Part II: Ray Diagrams

Please wait for the animation to completely load.

Often you will use ray diagrams in order to determine where an image of an object will be, whether it will be real or virtual, and whether it will be inverted or upright.  The animation shows an object arrow, a mirror, and a pink dot to show the focal point of the mirror.  You can move the object using the slider (position is given in meters) Restart.

1. One point source is attached to the object in the animation.  Move the point source up and down.  Where does the light converge? (Sketch an example of the position of the point source and the point where the light converges with the image.)

2. In order to sketch a diagram of the object, in addition to the lens and the approximate position of the image, you need to know where the light from every point on the object converges.  Instead of trying to draw a large number of the rays from many points on the object, we generally use three rays from the tip of the object.  As you move the object (with the slider) or move the point source, there is a ray that always passes through the focal point.  Describe that ray.  This is a ray generally included in a ray diagram.

3. Now switch to the "ray diagram" view.  Describe the other two rays.

4.  As you move the object, describe what stays the same for each ray even when the object is in a different position and the image is changing position and size.

5. Draw a diagram of an object in front of a concave mirror and sketch a ray diagram, labeling the three standard rays described below:

1. Ray travels from the object parallel to the optical axis reflects from the mirror and crosses the optical axis at the focal point.

2. Ray travels from the object through the focal point reflects from the mirror parallel to the optical axis.

3. Ray travels from the object and hits the mirror at its intersection with the optical axis reflects back at the same angle (relative to the optical axis) with which it entered the mirror.

6. Finally, move the object to a position between the focal point and the mirror.  Sketch a ray diagram.

7. What is the difference between the object with point source and ray diagram views?

Original exercise: Exploration 33.3 in Physlet Physics
Original credits: Exploration by Anne J. Cox.
© 2004 by Prentice-Hall, Inc. A Pearson Company