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# Ray Optics: Tracing the Path of Light Using a Laser (page 2)

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Source:
Author: Jerry Silver

### Mirrors at a right angle

Shine the laser on one of the mirrors and trace its path (by placing dots along the path and connecting them). The path should hit the second mirror, and then reflect off the second mirror. Try this for several angles of incidence on the first mirror. (If you like geometry, set the mirrors at an acute angle. Then, predict and test the angle of the outgoing ray for a given angle of incidence.)

### Convex lens

A convex lens is the one that is thicker at the middle than at the ends. Draw a centerline perpendicular to the axis of the lens. Trace the following paths: a) a straight line along the center line through the center of the lens, b) a line above the centerline running parallel to the centerline, c) a line below the centerline running parallel to it. Trace all the lines. Notice where the three lines cross. Measure that distance and put a dot on the centerline on the incident side of the lens that same distance from the lens. Direct the laser at any angle through that dot and trace its path through the lens. Try this for several angles.

### Concave lens

A concave lens is the one that is thinner at the middle than at the ends. Draw a centerline perpendicular to the axis of the lens. Trace the following paths: a) straight line along the center line through the center of the lens, b) a line above the centerline running parallel to the centerline, c) a line running below the centerline running and running parallel to it. Trace all the lines. How do these results compare with those from the convexlens?

### Semicircular lens

The semicircular lens has one circular side and one flat side. Place the circular side toward you. Trace the lens and draw a centerline on the flat side. Shine the laser at a 30-degree angle to that centerline, but hit the point where the centerline meets the flat side of the lens. This particular arrangement avoids refraction in the glass because the light comes in perpendicular to the tangent at the circular edge. In this case, the only refraction that occurs is at the glass-to-air interface. Observe what happens for different angles. Take it to the extremes of high and low angles of incidence.

### Rectangular prism

Draw a perpendicular line to one of the edges of the prism. (Make sure the edges you are using are clear and not frosted.) Direct the beam toward the point where the perpendicular meets the edge and trace the path of the laser through the prism at various angles.

### Right-angle prisms

There are two main types of right-angle prisms: 90°, 45°, 45° and 90°, 30°, 60°. Here is a challenge. Try it either by working out the light rays first or by just playing with the prisms and figuring it out by trial and error:

1. How can you direct a light ray through the prism and have a ray emerge at 90 degrees to the incoming ray (based only on total internal reflection)?
2. How can you direct a light ray through the prism and have a ray emerge at 180 degrees to the incoming ray heading back in the direction that it came from (also based only on total internal reflection)?