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
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