Explanation of light refraction

Why does light refract between the two mediums when transferring from one to another? And why is the refractive index different for different wavelenths/frequencies?

fawk3s

Although light can be thought of as individual photons, ie. particles of light, it also has wave properties. Refraction is a consequence of the wave nature of light. When light travels from one medium to another, its speed changes because it interacts with each medium differently since the atoms in each medium respond to the electric and magnetic fields of the light waves in specific ways that are determined by the atomic structure of the atoms in each material and the molecular structure of how those atoms are arranged.

So let's say that light is traveling from air to water. Since water interacts with light in such a way as to slow it down more than air, when the light wave moves between the air and water, its speed suddenly changes. Since the speed of any wave is its frequency times its wavelength, that means that one of those factors has to change in order for the speed of the wave to change. In the case of light, its frequency and therefore color, is set when it was emitted, but its wavelength can and does change (this happens in such a way that the sudden change in speed from one medium to another is matched by a proportional change in wavelength, otherwise the wave would experience a discontinuity... essentially a kink in the wave produced at the interface between the two media). The CHANGE in wavelength from on medium to another is greater for higher frequencies of light, therefore blue light refracts more than red and the rainbow has the pattern it does. This is why refractive index is wavelength/frequency dependent.

But as to why light bends rather than just continuing straight on when it slows down, I don't know. Someone please help me with that one.

Although light can be thought of as individual photons, ie. particles of light, it also has wave properties. Refraction is a consequence of the wave nature of light. When light travels from one medium to another, its speed changes because it interacts with each medium differently since the atoms in each medium respond to the electric and magnetic fields of the light waves in specific ways that are determined by the atomic structure of the atoms in each material and the molecular structure of how those atoms are arranged.

So let's say that light is traveling from air to water. Since water interacts with light in such a way as to slow it down more than air, when the light wave moves between the air and water, its speed suddenly changes. Since the speed of any wave is its frequency times its wavelength, that means that one of those factors has to change in order for the speed of the wave to change. In the case of light, its frequency and therefore color, is set when it was emitted, but its wavelength can and does change (this happens in such a way that the sudden change in speed from one medium to another is matched by a proportional change in wavelength, otherwise the wave would experience a discontinuity... essentially a kink in the wave produced at the interface between the two media). The CHANGE in wavelength from on medium to another is greater for higher frequencies of light, therefore blue light refracts more than red and the rainbow has the pattern it does. This is why refractive index is wavelength/frequency dependent.

But as to why light bends rather than just continuing straight on when it slows down, I don't know. Someone please help me with that one.

This is what I actually asked. I know WHY isnt a proper science question, but there must be an explanation, right?

fawk3s

In http://www.walter-fendt.de/ph14e/huygenspr.htm" [Broken] you can quite well see how light bends when it enters a medium with a different index of refraction. Basically, the explanation lies in the already stated fact that the phase velocity changes when light enters the new medium (look closely and you'll see this in the applet). It is also useful to use Huygens principle when explaining this phenomenon, which states that every point in the wave front acts as a point source. The interference between these "secondary" point sources causes the wave front to bend.

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Born2bwire
Gold Member
There are a couple ways of doing this. You can use classical wave physics to explain it. Huygen's principle is one as kloptok stated. Another is that the phase of the wave needs to be continuous across the boundary but the phase velocity changes across the boundary. This means that if the wave did not bend, then as the wave above and below the boundary travelled along the surface then the phase fronts would misalign as one would travel faster than the other. The solution is of course is that the direction of the wave shifts so that the new velocity along the boundary is the same as the old velocity along the boundary before entering the new medium.

And another way to derive it is via the conservation of momentum of the photon.

Thanks guys, I get it now !