Physics Review Question

Unit: optics
Year: 1991 Question#: 100
Question: A student's solution to an optics problem had a negative value for the focal length. The optical device in the problem was most likely a

(1 ) diverging (concave) lens
(2 ) converging (convex) lens
(3 ) converging (concave) mirror
(4 ) plane mirror

What is this question really asking?

Explanation by: Saba

[ Return to question menu ]













Answer 1

diverging (concave) lens

CORRECT


The focal length is negative because the principal ray is refracted away from the focal point on the side of the lens opposite to that of the image. This problem is actually really simple. The lens is called a diverging lens and this is the only thing that needs to be known to choose the right answer. return to top























Answer 2
converging (convex) lens

INCORRECT
With a converging lens, the principal ray is refracted so that it converge at the focal point on the side of the lens opposite to that of the image. return to top



























Answer 3
converging (concave) mirror

INCORRECT


With a converging mirror, the principal ray is reflected so that it converge at the focal point (think of spotlights and flashlights). return to top


























Answer 4
plane mirror

INCORRECT

The focal length of a plane mirror is infinite, neither negative nor positive. This is not even an option to the problem.
return to top


























What's this question really asking?

To have a negative value for the focal length, the principal ray, which originates parallel to the principal axis, has to refract away from the focal point on the same side of a lens (or on the opposite side of a mirror) from where the object is positioned. So, the question is really asking which mirror / lens results in diverging rays? 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.
Contact with comments.