You are to assume it as a point source and then solve....
The bulb doesn't emit light.... it is just placed at the bottom of the beaker
But if it doesn't emit light, then how is it a point source of light? What you are asking, then, is if I draw a dot with negligible diameter on the bottom of the beaker, how big does the dot appear from outside the water?
Chemists are physicists who don't do math.
