PASCO OS-8459 Beginning Optics System - Manual (Page 17)

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M o d e l N o . O S - 8 4 5 9 E x p e r i m e n t 6 : C o n v e x a n d C o n c a v e L e n s e s
17
Experiment 6: Convex and Concave Lenses
Purpose
In this experiment, you will explore the difference between convex and concave
lenses and determine their focal lengths.
Theory
When parallel light rays pass through a thin lens, they emerge either converging or
diverging. The point where the converging rays (or their extensions) cross is the focal
point of the lens. The focal length of the lens is the distance from the center of the lens
to the focal point. If the rays diverge, the focal length is negative.
Procedure
1. Place the light source in ray-box mode on a white sheet of paper. Turn the wheel
to select three parallel rays. Shine the rays straight into the convex lens (see Fig-
ure 6.1).
Note: The lenses used in this experiment have one flat edge. Place the flat edge on the
paper so the lens stands stably without rocking.
2. Trace around the surface of the lens and trace the incident and transmitted rays.
Indicate the incoming and the outgoing rays with arrows in the appropriate direc-
tions.
3. The point where the outgoing rays cross is the focal point of the lens. Measure
the focal length from the center of the lens to the focal point. Record the result in
Table 6.1.
4. Repeat the procedure with the concave lens. Note that in step 3, the rays leaving
the lens are diverging and do not cross. Use a ruler to extend the outgoing rays
straight back through the lens. The focal point is where these extended rays cross.
(Remember to record the focal length as a negative number.)
Required Equipment from Beginning Optics System
Light Source
Convex Lens from Ray Optics Kit
Concave Lens from Ray Optics Kit
Other Required Equipment
Metric ruler
Table 6.1: Results
Convex Lens Concave Lens
Focal Length
Incoming rays
Convex lens
Figure 6.1