Physics : A beam of light enters from air (a =1.0)

Subject: Physics    / General Physics
Question

A beam of light enters from air (a =1.0) into three layered surface. Each layer has different refractive index. Trace the path of the light beam as it goes through each layer.

na = 1.00

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n1 = 1.45

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n2 = 1.33

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n3 = 3.50

At what angle should the beam enter the surface so not refraction will be observed?