# Force on a Lens

1. Dec 5, 2008

### andrewm

1. The problem statement, all variables and given/known data
A laser of power P with wavelength $$\lambda$$ is directed through a lens (focal length f) off the optical axis by a distance d. What is the sideways force on the lens?

2. Relevant equations

Not sure. The average radiation pressure is I/c, where I is the intensity. But this does not seem useful.

2. Dec 5, 2008

### mgb_phys

You can just think of this classicaly.
Imaging a stream of particles being deflect downward by a certain angle - just calculate the resulting force on the deflector.

The angle you get form the focal length - the number of particles (photons) from the intenisty and the momentum form the wavelength