WAEC Physics 2017 Objective — Question 28

Question 28 of 50 from the West African Examinations Council (WAEC) Physics 2017 Objective paper, with the correct answer and a full explanation.

[Diagram: ray of light in air refracting into a denser medium] From the diagram, the correct equation for the refractive index n of the medium is

A. A. n = tani
B. B. n = sini/sin90°Correct
C. C. n = sini/cos(90°-i)
D. D. n = sin90°C

Explanation

i=r=[law of reflection]. From the diagram, r+90°=180° (angles on a straight line); r+r2=180°-90°=90° => r2=90°-i. The refractive index of medium = sini/sinr2 = sini/sin(90°-i) = sini/cosi = tani. (Note: the official answer key states B, n=sini/sin90°.)

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