Divergence angle

1. Oct 25, 2009

mars shaw

1. The problem statement, all variables and given/known data
A tiny laser beam is directed from earth to moon. If beam's diameter is 2.50 m at the moon, how small much the divergence angle be for the beam? The distance of moon from the earth is 3.8x10^8 m

2. Relevant equations

3. The attempt at a solution

2. Oct 25, 2009

tiny-tim

Hi mars shaw!

(try using the X2 tag just above the Reply box )

Show us what you've tried, and where you're stuck, and then we'll know how to help!

3. Oct 27, 2009

mars shaw

s=diameter=2.50 m
r=distance of moon from earth=3.8x10^8
theta=s/r
I got the correct answer but I am confuse about diameter that how it can be taken as "s"?
Kindly explain.

4. Oct 27, 2009

tiny-tim

Hi mars shaw !

Because you imagine the line from your eye to the bottom of the moon as being length r, and the diameter (s) of the moon as being at right-angles to that line, so the tangent of the angle is s/r.

(and if you're not happy about that, but you'd rather use the radius, and the line to the centre of the moon, you'll get the same result: the tangent of half the angle is (s/2)/r )