Question
When light travels from an optically denser medium to a rarer medium, why does the critical angle of incidence depend on the color of the light?
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Answer
Refractive indices are different for different wavelengths of colour. $\mu=\frac{a+b}{\lambda^2}$, hence the value of critical angle $\sin i_c=\frac{i}{\mu}$ is also different for different colours.
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