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JAMB Physics 1979 Question 8

Question 8

Refraction of Light

PQR is an equilateral glass prism of refractive index 1.5. Light incident normally on face PQ as shown in Fig. 1 will

Diagram for this question
  1. A.undergo total internal reflection at face PR and then emerge normal to the face QR
  2. B.emerge on face PR
  3. C.undergo total internal reflection at face PR and emerge deflected away from the normal on face QR
  4. D.undergo two successive total internal reflections and emerge at face PQ
  5. E.be completely reflected back along its path.
Show the answer and explanation

Answer: A (undergo total internal reflection at face PR and then emerge normal to the face QR)

Explanation

The ray enters PQ without bending and meets PR at 60°, which is more than the critical angle (about 42° for n = 1.5), so it is totally reflected. In an equilateral prism the reflected ray then meets QR along the normal and leaves without bending.