# Doppler shift with light

1. Dec 11, 2004

### daveed

do you get a doppler shift when your moving towards a light source? like, i know light gets redshifted with expanding space and gravity, but if light always approaches you at the same speed, how would frequency change there?

2. Dec 11, 2004

### Staff: Mentor

Absolutely. The relativistic Doppler effect is this:
$$\nu = \nu_0 \sqrt{(1 + v/c)/(1 - v/c)}$$
Where v is the relative speed of source and observer. If the source is approaching the observer, v is positive.

3. Dec 11, 2004

### dextercioby

The formula that Doc gave u is to be used,in all 4 possible cases:u're approaching the light sourse,the light source is approaching u,u're going away from the light source,the light source is going away from u.
It maight be silly to think about 4 possible cases,when there are actually 2.Yes,there are only 2,but in the case of light.Try to apply classical kinematics to sound waves,and u'll find 4 different formulas for each of the cases.Actually 2 formulas,once u've gotten the one,let's say,the sound wave's approaching u,change the sign of the speed in the formula gotten and u'll find the case in which the sound wave is moving away from u.

EDIT:And it's called (for light,that is) "Doppler-Fizeau effect".Doppler discovered it for the sound waves,bu Hippolyte Fizeau (one of the ones who measured the speed of light accuretely) who discovered it for light.

Last edited: Dec 11, 2004