Concave Mirrors

Question

Light from a star is collected by a concave mirror. How far from the mirror is the image of the star if the radius of curvature is 150 cm?

Answer 1

Since the star is very very far from the mirror, the star will be imaged at the focal point of the mirror which is located midway between the center of curvature and the mirror vertex.  Since the radius of curvature is 150 cm, the focal point is 75cm from the vertex.