The secondary mirror of a telescope whose primary mirror is 4 m in diameter has a diameter of 0.5 m. What fraction of the incoming light is obstructed by this secondary mirror?
A) about 1.6%
B) about 1/8, or 12.5%
C) about 35%
D) about 16%

Question

Quizplus · Accepted Answer

The fraction of light obstructed is the area of the secondary mirror divided by the area of the primary mirror. The area of the secondary mirror is (0.5/4)^2 = 0.0625, which is 6.25% of the primary mirror's area.

The Secondary Mirror of a Telescope Whose Primary Mirror Is