PASCO OS-8515C Basic Optics System - Manual (Page 38)

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B a s i c O p t i c s S y s t e m Experiment 13: Focal Length and Magnification of a Concave Mirror
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2. Use the Equation 13.1 to calculate the focal length.
f = _______________
Part II: Object Closer Than Infinity
In this part, you will determine the focal length of the mirror by measuring several
pairs of object and image distances and plotting 1/d
o
versus 1/d
i
.
Figure 13.2
Procedure
1. Place the light source and the mirror on the optics bench 50 cm apart with the
light source’s crossed-arrow object toward the mirror and the concave side of the
mirror toward the light source. Place the half-screen between them (see Figure
13.2).
2. Slide the half-screen to a position where a clear image of the crossed-arrow
object is formed. Measure the image distance and the object distance. Record
these measurements (and all measurements from the following steps) in Table
13.1.
3. Repeat step 2 with object distances of 45 cm, 40 cm, 35 cm, 30 cm, 25 cm.
4. With the mirror at 25 cm from the light source and a clear image formed on the
half-screen, measure the object size and image size. To measure the image size,
hold a small scrap of paper against the half-screen and mark two opposite points
on the crossed-arrow pattern (see Figure 13.3). If at least half of the pattern is not
visible on the screen, have your partner slightly twist the mirror to bring more of
the image into view. Remove the paper and measure between the points. Measure
the object size between the corresponding points directly on the light source.
Table 13.1: Image and Object Distances
d
o
d
i
1/d
o
1/d
i
Image Size Object Size
50.0 cm
45.0 cm
40.0 cm
35.0 cm
30.0 cm
25.0 cm
Light source Half-screen Mirror
50cm
Image
size
Figure 13.3