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 [ Return to question menu ] 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. return to top 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: return to top 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: return to top 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 |
[Physics Lessons] [Physics Review] [Physics Links]
This web site is designed and maintained by Science Joy Wagon and may
not be reproduced or redistributed without written permission from Science Joy Wagon. |