# Homework Help: Beat Frequency for a Laser!

1. Sep 17, 2009

### Hells_Kitchen

1. The problem statement, all variables and given/known data
A laser emits a monochromatic beam of wavelength $$\lambda$$ , which is reflected normally from a plane mirror, receding at speed $$v$$. What is the beat frequency between the incident and reflected light?

I know that the $$f_{beat} = |f_2 - f_1|$$.

When the wave hits the plane mirror it has frequency
$$f_1 = \frac{v_0}{\lambda}$$.
Then, when it bouces off due to the doppler effect the wavelength becomes:

$$\frac{\lambda_1}{\lambda} = 1 - \frac{v}{c}$$.
Furthermore then,

$$f_2 = \frac{v}{\lambda_1} = \frac{v}{\lambda - \frac{\lambda v}{c}} = \frac{v*c}{\lambda*c - \lambda*v}$$.

So if i then find $$f_{beat} = |f_2 - f_1| = |\frac{v c}{\lambda c - \lambda v} -\frac{v_0}{\lambda}|$$ This result does not match the book result which is:

$$f_{beat} = 2*(\frac{v}{c})*v_0$$.

I was wondering if anyone could help me with this problem on what I have done wrong.

Thanks!

2. Sep 18, 2009

### tiny-tim

Hi Hells_Kitchen!

(what's v0? Do you mean ν0 ?)

I don't understand your formula for f2.

Hint: the image of the laser (on the other side of the mirror) is receding at a speed of … ?