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### Course: Class 10 Physics (India)>Unit 1

Lesson 8: Refraction of light

# Refraction and Snell's law

Refraction and Snell's Law. Created by Sal Khan.

## Want to join the conversation?

• Photons don't have any mass so techincally don't occupy any space.....
so how is it that the density of a medium causes the bending of light?
and why does the photon bend when it goes into a denser medium?
• Alright, let's take a beam of light, travelling from an optically rarer medium to an optically denser medium, say air and glass respectively. The beam of light is slower in a denser medium because it's movement is hindered by lots of particles (in the media), compared to a rarer medium. It doesn't have that much space to travel freely. The photons keep bumping into all sorts of particles in the medium.
When you shine the beam of light on the surface that separates the two media, the photons in the beginning of one side of the beam hit the surface first, right? So they're the first set of photons to become slower (entering denser medium). Since the photons in the beginning of the other side of the beam don't hit the surface at the same time, there's a kind of a lag, and the beam bends. If the beam didn't bend, the beam wouldn't even be a beam, as the two lines of photons wouldn't be in the same line. Which can't happen.

This also explains how the beam of light DOESN'T BEND when you shine the beam along the normal; the two sets of photons hit the surface at exactly the same time, so the light doesn't bend; it just becomes slower.

Hope this helps :)
• What if light is travelling parallel to the normal
• Considering the example which Mr Sal gave, i.e., of the car entering the muddy area from the road, if we take the path traveled by car to be the incident ray , the road to be the first medium, the angle of incidence to be 90 (equal to the normal angle) then we find that the front 2 wheels of the car reach the mud at the same time, so, it does not bend to its either side. It keeps travelling in the same line, i.e. along the normal or parallel to the normal. In the same way light does not bend towards the normal or away from the normal when it travels from one medium to another. Thank You!
• Till date my physics teacher taught me that speed of light is same in vacuum and air, but sal has something diff. to say here?
• The speed is not exactly the same but it is very, very close. So close that for most problems you can just treat it as the same. In most problem, we say c is 3*10^8 m/s, and the speed in a vacuum and the speed in air both round to that.
• What if the medium is vice versa ,first the slower medium and then passes through faster one? What would happen actually..??/// I am little bit of curious to know it...
• if light travels from a denser medium to a rarer medium then it actually bends away from the normal as the speed of light increases as it enters the rarer(hence faster) medium.
• What basically is Refractive Index?
• The refractive index tells you the speed of light in a given material. it is defined as n = c/v
where c is speed of light in vaccum, and v is velocity of light in the material.
The refractive index is used in (not defined by) Snell's law, which relates the angle of incidence to the angle of refraction when light passes from one material into another.
• A merely theoretical postulation: if we were to create a beam of light that is comprised of a single stream of photons, would this light beam undergo refraction? I ask this question because the theory explaining the occurrence of refraction always relies on the fact that one portion of a light beam will hit the second medium before another portion, implying that the light ray must have "width" to undergo refraction.
• It's not merely theoretical, it's a question testable by experiment, and the answer is yes, it will refract. You can see a demonstration of behavior similar to (but not the same as) this, in this video, where a beam of single photons diffracts. Diffraction is not refraction but it is another wavelike behavior that seems like it should be impossible for particles to do, but they do.
• what is refractive index
• It is the ratio of the speed of light in vacuum to the speed of light in a material, denoted n=c/v. Since the speed of light in vacuum is a universal speed limit, the refractive index is greater than 1.
• is there a proof for snells law?
• We don't do proofs in science. Perhaps you mean derivation rather than proof.

For Snell, it was an empirical observation, not a derivation.

It can be derived, however, if you start with a postulate that the light will follow the "path of least time". There are other ways to derive it as well. They all came after the law was expressed by Snell, not before.

https://en.wikipedia.org/wiki/Snell%27s_law#Derivations_and_formula