The light gathering power of a reflecting telescope is directly related to the area of the primary mirror: Area = pi x d^2 / 4
(Where d is the diameter of the mirror)
Set up a fraction to compare the light gathering power of a 10- meter diameter (Keck) telescope with that of a human pupil. The human pupil has a diameter of about 5 millimeters. How many times more intense is the light gathered by a Keck telescope, than light gathered by the human eye.