![]() ![]() If you move sufficiently far to the left or right, your friend will come into view (Figure 1 left and right). Think about what happens if a friend of yours is hiding from you by standing a few feet behind a large tree (Figure 1, center). We’re aware of parallax without even thinking about it, every time we move our heads. The left and right figures also indicate the points of view of your left and right eye when looking at two objects that lie right in front of your nose, one behind the other. The reverse is true in the left figures, where the point of view has moved to the left and the objects lie to the right of straight-ahead. At right, the point of view is moved to the right the faraway object appears to move by a smaller angle to the left, relative to straight ahead (black line in bottom right figure), than does the nearby red object. At center, the point of view sees the red object as blocking the violet one. ![]() Fig 1: Two objects (red and violet) seen from three points of view (black dots) the top figures show the three arrangements of the objects and points of view as seen from above, while the bottom figures show the objects as seen from the corresponding point of view. ![]() This is the principle by which bats determine the distance to food and obstacles, and is also the principle of radar. This extremely simple principle is used by our eyes and brains to form our three-dimensional “picture” of the world, and has been used by astronomers for centuries to determine the distances (or relative distances) from the earth to astronomical objects.Īnother common way to detect distance involves sending out a wave that travels at a known speed, and will bounce off an object and return to us the time that it takes for the return of the wave - the echo - then tells us the distance to the object. How do we detect depth - the distance from our location to another object? There are a few ways to do it, and one of the most common and easy to understand involves the geometrical fact of parallax. ![]()
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