A concave mirror has an object placed 40 cm in front of it. An image forms 60 cm in front of the mirror. What is the focal length of the mirror?
A) 60 cm
B) 120 cm
C) −120 cm
D) 100 cm
E) 24 cm

Question

Quizplus · Accepted Answer

The focal length (f) of a concave mirror can be found using the mirror equation: 1/f = 1/v + 1/u, where v is the image distance and u is the object distance. Given v = -60 cm (since it's a real image formed in front of the mirror, we take it as negative) and u = -40 cm (object distance is always taken as negative for mirrors), we have 1/f = 1/(-60) + 1/(-40) = -1/60 - 1/40 = -1/24. Therefore, f = -24 cm. Since focal lengths for concave mirrors are considered negative, the correct answer is 24 cm, acknowledging the absolute value for the context of the question options.

A Concave Mirror Has an Object Placed 40 Cm in Front