# Simple Physics Question

1. Oct 26, 2004

### stunner5000pt

A laser emits light at a wavelength of 632.8nm. The laser beams power is 1.0mW. Find the number of wave crests per second passing through nay point in the beam.

so f = $$\frac{c}{\lambda} = 2.1 * 10^ -15$$

(while you're at it tell me what's wrong with my code too, please)

f = c /lambda = 2.1x 10^-15 Hz

so then i know the frequency and the inverse of this is the period. The number of crests per second is the period (?) so the frequency gives the number of crests per second??

Last edited: Oct 26, 2004
2. Oct 26, 2004

### Galileo

Hi stunner,

You know the power, which is the energy per second passing by.
You know the energy of a single photon (??wave crests??) from $E=h\nu$.
Hope this is the info you need and may use.

3. Oct 26, 2004

### stunner5000pt

SO since P = E / t which is PER second and hf would give the energy per second.

So HF is the number of photons (crests) passing throiugh the beam at one second???

Am i right here? Or have i misinterpreted something

4. Oct 27, 2004

### Galileo

P (power) is energy per second (given).
hf is the energy per photon (given).
So the number of photons passing by per second is...