High School Physics Practice Test

A concave mirror has focal length f = 0.20 m. An object is placed at do = 0.30 m. What is di?

0.40 m 0.50 m 0.60 m 0.30 m

Correct Answer:

0.60 m

Explanation:

The key relation here is the mirror equation that links focal length, object distance, and image distance for a concave mirror. Using 1/f = 1/do + 1/di with f = 0.20 m and do = 0.30 m, you solve for di: 1/di = 1/f − 1/do = 1/0.20 − 1/0.30 = 5 − 3.333… = 1.666…, so di = 0.60 m. The positive di means the image is real and forms in front of the mirror. The magnification is m = −di/do = −0.60/0.30 = −2, indicating the image is inverted and twice as large as the object. Therefore the image distance is 0.60 m.

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