A very striking example of this is shown in Figure 18.12 where
a coin has been placed on the bottom of a bowl. The coin is not
visible at first. But if water is poured into the cup the coin
appears to be raised higher and, thus, is visible.
Another example of this which you may have already noticed is
illustrated in Figure 18.13. There a fish is swimming in an aquarium.
When viewed through the top, the fish seems closer to the top
than it really is; when viewed through the front, the fish seems
closer to the front than it really is. This means you can see
two fish although there is really only one. If you position yourself
carefully you may even be able to see three fish! You are seeing
three images of the same fish.
How does a plane surface like this produce an image? Figure 18.14
shows an object O located at a distance do below the surface of
the water. One ray of light is shown going straight up. This ray
of light strikes the water-air interface normal or perpendicular
to the surface and passes into the air without being bent. A second
ray strikes the water-air interface a horizontal distance x away
with an angle of incidence of i and is refracted so that it leaves
with an angle of refraction of r. This ray and the first one now
appear to have originated from position I-the image-located at
distance di below the surface. This image distance
di is the apparent depth of the thing
we are looking at.
As you can see from the figure, the apparent depth or image distance
is going to be less than the actual depth or object distance.
For rays at other angles or for other values of x, the image is
not located at the same place. You can even notice this in looking
at fish in an aquarium. At some angles the images of a fish received
by your two eyes may not coincide precisely and the fish may appear
blurred or simply "strange" or you may even feel slightly
dizzy. This lack of a well-defined position for the image is an
example of astigmatism.
Q: When you look at a fish in an aquarium, it is closer to the glass or farther from the glass than it appears to be?
A: The fish is farther away than it appears to be.