10 000 nanometer or even more so clearly they are larger than the wavelength of visible light we'll first analyze the scattering of the light by these larger particles that is the water droplets and the dust particles let's say white light from the sun is falling on larger particles such as dust particles for simplicity let's consider the white light to be made of the three primary colors only red green and blue let's focus our attention on only one dust particle the dust particle scatters all the colors equally so as you can see red green and blue are scattered equally in all the directions so the white light when scattered by this dust particle appears white in color an important point to note is that most of the light passes straight through so most of the sunlight just goes straight through and only a small percentage is scattered by the dust particles now when you shine a torch in a room full of dust what do you think will be the color of the beam of light that's right it's going to be white in color because the dust particles scatter all colors equally in all the directions so the beam will appear white now let's look at the smaller particles the air molecules for example nitrogen and oxygen molecules these particles are smaller than the wavelength of visible light so let's say white light from the sun is falling on the air molecules again we are taking the white light to be composed of red green and blue now let's focus our attention on a single air molecule the air molecule scatters the different colors differently and as you can see the blue color is scattered more than the green color and that's scattered more than the red color why because different colors have different wavelengths and shorter wavelengths are more scattered than larger wavelengths blue color has the smallest wavelength so it is scattered the most and red color has the largest wavelength so it's scattered the least in fact blue color is scattered 10 times more than red color this is known as preferential scattering and it happens in particles that are smaller than the wavelength of visible light such as air molecules so are you getting a clue why the sky appears blue to us we know that sunlight is white sunlight is made up of all the seven colors now when sunlight enters the atmosphere the light is scattered by the atmosphere the atmosphere is mainly made up of air molecules and they are trillions and trillions of air molecules as we discussed each air molecule is doing preferential scattering it scatters blue color more than the red and green color so blue color is scattered the most it's like the air molecules are playing with the blue colored powder and scattering it all around so whenever we look up at the sky we see blue color and that's why the sky appears blue to us what do you think the color of the sky would be if there was no atmosphere that's right black just like it appears to an astronaut because if there are no particles to scatter the light the sky would appear black to us the sun would still appear white so the blue color of the sky is an optical illusion it is due to the preferential scattering of light by the air molecules we know that the sky is blue but why are the clouds white think about what the clouds are made up of that's right water droplets and dust particles as we discussed these particles are larger in size larger than the wavelength of visible light so these particles scatter all colors equally so when the white light from the sun falls on the clouds all the colors are scattered equally and so the cloud appears white to us now let's look at why the sun appears red at sunrise and sunset but white at noon let's consider the sunset case obviously i have seen more sunsets than sunrises you need to wake up really early to see the sunrise to analyze we need to zoom out quite a bit so imagine you're standing on the earth like me and looking at the sunset during sunset the sun is near the horizon as you can see the sunlight has to travel the greatest distance through the atmosphere to reach us since sunlight has to travel through such a large distance in the atmosphere a lot of scattering of light will happen we know that sunlight is made of white light the seven colors but for simplicity let's consider the three colors red green and blue light now blue light has the shortest wavelength so it gets scattered the most let's say it gets scattered so much from our line of sight that no blue light reaches our eyes similarly green light is also scattered a lot and it does not reach our eyes red light which has the largest wavelength reaches our eyes because it is scattered the least so what will be the color of the sun that's right the sun appears red to us of course this is an exaggeration small amount of blue and green light also reaches our eyes but red light reaches us the most since it is scattered the least and that's why the sun appears red during sunrise and sunset but the amount of red light that reaches our eyes is much more than the other colors so the sun appears reddish at sunrise and sunset now why do you think the sun appears white to us when it is above us let's say at noon once again let's zoom out and take a look as you can see at noon the sunlight has to travel through a much smaller distance in the atmosphere to reach our eyes again blue color is scattered the most green color is scattered less and red color is scattered the least since sunlight has to travel through a much smaller distance the percentage of blue and green color that scattered is very small so all the colors of the sunlight reach our eyes the difference in intensity of the different colors is small and that's why the sun appears white to us at noon now let's talk about danger signal lights have you seen these lights at the end of aeroplanes or on the top of a tall building so that the planes know that there's a tall building here what is the color of these lights that's right they're always red in color why is it because red is the color of danger or caution no let's discuss the physics behind the choice of color here imagine there are three danger signal lights at the top of a tall building red green and blue light and they are of equal intensity let's say a plane is flying at a far distance which light do you think would be most visible to the pilot that's right it's going to be the red light because red color has the largest wavelength so it's scattered the least by the air molecules red light can travel the furthest distance so that's why danger signal lights are always red in color because it's due to the preferential scattering of light by the air molecules as we discussed in this video the blue color of the sky and the red color of the sun at sunrise and sunset these are all optical illusions optical illusions due to the scattering of light and next time you see the red danger signal lights at the end of an aeroplane or at the top of a building do remember why they are red in color because red color is scattered the least which reminds me do hit the red subscribe button 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