Science (086) — AI-generated practice question

Cause of lateral shift:
When a ray strikes the glass slab obliquely at surface AB (air to glass), it bends towards the normal (denser medium). At the opposite surface CD (glass to air), it bends away from the normal by an equal and opposite amount. Since the two refracting surfaces AB and CD are parallel, the bending at each surface is equal and opposite, so the emergent ray is parallel to the incident ray but displaced sideways — this sideways displacement is called lateral shift.

Why shift vanishes for normal incidence:
When a ray hits the slab perpendicularly (angle of incidence = 0°), it passes straight through both surfaces without any bending at either interface. Since there is no change in direction at AB or CD, no lateral displacement occurs and the shift becomes zero.

Source: Chapter 9, Section 9.3.1 — Refraction through a Rectangular Glass Slab

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• Examiners expect two distinct parts: (1) cause of lateral shift and (2) normal incidence case.
• Key phrase to use: "equal and opposite bending at parallel surfaces."
• The textbook explicitly states: "The extent of bending… at the opposite parallel faces AB and CD… is equal and opposite. This is why the ray emerges parallel to the incident ray. However, the light ray is shifted sideward slightly."
• For normal incidence, angle of incidence = 0°, so angle of refraction = 0° — no bending, no shift. The textbook hints at this: "What happens when a light ray is incident normally to the interface?"
• Avoid writing about refractive index formula unless asked — it wastes word count here.