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Q33
(CDS-I/2016)
Science & Technology › Basic Science (Physics, Chemistry, Biology) › Geometrical optics
Answer Verified
A myopic person has a power of - 1.25 Dioptre. What is the focal length and nature of his lens ?
Result
Your answer:
—
·
Correct:
D
Explanation
The power of a lens (P) is defined as the reciprocal of its focal length (f) in meters, expressed by the formula P = 1/f [1]. For a lens with a power of -1.25 Dioptres, the focal length is calculated as f = 1/P = 1/(-1.25) = -0.8 meters. Converting this to centimeters, -0.8 m equals -80 cm. According to standard sign conventions, a negative focal length or negative power signifies a diverging or concave lens [2]. Myopia (near-sightedness) is a vision defect where distant objects appear blurry because the image forms in front of the retina; it is corrected using a concave lens to diverge light rays and shift the focus onto the retina. Therefore, a person with -1.25 D power requires a concave lens with a focal length of 80 cm.
Sources
- [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.8 Power of a Lens > p. 157
- [2] 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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