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Asked in GUJCET 2020 · Lens maker's formula and power
Given: a lens of refractive index 1.5 immersed in a liquid.
Idea: we see a lens only because light bends at its surfaces; it disappears when it stops bending light at all, that is when its power is zero.
In a medium, 1/f = ((nₗₑₙₛ)/(n_liquid)-1)(1/(R₁)-1/(R₂)).
The radii cannot be zero, so the only way to get 1/f = 0 is (nₗₑₙₛ)/(n_liquid) = 1.
That means n_liquid = nₗₑₙₛ = 1.5: no refraction at either surface, nothing to mark the boundary, and the lens cannot be seen.
So the liquid must have n = 1.5.
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