# Refraction (Help)

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#1
Hi, I'm in school right now. (Year 9.) We've just done refraction in Physics and my teacher has taught us nothing, I'm really struggling to understand and I've searched it up but it still isn't clicking. Could anyone briefly explain the concept of refraction (simply, if possible?) It would be much appreciated!
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6 months ago
#2
First, what is refractive index ? Light travels at different speeds in different materials, the refractive index tells you how much slower light goes in a medium. So the refractive index n of vacuum is n=1 and light travels at c, the refractive index of glass is about n=1.5 so the speed of light in glass is slower and about c/1.5.

Now the refraction bit. When a light ray passes from one medium to another with different refractive index the ray path can get bent (or reflected), how much depends on the ratio of refractive indices and the angle it hits the boundary at. It follows Snell’s law n_1.sin (theta_1) = n_2.sin(theta_2) see the link below for how the angles are defined relative to the surface normal.

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

Other things can happen as well, some of the ray will bounce off the interface like a mirror, how much depends on Fresnell reflection. This is why you can see a relfection in a window, even both the air and glass are perfectly transparent.

https://en.wikipedia.org/wiki/Fresnel_equations

Also for light hitting a boundary at the right angle, Sin(theta) in Snell’s equation would need to be bigger than 1 .... which is unphysical. That angle is called the “critical angle” and beyond it, the boundary acts like a mirror and all the light is reflected rather than being refracted. This is how optical fibres guide light.

"Simple" optical components like prisms change the direction of a light ray, but the refractive index is also a function of wavelength, so they do this different amounts for different colours. This is why a prism splits white light up into a spectrum, that’s called “dispersion”.

Curved surfaces like a lens bend rays different amounts as the angle they hit the lens at varies from place to place. They all intersect down stream at the focal length of the lens.
Last edited by Mr Wednesday; 6 months ago
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#3
(Original post by Mr Wednesday)
First, what is refractive index ? Light travels at different speeds in different materials, the refractive index tells you how much slower light goes in a medium. So the refractive index n of vacuum is n=1 and light travels at c, the refractive index of glass is about n=1.5 so the speed of light in glass is slower and about c/1.5.

Now the refraction bit. When a light ray passes from one medium to another with different refractive index the ray path can get bent (or reflected), how much depends on the ratio of refractive indices and the angle it hits the boundary at. It follows Snell’s law n_1.sin (theta_1) = n_2.sin(theta_2) see the link below for how the angles are defined relative to the surface normal.

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

Other things can happen as well, some of the ray will bounce off the interface like a mirror, how much depends on Fresnell reflection. This is why you can see a relfection in a window, even both the air and glass are perfectly transparent.

https://en.wikipedia.org/wiki/Fresnel_equations

Also for light hitting a boundary at the right angle, Sin(theta) in Snell’s equation would need to be bigger than 1 .... which is unphysical. That angle is called the “critical angle” and beyond it, the boundary acts like a mirror and all the light is reflected rather than being refracted. This is how optical fibres guide light.
Thank you so much! This is really, really helpful.
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6 months ago
#4
(Original post by c.o.r.a)
Thank you so much! This is really, really helpful.
The cool bit is that it is explained by a something called “the principle of least time”, somehow the light “knows” which path to take to minimise its travel time ..... that’s “freaky quantum mechanical stuff” you learn about at university ..
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