Question

A plate of glass with parallel faces having a refractive index of 1.49 is resting on the surface of water in a tank. A ray of light coming from above in air makes an angle of incidence 32.5 ∘ with the normal to the top surface of the glass. What angle θ3 does the ray refracted into the water make with the normal to the surface?

Use 1.33 for the index of refraction of water. Express your answer in degrees.

Answer 1

Just use Snell’s law. You don’t need to worry about the glass, because the final angle will be the same as if the light passed from the air to the water directly. This might not be obvious to you, but it should make sense, because the light will have to speed up or slow down when it hits the water, and it doesn’t matter that the glass is there or not – the only thing that matters is the relative refractive indices.

n1sin(θ1) = n2sin(θ2)

1 * sin(32.5) = 1.33 * sin(θ)
sin(θ) = 0.5373 / 1.33
θ = 23.83°

23.83°