# Can't get started

1. Dec 10, 2004

### nannygreen

I need some help on how to start this problem and I think I can work it out.
A laser beam is directed at the moon 380,000 km from earth. The beam diverges at an angle of 1.8 x 10-5 rad. How large a spot will it make on the moon?

2. Dec 10, 2004

### BobG

That would be you equation for finding the length of an arc.

$$s=r\theta$$

Technically, that gives you the diameter on a flat moon.

You'd then have to figure out how big of an angle that is based on the Moon's radius and use your equation of the length of an arc once again. Considering the size of your spot as compared to the radius of the Moon (1738.1 km), it's not worth it (which is why the radius of the moon wasn't given). For the number of significant digits given, accounting for the curvature of the Moon winds up being the same as for a flat moon.