Final answer:
Objects under water appear closer than they actually are due to refraction of light at the water's surface.
Step-by-step explanation:
When light passes from one medium to another, it changes direction. This phenomenon is known as refraction. When light travels from air to water, it speeds up and bends towards the normal, causing objects to appear closer to the surface than they actually are.
This can be observed when looking at a submerged object from above the water surface. For example, a fish in a pond seems to be closer to the surface than it really is, due to the refraction of light at the water's surface.
The magnitude of refraction depends on the refractive index of the two media involved, which is a measure of how much the speed of light is reduced when passing through.
Learn more about Refraction of light