Shown in the photograph below is a small fuse box as seen through a magnifying glass.

 

Now suppose that the entire magnifying glass and fuse box assembly were placed into the tank of water seen at the rear of the picture behind the assembly.

After the magnifying glass/fuse box assembly is placed into the water the letters on the fuse box will appear:

• (a) larger.
• (b) smaller.
• (c) the same size.

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 The answer is (b): the letters on the fuse box will appear smaller when the lens/fuse box system is immersed into water, as seen by comparing the original photograph, at the left, with the photograph at the right, in which the system is immersed.

 

When the lens is immersed in water it no longer attains its normal focal strength, because the medium in which it is immersed has an index of refraction nearly equal to that of the glass in the lens. Light rays no longer bend as much at the interfaces where they enter and exit the lens.