You learned about the general concept of parallax in Chapter 2, and the video offers a review of the basic ideas. To check your understanding of the concept, which of the following is a valid way of demonstrating parallax for yourself?

a. Get a camera, and photograph a person who is running back and forth.
b. Hold up your hand in front of your face, and move it slowly back and forth.
c. Hold up your hand in front of your face, and alternately close your left and right eyes.
d. Look up at a star, and notice how it moves toward the western horizon during the night.

The correct answer is:

c. Hold up your hand in front of your face, and alternately close your left and right eyes.

Explanation:

Parallax is the difference in the apparent position of an object when viewed with different lines of sight. In measurement and visual perception, parallax errors occur, giving the viewers different measurements of the object in question. In this example, holding up a hand in front of the face, and alternately closing the left and right eyes will cause a difference in the lines of sight/vision, and angles of inclination, thereby changing the relative position of the object when viewed, even though the object is stationary. To make up for this differences in animals, stereopsis occurs, which entails the overlapping of the fields of vision of both eyes to cancel out the effect of parallax.