EMET-245 Laser Foundations
Fall 2014
Name:____________________
Laboratory 3: Diffraction Grating
Required Equipment
HeNe Laser(s) (two different wavelengths recommended)
Diffraction grating (15,000 lines per inch)
White foam board or a wall
Other Required Equipment
Calculator
Graph paper
Pencil or pen
Ruler
Masking tape
Purpose
The purpose of this experiment is to determine the wavelength of two HeNe lasers using a
diffraction grating.
Theory
If the diffraction angle θm (in Figure 1) can be measured for a particular order m and the grating
spacing d is known, the wavelength of the light can be calculated using the relationship
described by:
mλ = dsinθm
Where:
λ is the wavelength of the incident light in meters (or nanometers, nm)
d is the spacing between lines on the grating in meters (or nanometers, nm)
m is an integer that takes on the values 0, 1, 2, …., and
θm is the diffraction angle for a particular diffraction order m
Figure 1
Note: The diffraction grating we are using is 15,000 lines per inch, this will need to be converted
to nanometers, nm. First, convert lines per inch to lines per mm.
Source: OP-TEC Course 1, Module 1-1, Laboratory 1-1B
Page 1
EMET-245 Laser Foundations
Fall 2014
Name:____________________
Procedure
1. Position the laser so the beam goes straight through the diffraction grating, perpendicular to
the grating surface, onto the white foam board or wall. There it produces a center spot with
diffracted spots on both sides as shown in Figure 1.
NOTE: The diffraction grating and white foam board/wall should be as parallel to each other as
possible.
2. Measure the vertical distance from the grating to the white foam board/wall. Record this as L.
3. Measure the distances from the central spot to the center of the first diffracted spots on both
sides. Average these two distances and record the average as Δy. For these nearest diffracted
spots, m = 1.
4. Calculate the diffraction angle using θm by drawing (with a straight edge on your report) a right
triangle and using right triangle trigonometry.
5. Calculate the wavelength of the two lasers’ light using this first-order diffraction angle.
6. Repeat steps 1-5 three times and average the wavelengths.
7. Repeat steps 1-6 with a second HeNe laser.
Source: OP-TEC Course 1, Module 1-1, Laboratory 1-1B
Page 2
EMET-245 Laser Foundations
Fall 2014
Name:____________________
Lab Notes, Data and Calculations
Source: OP-TEC Course 1, Module 1-1, Laboratory 1-1B
Page 3