Name ____________________________
Determining the Wavelength of Laser Light Lab
(HS5.4.3.4, HS1.6.1, HS5.4.3.3)
Purpose: To determine a tiny measurement like the wavelength of the laser light by using knowledge
of the geometric relationship between variables involved in producing a light interference
pattern on a screen.
n = X
d L
.
Discussion: Referring to the diagram below, if:
 the distance L from a diffraction grating to a screen is measured,
 the separation distance d between diffraction grating slits is known, and
 the distance between bright fringes (maxima) X is measured,
 the wavelength of the light projected through the diffraction grating can be found using the
geometry of similar triangles:
Screen
X
Bright fringes (maxima)
Diffraction grating
L
Laser light
laser
Testable Question: Can we use geometric patterns to determine the wavelength of light and other
tiny wavelengths?
Hypothesis: If __________________________________________________________________
Then _______________________________________________________________
Materials: laser of known wavelength, various diffraction gratings, string, meter stick, flat screen
Procedure:
 Select a diffraction grating to use and calculate the value d for the spacing between slits.
 Set up laser with the selected diffraction grating next to aperture so that the interference
pattern appears on a screen a few meters away.
 Measure the exact distance L from the aperture to the central bright fringe (central maximum)
 Measure the exact distance X from the central maximum to one of the first order maxima.
 Calculate the wavelength of the laser light using your measurements and .
n = X
 Compare your measured value to the accepted value given below by calculating
d L
the percent difference of your average as indicated in question #2 below
 Answer the questions.
Data: Actual wavelength of the laser light = 632 nm = _____________________ meters
Distance L from slits to screen = _____________ meters
Diffraction Grating
Lines/mm
Separation of slits
d (meters)
Maxima separation
X (meters)
Calculated laser
Wavelength
(meters)
% Difference
of calculated
and accepted
100
300
600
Average
Values
Sample Calculations:
Quantity
d (m)
(m)
% error
Formula Used
Substitution
Solution w/Units
Analysis:
1. The accepted value for the wavelength of the laser light used was 632 nm. This means that you
could fit 10,000 wavelengths of this light end to end on a millimeter wide length (about the length
of the white part of your nail at the tip if you keep your nails closely trimmed). Can you think of
any other way to measure that kind of length? Explain why or why not.
2. Describe what happened to the separation X between the maxima projected on the screen as the
distance d between the slits decreased.
3. Describe what would happen to the separation X between the maxima projected on the screen as
the distance L between the slits and the screen decreased.
4 What would happen to the separation X between the maxima projected on the screen as
the wavelength decreased (use blue light instead of red).
5. What is your answer to the Testable Question? Why?
6. List and describe quantitatively (show by calculations of the effect your chosen error would have
on the percent difference of your wavelengths) the effect of three things that could possibly have
caused the % difference you found for your wavelengths.
a.
b.
c.