Diffraction Grating Activity
Physics
Name:
Period:
A diffraction grating is a series of thin, closely-spaced slits, which allow light waves to pass through.
As the waves spread out on the other side of the grating, they can overlap so that they are in phases (with
all of the different waves crests lining up) or out of phase (with the different waves lining up so that
one’s crest matches another’s trough). Which one happens depends on where you and the difference in
the distance between different slits. At the right angle, the waves coming from each slit are exactly one
wavelength behind the waves coming from the next closer slit and those waves line up to produce
constructive interference and a bright light. At other angles, the waves from one slit are exactly one-half
wavelength behind the waves coming from the next closer slit and those waves combine to produce
destructive interference and darkness. These regions alternate according to two simple formulae:
Maxima: d sin = m
where m is the number of the maximum
1
Minima: d sin = (m-2)

where m is the number of the minimum
In this lab we will investigate the diffraction pattern formed by different diffraction gratings and use it to
find the wavelength of a laser pointer and
the spacings of the slits in a pair of
diffraction grating glasses.
Materials
Laser Pointer
5300 line/cm Grating
Meter Sticks Clipboard
Diffraction Grating Glasses
Procedure:
Part 1: Finding the Wavelength of Your Laser Pointer
1. Arrange your materials as shown in the diagram above.
·Your laser pointer should be resting flat, elevated off the table so that you can shine it through the
diffraction grating that is right in front of the laser pointer.
·The diffraction grating should be standing perpendicular to the surface and square to the laser beam,
oriented so that the maxima and minima in the diffraction pattern are to the side of the 0th order
maximum, not above or below it.
·You should tape a piece of unlined, white paper onto the clipboard to use as a screen to observe that
diffraction pattern. Hold the screen in landscape orientation and be sure that the screen is
perpendicular to the tabletop and square to the oncoming laser beam.
2. On the diffraction grating slide, find the groove density and use that to calculate the spacing between
grooves. Record your result in the space provided above the data table.
3. Turn on the laser pointer and aim it through the diffraction grating. Hold the screen in landscape
orientation, as far away from the diffraction grating as you can get and still see two maxima on either
side of the 0th maximum. Measure and record the distance from the diffraction grating to the screen.
4. Make a light pencil mark where each maximum appears on the screen. Be careful not to move the
screen while you do this.
5. Turn off the laser pointer and remove the screen. Measure the distance from the 0th maximum to the
1st and 2nd maxima, which you marked on the paper.
6. Measure the distance from the 0th maximum to the 1st and 2nd minima. Take the minimum to be the
spot exactly between the two adjacent maxima.
7. Based on your measurements, calculate the angle to each maximum and minimum.
8. Use the angle and the appropriate formula to calculate the wavelength of that light wave that created
that dot. All of your measurements should agree, but don’t be surprised if they are not exact.
9. Take the average value of the wavelengths you calculated.
10. Find the value of the wavelength printed on your laser pointer and calculate the percent error
between your average wavelength value and the manufacturer’s expected wavelength.
Data Table: Finding the Wavelength of Your Laser Pointer
Grating Spacing: d =
Spot
Length from Grating to Screen:
Distance From
Center
Angle
Wavelength ()
2nd Maximum Left
2nd Maximum Right
2nd Minimum Left
2nd Minimum Right
1st Maximum Left
1st Maximum Right
1st Minimum Left
1st Minimum Right
Part 2: Finding the Spacing of the Slits on a Diffraction Grating
1. Setup your materials, exactly the same as you did before, but replace the diffraction grating with a
pair of toy glasses.
2. Repeat the procedure for Part 1. The only difference will be that the glasses will make a grid of dots
(because it has grooves marked in it in both the horizontal and vertical direction). Pick the horizontal
line of dots that includes the brightest dot and just use those dots. Ignore the other lines of dots.
Data Table: Finding the Spacing of the Slits on a Diffraction Grating
Wavelength:  =
Spot
2nd Maximum Left
2nd Maximum Right
2nd Minimum Left
2nd Minimum Right
1st Maximum Left
1st Maximum Right
1st Minimum Left
1st Minimum Right
Length from Grating to Screen:
Distance From
Center
Angle
Groove Spacing (d)