1. Dec 8, 2005

### ashkash

A 200 MW laser pulse is focused with a lens to a diameter of 2.0 micrometers.

Part A:
What is the laser beam's electric field amplitude at the focal point?

Part B:
What is the ratio of this electric field to the electric field that keeps the electron bound to the proton of a hydrogen atom? The radius of the electron's orbit is 0.053 nm.

I was able to get Part A correct. I used I = P/A to find the intensity using the power and area and then used E = sqrt( (2* I) / (c * epsilon_0)) to find the amplitude of the electric field and I got a numerical value of 2.19×10^11 V/m.

Part B is where I need help. I tried finding the electric field like I did in part A using the new radius given (0.053 nm) and then divided my answer from part A by this to get the ratio, but this does not work. What am I doing wrong?

2. Dec 8, 2005

### Chi Meson

You got the hard part! Part B is much simpler. Use the standard electric field equation for a point charge: E=q/(4pi epsilon r^2)

3. Dec 8, 2005

### ashkash

thanks for the help.