# Simple laser question

1. Oct 22, 2007

### timhunderwood

Hi I'm a student in school and I'm having a mental block on how to do this question. Could some give me a hint as to the principals I'm meant to consider /use.

Q) A red, 100mW, laser beam , wavelength 650nm, is incident normally on a perfectly black disk . Calculate the force on the disk.

I've never really had to relate photons to forces before. I tried consider change in momentum of photon is equal to impulse on black disk but that didn't work.

Thanks for the help

2. Oct 22, 2007

### mgb_phys

You are correct it's just the momentum of the photons ( = h / lambda )
You can treat the photons just like you were throwing tennis balls at the disc - but remember that the photons are absorbed by the black disc so it is an in-elastic collision.

3. Oct 22, 2007

### timhunderwood

i tried using change of momentum = impulse but I only need to use lambda or Power not both so i think I'm doing something wrong can someone show me a solution.

Thanks

4. Oct 22, 2007

### mgb_phys

You need both, the power tells you how many photons/sec hit the disc and the wavelength tells you how much effect they have.