Question

A lightbulb emits light that creates a shadow of a ball on a wall. If the radius of the ball is 12.3 cm, and the radius of the shadow is 29.5 cm, and the ball is 1.27 m away from the wall, how far is the lightbulb away from the wall? (For the present problem, you may assume that the light bulb is a"point source".)

Answer 1

view the setup from the side, with the line joining the light source and the center of the ball perpendicular to the wall.

Then, if the light is x cm away from the wall, using similar triangles,

x/29.5 = (x-127)/12.3
x = 217.8 cm