Q1. [1] straightforward initial-understanding
Where is the principal focus of a concave lens located relative to the lens, when parallel rays of light fall on it?
- A On the opposite side of the lens from the incoming light, where the rays actually meet
- B On the same side as the incoming light, where the diverging rays appear to come from
- C At the optical centre of the lens
- D At the centre of curvature on the opposite side of the lens
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
Answer: B
The principal focus of a concave lens is located on the same side as the incoming light, where the diverging rays appear to come from (virtual focus).
Source: Chapter 9, Section 9.3.5
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Explanation
- A concave lens is a diverging lens; it never actually converges rays, so its focus is virtual.
- The textbook states: "In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens" as the incoming light.
- Option A describes a convex (converging) lens where rays actually meet on the other side — wrong.
- Option C (optical centre) and D (centre of curvature, opposite side) are both incorrect locations for the principal focus.
- Remember: virtual focus = same side as object; real focus = opposite side.