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.

.

Discussion: Referring to the diagram below, if:

  • the distance L from a diffraction grating to a screen is measured,
  • the separation distance dbetween 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:

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 .
  • Compare your measured value to the accepted value given below by calculating

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

DistanceL 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

/

Formula Used

/ Substitution / Solution w/Units
d (m)
(m)
% error

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 describequantitatively (show by calculations of the effect your chosen error would have

on the percent difference of your wavelengths) theeffect of three things that could possibly have

caused the % difference you found for your wavelengths.

a.

b.

c.