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Q.
When a light ray from a rarer medium is refracted into a denser medium, its
Ray Optics and Optical Instruments
Solution:
When a light ray goes from a rarer to denser medium by definition of refractive index, its speed creases.
Its frequency is found to be invarant.
So, if velocity of light in a medium is $v$ : $v=f \lambda$
[by wave theory]
or $v \propto \lambda$
So, if $v$ decreases, $\lambda$ also decreases.