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WAEC Physics 2017 Paper

Question : 29

Total: 50

\theta

Solution:

Concept:

The ray grazing the surface on emerging means it leaves at

90^{\circ}

to the normal.

So the angle of incidence inside the prism is the critical angle.

Explanation:

For a ray going from the prism to air, Snell's law gives:

1.5 \sin \theta = 1 \times \sin 90^{\circ}

Since

\sin 90^{\circ} = 1

\sin \theta = \frac{1}{1.5} = 0.667

\theta = \sin^{-1}(0.667) = 41.835^{\circ}

Therefore,

\theta \approx 41.8^{\circ}

Answer:

The value of the angle is

41.8^{\circ}

, which is option A.

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