# Quick question about light waves

1. Feb 6, 2008

### cardinalboy

As a light wave enters from a material of lower refractive index (air, say) to a material of higher refractive index (glass, for example), the speed of the wave and its wavelength both decrease.

Let's say a light wave enters a pane of glass and then exits the other side of the glass, back into the air. What happens to the speed and wavelength as the light wave exits the material of higher index (glass) and enters into the material of lower index (air)? Does the wavelength and speed return to the values they had before entering the glass, or does the wave maintain the same wavelength and speed it had while in the glass?

Any help would be greatly appreciated. Thanks!

2. Feb 6, 2008

### Claude Bile

It would revert back to the previous wavelength and speed.

Assuming of course the media on both sides of the glass pane have the same refractive index.

Claude.

3. Feb 6, 2008

### greeniguana00

EDIT: Looks like Claude beat me to it

4. Feb 6, 2008

### cardinalboy

One more quick question...

Thanks for your help guys! I've got one more question....

Let's now say a light wave enters a pane of glass from the air, travels through the glass, and then exits the other side of the glass, where there is only water. As the wave enters from a material of lower refractive index (air) to a material of higher refractive index (glass), the speed of the wave and its wavelength both decrease.

When the light wave then exits the glass and enters the water, the wave is now entering from a material of higher refractive index (glass) to a material of lower refractive index (water). How does this change in refractive index (from higher to lower) affect the light wave's speed and wavelength in the water as compared to its wavelength and speed when in the glass? Do the wavelength and speed decrease even further after entering the water?

I know that when passing from one material to another, n(1)$$\lambda$$(1) = n(2)$$\lambda$$(2). Does this apply to both moving from a lower refractive index to a higher refractive index AND to moving from higher refractive index to a lower refractive index?

I ask because my book says that the light wave's wavelength in the water (after passing through the glass) is determined by dividing the light wave's wavelength IN AIR (before entering the glass) by the refractive index of water. Somehow this does not seem correct to me.

Again, any help would be greatly appreciated. Thanks much!

Last edited: Feb 6, 2008
5. Feb 6, 2008

Yes.

Why?

6. Feb 6, 2008

### Staff: Mentor

If the light goes through various media, then: $\lambda_1 n_1 = \lambda_2 n_2 = \lambda_3 n_3$, regardless of the direction in which the light moves or the order of the media.

Realize that the refractive index of air is extremely close to 1, the refractive index of vacuum, so $\lambda_{water} n_{water} = \lambda_{air} n_{air} = \lambda_{air}$.

Last edited: Feb 6, 2008
7. Feb 7, 2008

### Mephisto

hm! I knew the photon slows down, but i didn't know the wavelength changes too. Does this means that under-water everything you see from above the surface is shifted in wavelength? I mean, obviously from experience the colors are weird if you open your eyes underwater, but I thought this had to do with the fact that water is actually slightly blue, and hence absorbs some wavelengths more than others. Is the color weirdness a combination of this fact and the fact that the wavelengths of all photons coming from above the surface are different?

8. Feb 7, 2008

### Hootenanny

Staff Emeritus
:yuck: This is a common misconception that photons travel slower in more dense mediums, see our https://www.physicsforums.com/showpost.php?p=899393&postcount=4" [Broken] for more information.

Last edited by a moderator: May 3, 2017
9. Feb 7, 2008

### Staff: Mentor

It's the wave speed that changes, not the speed of a photon. Frequency doesn't change, so the wavelength must change as well.
The wavelength is different underwater, but that's not why it looks different. (What determines how it looks is the combination of frequencies that hit your retina.)
Any change in color seen underwater is due to the fact (as you note) that water is slightly blue along with absorption, scattering, dispersion, and similar effects--not simply due to the change of wavelength from being in water. After all, once the light gets in your eye, the wavelength it had when under water is irrelevant.