Board question: Define power of a lens. Find power of a lens whose focal length is 50 cm.
Paper: CBSE 2024 Main 31/5/2 Q25, 2 marks.
What the question asks: say what the power of a lens means, then work out the power of this lens from its focal length.
Key words: a lens bends light. A convex lens brings rays together; a concave lens spreads them apart. Focal length is the distance from the lens to its focus, the point where the rays meet.
Step 1, define: power of a lens is the degree of convergence or divergence it gives to light rays. As a formula, P = 1/f (spoken “1 by f”), with f in metres.
Step 2, the unit: power is measured in dioptres (D). Why metres: 1 dioptre is the power of a lens whose focal length is 1 metre.
Step 3, change the units: 100 cm make 1 m, so f = 50 cm = 0.5 m.
Step 4, work it out: P = 1/0.5 = +2 D. Why plus: the focal length has no minus sign, so it is positive, which means a convex lens.
Answer, marking scheme (CBSE 2024 Main 31/5/2 Q25): definition (1); P = 1/f = 100/50 = 2 D (1).
The trap: putting the focal length in centimetres. 1/50 gives 0.02, which is wrong. Change f to metres first.
Quick recall (Light): Absolute refractive index? → n = c/v
Comment challenge (CBSE 2025 Main 31/1/1 Q36(a)(iii)): the focal lengths of two lenses A and B are 10 cm and 20 cm. Which one will show a higher degree of convergence or divergence? Give a reason. Answer in the pinned comment.
References: NCERT Class 10 Science, Ch 9 Light – Reflection and Refraction, sec 9.3.6 Power of a lens (P = 1/f with f in metres; SI unit dioptre, 1 D = 1 m⁻¹; the power of a convex lens is positive and of a concave lens negative) and Fig 9.12 (rays parallel to the axis meet at the focus); CBSE Class 10 board questions and marking schemes.
Narration uses a licensed AI voice (Azure AI Speech, Indian English). Script and physics research by ScoreSpark.
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Board question (CBSE 2024 Main 31/5/2 Q25, 2 marks): Define power of a lens. Find power of a lens whose focal length is 50 cm.
Board answer (CBSE marking scheme 2024 Main 31/5/2 Q25): power of a lens is the reciprocal of its focal length in metres, or the degree of convergence or divergence of light rays achieved by a lens (1). P = 1/f = 100/50 = 2 D, the same as 1/0.5 m = +2 D (1).
Comment challenge (CBSE 2025 Main 31/1/1 Q36(a)(iii), 1 mark): the focal lengths of two lenses A and B are 10 cm and 20 cm. Which one will show a higher degree of convergence or divergence? Give a reason.
Answer: lens A, the one with focal length 10 cm. Reason: power is 1 by the focal length in metres, so the lens with the smaller focal length has the larger power (A: 1/0.1 m → 10 D; B: 1/0.2 m → 5 D). CBSE 2025 Main 31/1/1 marking scheme: "The lens with focal length 10 cm" ½; "less focal length, more converging/diverging power" ½.
Quick recall (Light): Absolute refractive index? → n = c/v (the speed of light in vacuum by its speed in the medium)