Physics Review Question

Unit: Optics
Year: 1997 Question#: 88
Question: A spherical concave mirror is used in the back of a car headlight. Where must the bulb of the headlight be located to produce a parallel beam of reflected light?

(1 ) between the principal focus and the mirror
(2 ) beyond the center of curvature
(3 ) at the principal focus of the mirror
(4 ) at the center of curvature of the mirror

What is this question really asking?




Explanation by:Alexey S

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Answer 1
between the principal focus and the mirror

INCORRECT
If the bulb is placed at the location of the object, the rays will be diverging from the mirror, and a virtual image will be formed behind the mirror.
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Answer 2
beyond the center of curvature of the mirror

INCORRECT

The rays from the bulb will concentrate between the principal focus and the center of curvature of the mirror. return to top



























Answer 3
at the principal focus of the mirror

CORRECT

Rays through the principal focus reflect parralel to the mirror, di = INFINITY from the formula:





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Answer 4
at the center of curvature of the mirror

INCORRECT

The rays emitted from the center of curvature would reflect back through it. When do is 2F, di = 2F as well coming from the formula:





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What's this question really asking?

The question asks from which point the rays emmitted reflect parallel to the principal axis. This means that the distance to the image, di is infinity. You need to use the formula:

Or just know that the rays through the principal focus reflect parallel to the mirror. return to top

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