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'O' Level Standard Questions - Refraction

Q4. A fish under water looks upwards and sees the setting sun. What is the angle between the horizontal and the ray of light which is entering the fish's eye?

Take the refractive index of the water as being 1.33.

This question assumes that you understand that to see the sunset the fish would have to see rays that skim the water's surface... that the angle that the setting sun's rays in air make with the normal is virtually 90o.

Snell's Law: n1sin θ1 = n2sin θ2

nairsin θair = nwatersin θwater

1 x sin 90o = 1.33 x sin θwater

sin θwater= 1/1.33 = 0.752

θwater= 49o

This is the angle that the refracted ray that enters the fish's eye makes with the normal to the water surface.

 

Therefore the angle the ray makes with the water surface will be 90o - 49o = 41o