A lens will be invisible in vacuum when the refractive index of the lens is
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The index of refraction of the lens will remain the same (the medium remains the same, whether in a vacuum, in air, immersed in water, etc.), but the focal length will increase.
The index of refraction of water is greater than that of air, so light entering through the lens will be refracted at a smaller angle with respect to the horizontal of the lens than it would in air.
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