Science (086) — AI-generated practice question

A lens has two principal foci because light can enter from either side. F₁ is the focus for light entering from the right, and F₂ is the focus for light entering from the left. Since refraction works in both directions, one focus alone would be insufficient to describe the lens's behaviour for all cases.

Both foci are equidistant from the optical centre only when the medium on both sides of the lens is the same (e.g., the lens is surrounded by air or water on both sides). This tells us that the medium has a uniform refractive index on both sides, so the speed of light — and hence the degree of refraction — is identical on each side.

Source: Chapter 9, Sections 9.3.4 and 9.3.5

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• Examiners expect you to state why two foci exist (reversibility/two directions of light) and the condition for equal focal lengths (same medium on both sides).
• Linking equal focal distances to same refractive index / same medium on both sides scores the third mark.
• The source passages show F₁ and F₂ marked on either side of the lens in all ray diagrams and tables, confirming light travels both ways through a lens.