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Question
The wavelength of light in a medium is \[\lambda = \lambda_0 /\mu,\] where \[\lambda \] is the wavelength in vacuum. A beam of red light \[\left( \lambda_0 = 720\text{ nm} \right)\] enters water. The wavelength in water is \[\lambda = \lambda_0 /\mu = 540\text{ nm.}\] To a person under water, does this light appear green?
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Solution
Colour of light will depend only on the frequency of light and not on the wavelength of the light. So, light will appear red to an observer under water.
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