The radii of curvature of the faces of a double convex lens are 10 cm and 20 cm. The refractive index of the glass is 1.5. What is the power of this lens (in units of dioptre) ?
+7·5 D
–7·5 D
+2·5 D
+5·0 D
Answer is Wrong!
Answer is Right!
This question was previously asked in
UPSC NDA-1 – 2017
Using the lensmaker’s formula:
1/f = (1.5 – 1) * (1/0.1 – 1/(-0.2))
1/f = 0.5 * (1/0.1 + 1/0.2)
1/f = 0.5 * (10 + 5)
1/f = 0.5 * 15
1/f = 7.5 m⁻¹
Power P = 1/f = 7.5 Dioptre (D). Since the focal length is positive (1/7.5 m), it is a converging lens, which is expected for a double convex lens in air.