# Doppler effect of light

1. Mar 14, 2009

### cragar

I remember reading in a book that if we approached a red light
really fast it would appear green to us due to the Doppler effect of light
does any one know how fast we would need to travel for this to happen.

2. Mar 14, 2009

### jeffreydk

If you are approaching a light source, then you can compute the shift in frequency of light by

$$\nu_O=\nu_S\sqrt{\frac{1-v/c}{1+v/c}}$$

where $\nu_O$ is the frequency seen by the observer, $\nu_S$ is the frequency emitted by the source of the light, c is the speed of light, and v is the speed that you're traveling toward the source.

3. Mar 14, 2009

### cragar

sweet that helps , thank-you

4. Mar 14, 2009

### jeffreydk

5. Mar 14, 2009

### v2kkim

The equation above seems to have a typo, because the equation says when source approaches the observer the freq decreases , which is wrong.

6. Mar 14, 2009

### jeffreydk

Ah yes there is a typo--for approaching the signs should be reversed

$$\nu_O=\nu_S\sqrt{\frac{1+v/c}{1-v/c}}$$