Science (086) — AI-generated practice question

A flat glass slab has two parallel flat surfaces. When light enters the first surface, it refracts and bends. However, at the second parallel surface, it refracts again by an equal and opposite amount. As a result, the emergent ray comes out parallel to the incident ray — only laterally shifted. Since the rays are never made to converge or diverge, the slab cannot act as a lens. A lens works because its curved (spherical) surfaces refract rays by different amounts at different points, bringing them to a focus or spreading them out.

Source: Chapter 9, Section 9.3.1 (Refraction through a Rectangular Glass Slab) and Section 9.3.3 (Refraction by Spherical Lenses)

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• The key physics is that parallel surfaces cancel each other's bending effect, so there is no net convergence or divergence — only lateral displacement.
• Examiners want you to explicitly state: (1) two refractions are equal and opposite, (2) emergent ray is parallel to incident ray, (3) therefore no focusing/diverging action.
• Contrast this with a lens: its curved surfaces bend rays by varying amounts at different heights, creating a principal focus.
• Do not just say "it has flat surfaces" — explain why flat surfaces prevent convergence/divergence.