A light ray strikes the flat surface of a rectangular glass slab at exactly 90° (i.e., along the normal). Wil…

Question

A light ray strikes the flat surface of a rectangular glass slab at exactly 90° (i.e., along the normal). Will it be refracted as it enters the glass? Justify your answer using the laws of refraction.

Accepted Answer

## Model Answer No, the light ray will **not be refracted**. When a ray strikes the surface along the normal, the angle of incidence (i) = 0°. By Snell's law: $$\frac{\sin i}{\sin r} = 	ext{constant} \Rightarrow \frac{\sin 0°}{\sin r} = 	ext{constant} \Rightarrow \sin r = 0 \Rightarrow r = 0°$$ Since the angle of refraction is also 0°, the ray passes straight through without bending. *Source: Chapter 9, Section 9.3.1 & Snell's Law (9.3.2)* --- ## Explanation - Examiners expect you to **state the conclusion** (no refraction) and **justify using Snell's law** with the substitution i = 0°. - Simply saying "it goes straight" without the law earns partial credit only. - Note the textbook itself asks students to find out what happens at normal incidence — this is the expected answer to that implicit question.

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