# Phase velocity wave packets Modern Physics

1. Feb 17, 2010

### richard7893

1. The problem statement, all variables and given/known data

The phase velocity of each wavelength of white light moving through or-
dinary glass is a function of the wavelength i.e. glass is a dispersive medium. What is the general dependence of phase velocity on wavelength in glass? Is dvp/dlamda positive or
negative? Why?

2. Relevant equations

phase velocity= frequency * lamda lamda= lamda not / n

3. The attempt at a solution
I said the phase velocity will be lower when light enters glass because the wavelength will be smaller. However I am unsure whether dvp/dlamda will be positive or negative. It could be negatve because the change in phase velocity has decreased when light enters the glass. On the other hand it could be positive beause the frequency and wavelength are always positive numbers and taking dvp/dlamda should always yield a positive number. I am very unsure about this part of the question.