A concave mirror has a focal length of 20 cm. What is the position (in cm) of the object if the image is upright and is two times larger than the object?
A) 30
B) 20
C) 10
D) 40
E) 60

Question

Quizplus · Accepted Answer

Since the image is upright, the object must be located beyond the focal point. Using the magnification equation, M = -i/o = 2, where i is the image distance and o is the object distance. Solving for o, we get o = -i/2. We also know that f = 20 cm. Using the mirror equation, 1/f = 1/o + 1/i, we can substitute in the value of f and o to get 1/20 = 1/o + 1/i. Simplifying, we get i = -40 cm. We can then substitute this value for i into the equation o = -i/2 to find that o = 20 cm. Therefore, the object must be located 20 cm away from the mirror. The answer is C.

A Concave Mirror Has a Focal Length of 20 Cm