Unit: Optics
Year: 1992 Question#: 95
Question: The diagram below represents two rays of red light passing through a converging lens.
if the two rays were blue light, f would be:
(1 ) shorter
(2 ) longer
(3 ) the same
What is this question really asking?
Explanation by: Derek K
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Answer 1
shorter
CORRECT
Blue light has a shorter wavelength than red light and will therefore travel at a slower speed in the lens. This slower rate of travel will cause the blue light to be bend more than the red light, this will cause a decrease in focal length. This change in wavelengths can be nicely seen in a prism; also in prism the the shorter wavelengths can be seen to bend more dramatically.
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Answer 2
longer
INCORRECT
Blue light has a shorter wave length than red light and will therefore travel at a slower speed in the lens. This slower rate of travel will cause the blue light to be bend more than the red light, this will cause a decrease in focal length. This change in wavelengths can be nicely seen in a prism; also in prism the the shorter wavelengths can be seen to bend more dramatically.
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Answer 3
the same
INCORRECT
Blue light has a shorter wave length than red light and will therefore travel at a slower speed in the lens. This slower rate of travel will cause the blue light to be bend more than the red light, this will cause a decrease in focal length. This change in wavelengths can be nicely seen in a prism; also in prism the the shorter wavelengths can bee seen to bend more dramatically. A piece of test taking advice: you can be pretty sure that some change will occur so don't chose this answer.
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Answer 4
only three possible answers to this question return to top
What's this question really asking?
This question is asking about differences in how light bends due to different wavelengths in a lens. Blue light has a shorter wave length than red light and will therefore travel at a slower speed in the lens. This slower rate of travel will cause the blue light to be bend more than the red light, this will cause a decrease in focal length. This difference in amount of bend can be nicely observed in a prism. return to top
