# Beat frequency

1. Nov 8, 2005

### thenewbosco

There are two parts to this but i solved the first part:
in part a) i was to show, for an electomagnetic wave reflected back to its source from a mirror approaching at speed v, that the reflected wave had frequency $$f=f_{source}\frac{c+v}{c-v}$$ where fsource is the source frequency and c is the speed of light.

now i am asked: when v is much less than c, the beat frequency is much smaller than the transmitted frequency. In this case use the approximation $$f + f_{source}\approx 2f_{source}$$ and show that the beat frequency can be written:

$$f_{beat}=\frac{2v}{\lambda}$$

i dont know how to go about this. I was thinking to solve for fsource and put it into the equation from part a. but this doesnt work...any help

2. Nov 9, 2005

### thenewbosco

no help on this...........