Question map
Not attempted Correct Incorrect ★ Bookmarked
Loading…
Q51 (NDA-II/2025) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Geometrical optics Answer Verified

The focal length of a concave lens is 0·5 m. The power of the lens is

Result
Your answer: —  ·  Correct: D
Explanation

The power of a lens (P) is defined as the reciprocal of its focal length (f) when measured in meters, expressed by the formula P = 1/f [1]. The SI unit for lens power is the dioptre (D). According to standard sign conventions in optics, the focal length of a convex (converging) lens is positive, while the focal length of a concave (diverging) lens is negative. In this problem, the lens is concave with a focal length of 0.5 m. Applying the sign convention, f = -0.5 m. Substituting this into the power formula gives P = 1 / (-0.5 m), which equals -2.0 D. Therefore, a concave lens with a focal length of 0.5 m has a power of -2.0 D, indicating its diverging nature.

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.8 Power of a Lens > p. 157
How others answered
Each bar shows the % of students who chose that option. Green bar = correct answer, blue outline = your choice.
Community Performance
Out of everyone who attempted this question.
50%
got it right
✓ Thank you! We'll review this.

SIMILAR QUESTIONS

CAPF · 2022 · Q25 Relevance score: 8.20

The magnitude of focal length of a concave lens is 2 m. What is the power of the lens?

NDA-I · 2013 · Q87 Relevance score: 7.37

An optician prescribes a power = -0.5 dioptre. The corresponding lens must be a

NDA-II · 2018 · Q60 Relevance score: 6.72

If the focal length of a convex lens is 50cm, which one of the following is its power?

CDS-II · 2006 · Q9 Relevance score: 6.40

The power of lens is -2 D. What is its focal length ?

NDA-I · 2018 · Q55 Relevance score: 6.20

Two convex lenses with power 2 dioptre are kept in contact with each other. The focal length of the combined lens system is