# How do I show that dispersion provides evidence of the wave like nature of light?

1. Mar 25, 2004

### apples and pears

And yeah, so how?

Thank you very much..

2. Mar 26, 2004

Look up Thomas Young's two-slit experiment.

3. Mar 26, 2004

### apples and pears

I have done that, and I also know that his experiment does support the wave theory of light, but how does this relate to dispersion and the splitting of light into colours with different wavelengths?

Also, is dispersion and diffraction the same thing? I'm confused...

Last edited: Mar 26, 2004
4. Mar 26, 2004

Sorry, you typed "dispersion" and I read "diffraction."

Dispersion supports the wave nature because dispersion depends on the frequency of the light wave. Actually, dispersion depends on the index of refraction which in turn depends on the phase velocity which in turn depends on the frequency, but you get the idea.

If light were not a wave and exhibited only particle properties, it would not disperse.

5. Mar 26, 2004

### Staff: Mentor

No, they are different phenomena.

Wave theory explains refraction. Since light slows down when traveling through a medium (compared to vacuum), wave theory can explain why the light refracts (bends). Dispersion is due to different colors having slightly different speeds, and thus experiencing slightly different refractions.

6. Mar 26, 2004

### apples and pears

I understand a little bit on what you said, but I kinda lost you when you said "phase velocity". Would you care to explain what that is? Thank you and sorry to be a pain.

Oh yeah, about material dispersion and modal dispersion:

Material dispersion - the spreading of a light pulse due to the dependence of the index of refraction on color (frequency).

Modal dispersion - the distortion of a light pulse due to the diverging path of the rays coming from the source.

Does anyone know what is the difference between spreading and distortion?

Last edited: Mar 26, 2004
7. Mar 26, 2004

That's a tricky one.

The phase velocity is basically how quickly a given crest or node or static point on a wave is travelling in space. It's defined as

$$v_p = \frac{\omega}{k}$$
where omega is the angular frequency and k is the wavenumber.

It sounds like material dispersion is like splitting the colors and modal dispersion is things be out of focus.