# Can someone help me understand the (basic) physics behind white dispersion in prisms?

Not really a homework question, but I don't understand the concept. How do I know when the light bends upward or downward? How do I know qualitatively how much the red and violet lights disperse apart?

I know it has something to do with Snell's law, but I can't seem to get the necessary equations to predict what happens. My teacher hasn't explained it, nor does my book.

tiny-tim
Homework Helper
How do I know when the light bends upward or downward?

Light always is nearer the perpendicular in the denser medium.
How do I know qualitatively how much the red and violet lights disperse apart?

It depends on the speed of red and violet light …

cacluating those speeds is way too difficult for your course

Oooh, ok I understand the answer to my first question now, thanks :)

On the 2nd part, I figured as much, but I'm not trying to calculate exact speeds. For example, if I were to compare the dispersion of colors in Air-Prism vs. Water-Prism, how do I determine if the colors expand or contract? That's where I tried to find a formula combining wavelength and index of refraction, only to get confused and wind up here. =/