Diffraction of Visible Light
Equipment:
Pasco optical bench
masking tape
Pasco laser
Pasco SS slide holder
!Safety!: Do not look into the laser beam directly and do not point the laser beam toward anyone’s
eyes. This can cause permanent vision damage.
Introduction:
The famous scientist Isaac Newton considered light to be made up of small particles. In many
ways it does behave as if it were made up of particles and Newton’s ideas were believed for many
years. However, evidence that light behaved more like waves was found in diffraction and
interference experiments. After these experiments had been repeated many times around the world, it
seemed that the wave picture had won out. That was only until Einstein explained a curious
phenomenon called the photoelectric effect in 1905. In this effect it was found that light had a definite
discrete behavior: it came in chunks of a definite size. Thus light was a wave and a particle, proved!
The modern view of light is that it consists of “wave packets” which are called photons, a
word coined by Einstein. For some phenomenon these packets behave like a single entity. In this case
we can view light as a particle. For other phenomenon, the underlying wave nature inside the package
is dominant, and we must treat light as a wave. A curious double behavior with one major clause -light can exhibit only one behavior (particle or wave) in any given experiment.
In this lab we will utilize the wave nature of photons by diffracting red laser light through a
single narrow opening cut into a piece of metal. In the second part of the lab we will let the laser
beam diffract itself around a human hair. In both cases we are going to record the light patterns that
fall on a piece of paper.
The light pattern we will observe is made up of bright and dark fringes. These bright (BF) and
dark fringes (DF) surround a bright middle spot called the central maximum. The first dark fringe that
occurs around the central maximum is called the m = 1 dark fringe. And the first bright fringe that
occurs around the central maximum is called the m = 1 bright fringe. Next further out from the central
maximum are the m = 2, 3, ... BFs and DFs.
The relevant formulas for single slit or obstruction diffraction are:
SS-DF
a sin  m
m = 1, 2, 3, ….
SS-BF
a sin   (m  21 )
m = 1, 2, 3, ….
In these formulas, a is the width of the single slit opening,  is the angle of the minima or maxima
from the line defined by the laser beam (it does not matter what side of the central maximum it is),
and  is the wavelength of the monochromatic (single wavelength) light. This value will be given to
you by your lab instructor or will be indicated on the laser you are using.
x(fringe)

slit
laser
L = slit to screen distance
Procedure for Slit
 Screen: Measure and record the distance from the slit to the screen. A piece of paper taped to a
vertical surface will suffice. One meter is a convenient distance
 Laser/Slit: Arrange the laser and diffraction slit so that the beam passes through the center of the
slit.
 Central Maximum: Mark the CM with a long vertical line.
 DFs: (|) Mark the DFs with short vertical lines.
 BFs: (-) Mark the BFs with short horizontal lines.
Procedure for Hair
 Replace the slit with a hair taped across the holder. Note any change in L if it occurs.
 Determine the DFs and BFs produced by the laser light passing over the hair.
Questions
1. Characterize the accuracy and precision of your dark and bright fringe data for the slit. Assume
the given value for the slit is true. Explain any large discrepancies.
2. Characterize the precision of your dark and bright fringe data for the human hair. Do you believe
your result? If so, why? If not, why not?
Data Table 1: Single Slit
Width of slit marked on slide __________________
Measured slit to screen distance L = ____________
Wavelength of Laser Light ___________________
fringe order, “m”
1
distance
to
dark
fringes (x)
distance to bright
fringe (x)
Calculations Table 1: Single Slit
fringe order (“m”)
1
L
(m)
dark: a 
x
L
(m  1 )
bright: a 
2
x
2
3
4
5
6
2
3
4
5
6
Average value of width for dark fringes +/- standard error ____________________
Average value of width for bright fringes +/- standard error ___________________
Data Table 2: Human Hair
Distance L = ____________
fringe order, “m”
1
distance
to
dark
fringes (x)
distance to bright
fringe (x)
2
Calculations Table 2: Human Hair
fringe order (“m”)
1
2
L
(m)
dark: a 
x
L
(m  1 )
bright: a 
2
x
3
4
5
6
3
4
5
6
Average value of width for dark fringes +/- standard error ____________________
Average value of width for bright fringes +/- standard error ___________________