Question:

A laser beam is directed at the Moon, 380,000 km from Earth. The beam diverges at an angle $\theta$ (Fig. 8–40) of $1.4 \times 10^{-5} \textrm{ rad}$. What diameter spot will it make on the Moon?

Source: Giancoli, Douglas C., Physics: Principles with Applications, 7th Edition, 2014.

Quick Answer:

$5.3 \textrm{ km}$

## Comments

wait but isn't the arclength not the diameter itself though? wouldn't you have to set it to 2pi (because you found the arclength of the crater it would be 180 degrees) and then solve for r, and follow up with multiplying it by 2?