Homework Help: Physics

Posted by Lynn on Monday, February 21, 2011 at 10:59pm.

A 100.0 W light bulb hangs from the ceiling. What is the visible light intensity on the floor, 2.1 m directly below the bulb, if 10% of its output is visible light?

So I am attempting to use: I= Power/area
however, I don't have a radius. I know 10% of 100 is 10 so that would be the intensity for that and that the refraction for air should be used but I am missing something...

HELP :)

Physics - drwls, Monday, February 21, 2011 at 11:35pm
The radius is R = 2.1 m. The light is distributed uniformly over a sphere of that radius, at that distance. So divide 10 W by 4*pi*R^2 and you will have the visible light irradiance.

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