Unit: Optics
Year: 1994 Question#: 91
Question: In the diagram below, a crown glass converging lens has foci F and F'. An object is places at a distance slightly less than the focal length from the lens, and a virtual image is produced.
The object remains in the same position and the crown glass lens is replaced with another lens of identical shape. If the new lens produces a real image of the object, the new lens is most likely made of
(1 ) water
(2 ) Lucite
(3 ) fused quartz
(4 ) flint glass
What is this question really asking?
Explanation by: Lisa K
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Answer 1
water
INCORRECT
water has a smaller index of refraction than crown glass. The reason this doesn't work is because material with a smaller index of refraction doesn't have the strength to pull all of the rays in. Therefore, it diverges.
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Answer 2
Lucite
INCORRECT
Lucite also has a smaller index of refraction than crown glass. See diagram in Answer 1. return to top
Answer 3
fused quartz
INCORRECT
fused glass also has a smaller index of refraction than crown glass. See diagram in answer 1. return to top
Answer 4
flint glass
CORRECT
Flint glass does have a greater index of refraction and therefore has the strength to pull all of the rays into a converging image.
return to top What's this question really asking?
This is really asking which one of the materials has a higher index of refraction than crown glass. This is explained in the answer explanations. return to top
