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Light of wavelength lambda (in air) enters a medium of refractive index mu. Two points along the light path inside the medium are separated by a distance x. What is the phase difference between these points?

1. 2*pi*mu*x/lambda
2. 2*pi*x/(mu*lambda)
3. 2*pi*(mu - 1)*x/lambda
4. 2*pi*x/((mu - 1)*lambda)

Correct answer: 2*pi*mu*x/lambda

Solution

In the medium the wavelength shortens to lambda' = lambda/mu. The phase difference over a path x is (2*pi/lambda')*x = (2*pi*mu/lambda)*x = 2*pi*mu*x/lambda.

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