Seeing in Three Dimensions
Did you ever use 3-D glasses to watch a movie, like the boy inFigure20.12? If you did,
then you know that the glasses make people and objects in the movie appear to jump out
of the screen. They make images on the flat movie screen seem more realistic because they
give them depth. That’s the difference between seeing things in two dimensions and three
dimensions.
Figure 20.12: This boy is wearing 3-D glasses; when you look at objects and people in the
real world, your eyes automatically see in three dimensions. ( 37 )
We are able to see in three dimensions because we have two eyes facing the same direction
but a few inches apart. As a result, we see objects and people with both eyes at the same
time, but from slightly different angles. Hold up a finger a few inches away from your face,
and look at it first with one eye and then with the other. You’ll notice that your finger
appears to move against the background. Now hold up your finger at arm’s length, and look
at it with one eye and then the other. Your finger seems to move less against the background
than it did when it was closer. Although you aren’t aware of it, your brain constantly uses
such differences to determine the distance of objects.
Seeing in Color
For animals like us that see in color, it may be hard to imagine a world that appears to be
mainly shades of gray. You can get an idea of how many other animals see the world by
looking at a black-and-white picture of colorful objects. For example, look at the apples on
the treeFigure20.13. In the top picture, they appear in color, the way you would normally
see them. In the bottom picture they appear without color, in shades of gray (Figure20.14).