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Asked in GUJCET 2011 · Lens maker's formula and power
Given: a concave-shaped pocket of air, surrounded by water.
Idea: what a lens does depends on how its material compares with what surrounds it, through 1/f=((nₗₑₙₛ)/(n_medium)-1)(1/(R₁)-1/(R₂)).
For glass in air the bracket is 1.5-1>0, so a concave shape diverges.
Here the material is air and the medium is water: (1.00)/(1.33)-1<0.
The shape factor is unchanged, but the bracket in front of it has flipped sign, so the concave shape now converges.
The air bag therefore acts as a convergent lens in water.
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