Question

A person is reading a newspaper using the light from an unshaded 100-watt light bulb which is placed on a table 3 meters away. If the table is now moved to a distance of 9 meters away from the person, the brightness of the light on the newspaper is now _________ the brightness before?

Answer choices:

A. 3 times

B. 1/3 of

C. 1/9 of

D. 9 times

E. 1/27 of

F. 27 times

Answer 1

The brightness follows inverse square relationship with the distance.

for brightness b and distance d:

                $b\propto \frac{1}{d^{2}}$

so if distance is increased 3 times, brightness will decrease by 9 times

so option C is correct