NEET Physics · Optics

Refraction, the thin lens and the lens maker's formula

Taught in NCERT Class 12 Physics, Chapter 9: Ray Optics and Optical Instruments.

2 verified questions: Easy 0 · Medium 1 · NEET level 1

Sample questions

Sample 1Medium

A thin biconvex lens of refractive index 1.5 has both radii of curvature 20 cm. Its focal length when immersed in a liquid of refractive index 1.25 is

A
20 cm
B
25 cm
C
50 cm
D
40 cm

Show the answer

The answer is C.

In the liquid, 1/f = (1.5/1.25 − 1)(1/20 + 1/20) = 0.2 × 0.1 = 0.02 cm⁻¹, so f = 50 cm.

A.
20 cm is the focal length in air.
B.
25 cm only multiplies the air focal length by 1.25.
D.
40 cm uses (1.5 − 1.25) instead of (1.5/1.25 − 1) in the lens maker's formula.

NCERT: Class 12 Physics, Chapter 9, Refraction at spherical surfaces and by lenses (lens maker's formula)

Sample 2NEET level

A thin biconvex lens of glass of refractive index 1.5 has both radii of curvature equal to 20 cm. It is completely immersed in water of refractive index 4/3. The focal length of the lens in water is

A
8.9 cm
B
20 cm
C
60 cm
D
80 cm

Show the answer

The answer is D.

By the lens maker's formula, 1/f = (n₂₁ − 1)(1/R₁ − 1/R₂), where n₂₁ = 1.5 ÷ (4/3) = 9/8 is the index of glass relative to water. So 1/f = (1/8)(2/20) = 1/80 and f = 80 cm, four times the 20 cm focal length in air.

A.
Uses n₂₁ = 9/8 but leaves out the −1: 1/f = (9/8)(1/10).
B.
20 cm is the focal length in air, with n₂₁ = 1.5.
C.
Subtracts the indices, 1.5 − 4/3 = 1/6, instead of taking their ratio.

NCERT: Class 12 Physics, Chapter 9 (Ray Optics and Optical Instruments), Refraction by a lens: lens maker's formula

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Refraction, the thin lens and the lens maker's formula

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