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Question
How does focal length of a lens change when red light incident on it is replaced by violet light? Give reason for your answer.
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Solution
The refractive index of the material of a lens increases with the decrease in wavelength of the incident light. So, focal length will decrease with decrease in wavelength according to formula.
`1/f = (mu - 1)(1/R_1 - 1/R_2)`
Thus, when we replace red light with a violet wavelength decrease and hence the focal length of the lens also decrease.
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