What Causes Refraction to Occur?

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In summary, light will always bend towards the normal when entering a higher refractive index medium. The direction change is specific and always occurs. We don't know why light will always bend towards the normal, but we do know that it is due to the wavefront changing its direction.
  • #1
austinv
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I'm very curious as to what actually causes light to bend when it encounters a medium with a different refractive index. Of course the speed of light changes, and this speed change is accompanied by a corresponding wavelength change so that the frequency remains the same and there is no discontinuity at the interface between media... but why does the light also change direction? And the fact that the direction change is specific and always occurs is interesting. Do we have any understanding as to why light will always bend toward the normal when entering a higher RI medium?

And why is it only when light impinges upon a different medium at an angle? For instance, light striking a surface orthogonally does not change direction.

Thank you very much!
 
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  • #2
Hi Austinv and welcome to the forum!

A wave front approaching media from an angle will produce the situation where the edge of the wave contacting and penetrating the media first will be slowed while the wave edge outside the media will still travel at its original velocity. The difference in velocity between the 2 edges of the wavefront and the fact that momentum must be conserved along the wavefront will effectively pull the entire wave front towards the direction of the edge that contacted the media first.
 
  • #3
Hi and thanks for your reply!

That actually makes total sense, but due to the extremely small scale of the wavelength of visible light, compared to the extent of the surface of a piece of glass for example, is it really true that the bending is occurring because the inside edge of the wave is in the medium and the outside edge is not?

The degree of curvature on the surface of a glass lens compared with a wavelength of visible light seems like there would be no difference between the light striking it straight on versus at an angle.
 
  • #4
The surface of the media doesn't need to be curved. Even with a flat surface the inside edge of the wave front will contact the media first. Another way to analyze the situation is to do ray tracing and invoke Fresnel's principle. You should end up with Snell's Law.
 

1. What is refraction?

Refraction is the bending of light as it passes through different mediums, such as air, water, or glass.

2. What causes refraction to occur?

Refraction occurs when light passes through a medium with a different optical density. This causes the light to change speed and direction, resulting in the bending of the light.

3. How does the angle of incidence affect refraction?

The angle of incidence, which is the angle at which light hits the surface of a medium, determines the amount of refraction that occurs. A steeper angle of incidence will result in a greater amount of refraction.

4. Why do objects appear distorted when viewed through a curved surface, like a glass of water?

When light passes through a curved surface, such as a glass of water, the angle of incidence varies at different points on the surface. This causes the light to bend at different angles, resulting in distortion of the object seen through the curved surface.

5. How does the speed of light in different mediums affect refraction?

The speed of light in a medium is directly related to the optical density of that medium. The greater the optical density, the slower the speed of light. This difference in speed is what causes refraction to occur when light passes from one medium to another.

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