Adjusting a photographer's lighting. The illumination from a light source is inversely proportional to the square of the distance from the light source. Ansel is taking a portrait with his light...

Adjusting a photographer's lighting. The illumination from a light source is inversely proportional to the square of the distance from the light source. Ansel is taking a portrait with his light source set 4 feet from his subject and finds that the illumination is twice as bright as it should be. If he wants to reduce the illumination to one-half of what it is now, at what distance should he place the light?

Include a sketch of the situation in your answer, along with some sentences explaining your approach to the problem.