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Physics Lab
Science Olympiad 2005
The Ohio State University
April 16, 2005
Team Name____________________________________________________
Your Names____________________________________________________
Activity Time: 45 Minutes
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Geometric Optics: In this lab you will measure the imaging properties of a simple
converging lens and confirm the lens equation.
Lens Equation (thin lenses):
1
1
1


Do Di F
where
F is the focal length of the lens
Do is the distance from the object to the center of the lens
Di is the distance of the image formed from the center of the lens
Procedure: Set up the converging lens near the center of the optical table rail. Set up the
object light source (arrows) on one side of the lens, and the paper to focus the real image
on the other side. For each fixed object distance move the paper back or forth until a clear
image is seen, and record the data. Repeat the procedure at another object distance. Try to
cover a large range of object distances. Get data for at least three data points.
Measurement
1
2
3
Do
Di
Analysis :
1. (10 points) Show explicitly that your data agrees with the lens equation. You may
do this using the table above, graphically, or any way you choose. Explicitly
explain your procedure. Your analysis must be convincing, and backed by
calculations(if you need more space use the back of this paper).
There are many way to do this. We are looking for a convincing set of measurements and a
complete data analysis. Points should be given for quality of the analysis.
The simplest is to calculate f or 1/f for the three data points and compare them. This method would
even be more convincing if the students plot the resulting f or 1/f as a function of Do or Di.
One can also show 1/Do vs 1/Di yields a straight line.
One can also calculate f and predict using the formula a theoretical Di given Do, and compare this
result with the measure Do.
2. (10 points) What is your best estimate of the focal length of this lens?
F=
(in centimeters)
The student should give the average f for the three measurement. The correct focal lenth is
close to 20 cm. If it is very far from this major points should be deducted.
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3. (5 points) What is the approximate uncertainty (error) in your focal length
measurement? (If calculate describe or show work.)
F 
(in centimeters)
The best answer would be to do an RMS of the f for the three measurements.
Second would be to guesstimate the size of the error from the three
measurements. The answer should be about 0.1cm.
4. (5 points) What are the possible sources of uncertainty (error)? (Discuss listing at
least two sources of error in the measurement.)
In order of size the uncertainties are: 1) image focus positioning error, 2) scale error
from reading the ruler, 3) angular tilt of the lense. These should all be given full
points.
Less good answers (less points): 1) temperature expansion,…
Calculation and data taking errors are not uncertainties but mistakes.
5. (5 points) Shown at right is the index of refraction for the lens’ glass. Is the focal
length of a lens greater for red light than or blue light? How does this effect the
image.
Red light has longer focal length. This chromatic
aberration will cause a bluring of the focus, as well as
color distortion near edges.
6. (5 points) If the source is moved between the focal point and the lens, discuss the
properties of the resulting image.
There is no real image formed. The image becomes virtual and is on the
same side of the mirror as the object. It is also upright and not inverted
like the real image.
7. (5 points) When the light wave enters the glass lens from the air which of the
following properties remains constant (circle the correct answer):
a) velocity b) wavelength c) frequency d) amplitude
e) none of the above f) all of the above
8.
(5 points) If you stick two converging lenses (focal lengths F1 and F 2 ) close
together, the lens combination will have a focal length , F, equal to:
a) F1  F2
b) ( F1  F2 ) / 2
c) F1 xF2 /( F1  F2 )
d) none of the above
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Physical Optics:
Laser light is incident on a two slits spaced 0.2 mm apart (Young’s “double slit”). A
screen is located 7 meters down stream from the slits, and the following fringe pattern
is observed (note a cm/mm scale is included to help you make measurements):
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2
3
4
5
6
7
8
9
10
11 cm
For reference, a similar (but NOT identical) apparatus has been setup in the lab.
9. (20 points) What is the wavelength of the laser light?
10. (5 points) Describe how the pattern would change if in the above apparatus, the
slits were separated by 0.1 mm, but all of the rest of the apparatus remained the
same.
11. (5 points)Describe how the pattern would change if the original setup (0.2 mm
spacing) were immersed in water (assume the apparatus survives the immersion).
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12. (5 points) Light is an electromagnetic wave. What is a typical frequency for a
visible light wave?
13. (5 points) What is a typical energy for a visible photon (note: h  6.626  1034 J s
and c  3.00  108 m/s ).
14. (10 points) The work function of silver is 7.57  1019 J. The work function of
metallic sodium is 3.68  1019 J. Suppose you made photoelectric effect
measurements on these metals using several different wavelengths of light. What
would you observe? In particular, how would your measurements on silver differ
from those on metallic sodium.
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Tie Breaker (The following question will only be graded in the case of a tie between
teams.)
The table below shows the minimum voltage required to power Light Emitting Diodes of
a given color.
LED color
voltage
ordinary red
1.7 Volts
special red: high-brightness, high-efficiency, or low-current
1.9 Volts
orange or yellow
2 Volts
green
2.1 Volts
bright white, emerald green, and most blue-derived (phosphor) 3.4 Volts
430 nM bright blue
3.5-3.8 Volts
U.V.
4.6 Volts
Explain why this clear relationship between voltage and color position in the
electromagnetic spectrum exist.
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