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'O' Level Standard Questions - Refraction

Q6. A glass prism has three angles of 60°.

A ray of light falls on one of the faces and the angle of incidence is 49°. The ray is refracted and now travels parallel to the second face. When it reaches the third face it is refracted again and emerges from the prism.

Find

(a) the refractive index of the glass of the prism,

(b) the angle between the ray entering the prism and the ray leaving the prism.

 

Snell's Law: n1sin θ1 = n2sin θ2

nairsin θair = nglasssin θglass

1 x sin 49o = nglass x sin 30o

nglass = sin 49o/sin 30o

(a) nglass = 1.5

We can now find the angle of refraction at the glass/air interface:

nairsin θair = nglasssin θglass

sin θair = 1.5 x sin 30o

sin θair = 0.75

θair = 49o

(b) Therefore the ray changes direction by 38o and the angle between the ray entering the prism and the ray leaving the prism is 38o