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A glass lens with refractive index 1.60 has a focal length of +20 cm when placed in air. If the same lens is immersed in water of refractive index 1.33, its focal length will be

1. three times the air value
2. twice the air value
3. unchanged from the air value
4. none of these

Correct answer: three times the air value

Solution

f_water/f_air = (n_g - 1)/((n_g/n_w) - 1) = (1.60 - 1)/((1.60/1.33) - 1) = 0.60/0.203 = 2.95, i.e. about three times the air focal length.

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