SAT Subject Physics Practice Question 125: Answer and Explanation

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Question: 125

8. An object is placed 60 cm from a spherical convex mirror. If the mirror forms a virtual image 20 cm from the mirror, what’s the magnitude of the mirror’s radius of curvature?

A. 7.5 cm
B. 15 cm
C. 30 cm
D. 60 cm
E. 120 cm

Correct Answer: D

Explanation:

Because the image is virtual, we must write the image distance, si, as a negative quantity: si = –20 cm. Now, with so = 60 cm, the mirror equation gives

The focal length is half the radius of curvature, so

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