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## Physics library

### Course: Physics library>Unit 15

Lesson 1: Reflection and refraction

# 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? • 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... • 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. • what is refractive index • is there a proof for snells law? • When light travels from a denser medium to a rarer medium, it gains velocity, but to gain velocity wouldn't the photon need to gain energy to increases its velocity? If yes then from where would it get the energy? • This gets a bit complex. Light waves or photons are expressions of vibrations in the electromagnetic field. Light interacts with electrically charged objects. When light interacts with a material like glass it interacts with the electromagnetic field in the glass which causes interference between the electromagnetic oscillations of light and the charged particles in the material. The resulting fluctuation in the electromagnetic field has a slower phase propagation than the light wave had. When the fluctuation gets to the edge of the glass the charged particles in the glass no longer interfere with the lights fluctuation so the speed of its phase propagation is back to its normal speed. There is no need for a increase or decrease in energy for this to happen.
• suppose a ray of light travels from vacuum to glass at 90 degrees. In this condition, angle r would be 0. Applying snell's law we get
sin90/sin0 =c/v which would give 1/0=c/v but 1/0 is undefined. how can u explain this? 