Question

(a) Calculate the focal length of the mirror formed by the shiny back of a spoon that has a 3.00 cm radius of curvature. (b) What is its power in diopters?

Final Answer

- $-1.50 \textrm{ cm}$
- $-66.7 \textrm{ D}$

### Solution video

# OpenStax College Physics for AP® Courses, Chapter 25, Problem 55 (Problems & Exercises)

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Video Transcript

This is College Physics Answers with Shaun Dychko. The back of a spoon is a diverging mirror, it's a Convex Mirror, in other words, which means it's radius of curvature is negative. So when we're finding the focal length of this mirror it's going to be half of its radius of curvature and we're going to substitute negative 3.00 centimeters in place of

*R*and we divide that by two and get a focal length of negative 1.5 centimeters, its power will be the reciprocal of that focal length. So that's one over negative 1.5 times ten to the minus two meters and we have to put this focal length and units of meters in order to get diopters in our answer for power. And we have negative 66.7 diopters.