PHYSICS 124 EXPERIMENT NO. 7
INTERFERENCE AND DIFFRACTION
Goal: The purpose of this laboratory is to study the phenomena
of diffraction and interference. In Part I, you will observe the
diffraction of light by a human hair (similar to the diffraction on
a narrow slit), and in Part II, both interference and diffraction
of light by various types of slit arrangements.
Part 1
Equipment:
1 Micro-meter
1 Helium-Neon laser with a wavelength of 632.5 nm.
1 Slide Frame to mount the hair.
Paper to display the diffraction pattern on the wall.
1 Ruler
Procedure:
Take a hair and tape it into the frame vertically as shown in
the picture above. Point the He-Ne laser (the wavelength should be
labeled on the laser) at the hair about 10cm away from the hair and
about a meter away from the wall. A diffraction pattern should
appear on the paper once the laser is hitting the hair. The
diffraction pattern will consist of light and dark spots. In the
center there is a very bright circular spot do to the fact that the
laser beam width is larger than the diameter of the hair, and the
spots get less bright the further out they are. Mark the dark spots
on the paper to either side of the bright spot in the middle until
you have five pairs. Label the closest point (and its pair) to the
center 1, the second closest 2 and so on; this quantity is called
the order. With the meter stick, measure and record the distance,
D, that the hair is from the wall. Measure the distance between the
dark spots that are labeled with the same order, ½ of this distance
is x (labeled in the picture above).
To calculate the diameter of the hair, we use the diffraction
formula with the small angle approximation
sin(tan/
mbbbxD
lqq
=)»»
, m = 1, 2…..
Where m is the order, b is the diameter of the hair and λ is the
wavelength of the light. To find the diameter solve for b for each
value of m (the value for x changes with m). From these five values
of the diameter, calculate the average diameter and the error the
diameter.
Q1. What is the diameter of your hair? Be sure to quote error
estimates on your result.
Q2. Can you observe any ellipticity in your hair? How would you
determine this?
Q3. How precisely do you think you could measure the diameter of
your hair with a conventional micrometer? How does this compare
with your diffraction measurement?
Part 2
Equipment:
1 Incandescent Light Bulb with vertical filament sitting in a
box covered with red
(bottom) and blue (top) filters .
1 3” x 5” Glass Slide, mostly opaque, but with various slit
arrangements.
A) Single slit
Stand where you can see the filament. Hold the glass with the
slits close to your eye so that you are looking through the single
slit that is widest (A1). Move the glass so that you start looking
through smaller and smaller slits (each slit is a factor of 2
smaller than the one before it). In your lab book, record how what
you see changes as the width of the slit changes (i.e. brightness
and spread in diffraction)
Q4. Explain why what you observed occurred.
B) Double Slit
Repeat the same process as with the single slit, but with the
double slit column, starting with E1. The first one is actually a
single slit, then as you move down the glass the distance between
the two slits increases by a factor of two. Record in your lab book
the difference in the diffraction patterns as you move to larger
divisions in between the slits.
Q5. Explain why what you observed occurred.
C) Multiple Slits
The middle column is all the same width, but they all have a
different number of slits. Repeat the process from single slit
using the middle column and record your observations in your lab
book.
Q6. Explain why what you observed occurred.
Q7. Which one has the most slits and why?
D) Color
Look at the non-filtered filament from the other side of the box
from the red and blue filters through the glass with slits. Use the
silt pattern in the middle column that you determined to have the
most slits in it. Record your observations in your lab book.
Q8. Knowing that blue has the shorter wavelength and red the
longer wavelength, explain why what you observed occurred.
Write-Up
Your lab report must include:
1) Answers to questions 1-8
2) The tracing of the dark spots labeled with the order
3) Your observations from part 2
Micro-meter
10 m
Laser spot on paper
Paper to display diffraction pattern
Laser pointed at wall
Ruler
Frame holding hair
Hair
oriented perpendicular to paper plane
x
D
Laser
Wall
Glass with slits
Box with light
Blue and red filter
Unused Unused
Single Many Double
Row
1
2
3
4
5
Column A B C D E
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