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Asked in GSEB Board March 2020 · Lens maker's formula and power
Idea: a lens bends light only because its refractive index differs from that of the medium around it. In a medium the lens maker's formula reads 1/f=((nₗₑₙₛ)/(n_medium)-1)(1/(R₁)-1/(R₂)).
The lens disappears when there is no longer any step in refractive index at its surface, so that nothing is refracted or reflected there.
That happens when (nₗₑₙₛ)/(n_medium)=1.
Then 1/f=0: the lens has infinite focal length and light passes straight through it.
So the liquid must have the same refractive index as the glass, 1.47. The lens is still in the trough — there is simply nothing at its surface to see.
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