A lens becomes essentially invisible when fully immersed in a liquid if?

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1. A lens becomes essentially invisible when fully immersed in a liquid if?

The lens’s focal length is exactly zero

The lens’s focal length is some finite, nonzero value

The lens’s refractive index is less than the liquid’s

The lens’s refractive index exactly equals the liquid’s

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When a lens and the surrounding liquid share the same refractive index, light passes straight through the boundary without bending at all, making the lens effectively optically invisible.