A convex lens is immersed in a liquid whose refractive index is equal to the refractive index of the material of the lens. Then its focal length will .

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(4) become infinite. Given: a convex lens of refractive index n immersed in a liquid of the same refractive index n. Idea: inside a medium the lens maker's formula uses the relative index, (nₗₑₙₛ)/(n_liquid), not n itself. 1/f = ((nₗₑₙₛ)/(n_liquid)-1)(1/(R₁)-1/(R₂)) With nₗₑₙₛ = n_liquid the first bracket is exactly zero, so 1/f = 0 whatever the radii are. Zero power means an infinite focal length: light crosses both surfaces without bending, and the lens becomes invisible in the liquid. So the focal length becomes infinite.

A convex lens is immersed in a liquid whose refractive index is equal to the refractive index of the material of the lens. Then its focal length will ______.

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