PASCO OS-8515C Basic Optics System - Manual (Page 34)

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B a s i c O p t i c s S y s t e m E x p e r i m e n t 1 2 : F o c a l L e n g t h a n d M a g n i f i c a t i o n o f a T h i n L e n s
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2. Use the Thin Lens Formula (Equation 12.1) to calculate the focal length.
f = _______________
Part II: Object Closer Than Infinity
In this part, you will determine the focal length by measuring several pairs of object
and image distances and plotting 1/d
o
versus 1/d
i
.
Figure 12.1
Procedure
1. Place the light source and the screen on the optics bench 1 m apart with the light
source’s crossed-arrow object toward the screen. Place the lens between them
(see Figure 12.1).
2. Starting with the lens close to the screen, slide the lens away from the screen to a
position where a clear image of the crossed-arrow object is formed on the screen.
Measure the image distance and the object distance. Record these measurements
(and all measurements from the following steps) in Table 12.1.
3. Measure the object size and the image size for this position of the lens.
4. Without moving the screen or the light source, move the lens to a second position
where the image is in focus. Measure the image distance and the object distance.
5. Measure the object size and image size for this position also. Note that you will
not see the entire crossed-arrow pattern. Instead, measure the image and object
sizes as the distance between two index marks on the pattern (see Figure 12.2 for
example).
6. Repeat steps 2 and 4 with light source-to-screen distances of 90 cm, 80 cm, 70
cm, 60 cm, and 50 cm. For each light source-to-screen distance, find two lens
positions where clear images are formed. (You don’t need to measure image and
object sizes.).
Analysis Part A: Focal Length
1. Calculate 1/d
o
and 1/d
i
for all 12 rows in Table 12.1.
2. Plot 1/d
o
versus 1/d
i
and find the best-fit line (linear fit). This will give a straight
line with the x- and y-intercepts equal to 1/f. Record the intercepts (including
units) here:
y-intercept = 1/f = _______________
x-intercept = 1/f = _______________
Note: You can plot the data and find the best-fit line on paper or on a computer.
Light source Lens
Screen
1 m
Measure object or image
size between two
pattern features
Figure 12.2