Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

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Teacher's Lab
The Science of Light
IntroductionLight in ColorLaws of Light
  how it works
what's important

how it works

Like so many things having to do with perspective, parallax can be counterintuitive. The "car" movie shows how near things move faster. But sitting on the couch, near things move less. How can this be?

The difference is that, in the car, we and the window are moving together; but from the couch, the window is stationary while our point of view moves—from one eye to another. While we use the window to frame or view in both cases, the angles work differently depending on the situation.

In the illustration below, you can see the two situations; the "eyeballs" show the two points of view:


In the car situation (at left) you can see that as the car moves from position 1 to 2, the red square moves from the right side of the window to the left, while the peak stays in the left. The near object moves more.

On the other hand, in the couch situation, the peak moved from the far left edge of the window to near the right edge (even though it's "to the left," the right eye will see the peak as being near the right edge of the window frame). The red square still moves from the left half of the window to the right half as we change eyes—but not as far. So here, the near object moves less.

what's important

While in some ways, perspective and parallax are purely geometrical issues, students can benefit from looking at them in science class. Here are some reasons why:

First, it's an effect that everyone has seen, even if some haven't noticed it. And if we make careful observations, however qualitative, we can figure a lot out.

Second, scientists actually use parallax to find how far it is to distant objects, for example, to planets and stars.

Third, and perhaps most important, when you make a cartoon, and people are moving a long distance, you have to get the parallax right or the illustration will look funny.

Young children can look to see which objects appear to the right or left of other, more distant landmarks—and then move (their whole bodies) to see how the view changes.

Gradually, students become more sophisticated, opening one eye or the other, or observing the passing landscape from a car and explaining it qualitatively.

More advanced students can get quantitative, realizing that an apparent distance in their field of view is really an angle, and that they can study those angles or distances with trigonometry and with similar triangles.

Finally, there is a visual arts connection; namely, in still paintings with perspective and in animation with parallax.

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