Correct Answer: 3

Explanation

For a thin lens, the lens maker formula is:

1 / f = (μ − 1) (1/R₁ − 1/R₂)

In air, refractive index of lens μ = 1.5:

1 / f_air = (1.5 − 1)(1/R₁ − 1/R₂)

When the lens is immersed in water, effective refractive index becomes:

μ′ = μ_lens / μ_water = 1.5 ÷ (4/3) = 1.125

So focal length in water:

1 / f_water = (1.125 − 1)(1/R₁ − 1/R₂)

Taking ratio:

f_water / f_air = (1.5 − 1) / (1.125 − 1) = 0.5 / 0.125 = 4

Thus:

f_water = 4f

Difference in focal lengths:

f_water − f_air = 4f − f = 3f

Hence:

α = 3

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