This document provides instructions for an experiment to determine the wavelength of a helium-neon laser using diffraction gratings. Students will collect data from two different diffraction gratings at three distances, including the grating width, distance from grating to screen, and distance between interference maxima. They will then use the double-slit diffraction formula with their measurements to calculate the laser wavelength six times and average the results. The actual laser wavelength is given for comparison to the student's experimental value.

1. Carbon, Climate, and Laser Technology
• www.atd.ucar.edu/apol/biocomplexity
• www.eo.ucar.eduTeacher’s Guide
Subject Focus:
High school physics or
optics/photonics course
Materials & Preparation
Time:
One to two class periods
Teacher and Student
Materials:
• He-Ne Laser
• 2 diffraction grating slides
with known grating widths
(each w/ different widths)
• a screen to project upon
(a white box works well)
• measurement tools
• student data recording
form (optional as recording
in lab notebook may be
preferred)
National Science Content Standards Addressed:
National Science Content Standards A, E, and G
Learning Ojbectives:
Students will determine the wavelength of a helium neon laser. Students will collect
six different sets of measurements in this experiment and use these measurements to
solve for the wavelength of the laser.
Preparations:
Put out one or sets of a laser, screen and two diffraction gratings for student use.
The intent is to have the laser beam pass through a diffraction grating resuling in
an interference pattern on the screen. Either the screen or the grating will need
to be movable since each grating will be used three times at different distances
from the screen. Decide ahead of time which object will be moveable. Each
of the three distances should yield a clearly observable interference pattern.
The photos on the following page show visual representations of the materials,
setup, and interference patterns generated during the experiment. A graphic of
the electromagnetic spectrum with wavelengths specified is also shown.
Procedure:
Have students record the diffraction grating lines (mm) and convert them into
centimeters to determine the slit width (d). Students will also record the dis-
tance from grating to screen (L) and distance from a maximum bright spot to an
adjacent maximum bright spot (x). Two different gratings are to be used at three
different distances from the screen and measurements recorded. Students will
then use these measurements to determine the wavelength of the laser.
Connection to APOL Biocomplexity Project:
Lasers are of many wavelengths and each type of laser has a particular applica-
tion based on that wavelength. The lasers used in the APOL project are designed
to give a final output of 4.3 x10-6 meters. This is in the mid-infrared range and
is invisible to the eye. In this activity, a laser that has an output visible to the
eye will be necessary. The He-Ne laser is the most common type of laser in the
school environment.
Solution:
Most schools will use a standard red helium neon laser. The wavelength for this
laser is 633 nanometers. Some schools may have access to helium neon lasers of
different wavelengths. The results for those lasers should be as follows: Green:
543.5 nm, Orange: 612 nm, and yellow: 594 nm.
Wavelength of Laser Light

2. 2 of 2
Laser, grat ing, and screen
m grating
Laser, 300 lines/mm grating, and screen
Gra ting in po sition in from of laser
Pattern generated by laser and 300 lines/mm grating
Laser, 300 lines/mm grating, and screen
Laser, grating, and screen
Laser and two gratings
Grating in position in from of laser
Photos by Randal Albrandt
Electromagnetic Spectrum

3. 1 of 2
Diffraction Grating
(lines/mm)
Slit Width (d)
(cm/line)
Distance from
grating to screen (L)
(cm)
Distance from
maximum to
maximum (x)
Two different gratings must be used three times each.
Note that x can be measured from any bright spot to the adjacent bright spot. Also, the diffraction gratings are
in lines per mm. This must be converted to cm per line. This value corresponds to d in Diagram A.
Use the values of x, d, and L from your data table and the double slit formula L
dx ))((
=λ
to determine
the wavelength of the laser. Average your six computed values and determine a final average for the wavelength
of the laser.
L
dx ))((
=λ
L
dx ))((
=λ
L
dx ))((
=λ
L
dx ))((
=λ
L
dx ))((
=λ
L
dx ))((
=λ
Average
Wave-
length
Show all
substitu-
tions of
data into
formula
Final Value
(cm)
Final value
(microns)
Ask your teacher to provide you with the actual wavelength of the laser used in your experiment. Compare this
value with your experimental value. Discuss any aspects of the experiment that would explain any disagreement
with the actual value.
Wavelength of Laser Light
Student Data Form
To determine the wavelength of the laser:

4. 2 of 2
DiagramA