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Q25
(CAPF/2022)
Science & Technology › Basic Science (Physics, Chemistry, Biology) › Geometrical optics
Answer Verified
The magnitude of focal length of a concave lens is 2 m. What is the power of the lens?
Result
Your answer:
—
·
Correct:
B
Explanation
The power of a lens (P) is defined as the reciprocal of its focal length (f) in metres, expressed by the formula P = 1/f. The standard unit for power is the dioptre (D), which is equivalent to inverse metres. According to the Cartesian sign convention for spherical lenses, the focal length of a convex lens is positive, whereas the focal length of a concave (diverging) lens is always negative [1]. Given that the magnitude of the focal length for the concave lens is 2 m, we assign it a value of f = -2 m. Substituting this into the power formula yields P = 1 / (-2 m), which equals -0.5 dioptre. Therefore, the power of the lens is -0.5 D, reflecting its diverging nature.
Sources
- [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.6 Sign Convention for Spherical Lenses > p. 155
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