You wish to read from a single lightbulb in a lamp at your desk.In order to get the same brightness on the page from a 25 W bulb as you can from 100 W bulb,how much closer do you need to bring the lamp to your book? Assume that the light bulbs are equally efficient.
A) 1/2 the distance
B) 1/8 the distance
C) 1/4 the distance
D) 1/16 the distance

Question

Quizplus · Accepted Answer

The brightness is proportional to the power of the bulb and inversely proportional to the square of the distance. To achieve the same brightness with a 25 W bulb as a 100 W bulb, the distance should be reduced by a factor of √(100/25) = 2, which is half the distance.

You Wish to Read from a Single Lightbulb in a Lamp