Physics Review Question

Unit: Optics
Year: 1990 Question#: 97
Question: A convex (converging) lens can form images that are

(1 ) real, only
(2 ) virtual, only
(3 ) either real or virtual
(4 ) neither real nor virtual

What is this question really asking?

Explanation by: Brian L

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Answer 1
real, only


Recall that the thin-lens equation is 1/f = 1/di + 1/do, where f is the focal length, di is the image distance, and do is the object distance. The focal length of a converging lens is positive by definition. If this equation always yielded a positive image distance (image on the other side of the lens), then real images would be the only type possible. However, if the object distance is less than the focal length, a negative image distance will result (image on the same side of the lens), indicating a virtual image. return to top

Answer 2
virtual, only


By the same reasoning as for choice A, if the object distance is greater than the focal length, then a positive image distance (image on the opposite side of the lens) will result, indicating a real image. return to top

Answer 3
either real or virtual


As shown by the above two explanations, both positive and negative image distances can result from different object distances. Thus, both real and virtual images can be produced.
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Answer 4
neither real nor virtual


This answer can be immediately eliminated. A converging lens has to make some sort of image. return to top

What's this question really asking?

What kinds of image distances can you get with a converging lens? Positive, negative, or both?
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