Lighthouse Initiative for Texas Classrooms

Measuring the Wavelength of Light (Young's Lab)

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To measure the wavelengths of six colors of light and light produced by a laser.

Part I—Colors

  1. Place a light bulb at the end of a meter stick. Look at the bulb through the diffraction grating, and note the colored band of light off to your left.
  2. Have a lab partner move the index card over to the left of the bulb until the center of the card lines up with the center of one of the colored bands of light.
  3. Measure the distance x from the bulb to the index card where the colored band (antinode) was formed, and record in the table below.
  4. Calculate the wavelength of the color using λ xd / L, and its frequency.
  5. Repeat steps 1-4 for five other colors of light.
  6. Using the given values for the wavelengths of each color, calculate the percent error for each wavelength you measured.
Color x
Calculated λ f
Known λ
% Error
red   2.5x10-6       6.8  
orange   2.5x10-6       6.3  
yellow   2.5x10-6       5.8  
green   2.5x10-6       5.2  
blue   2.5x10-6       4.8  
violet   2.5x10-6       4.2  

Show one example of each type of calculation below:



% Error

Part II—Measuring the Wavelength of Laser Light

Physics, Wavelength of Light, Figure 1

1. Place the double-slit slide in front of the laser, so that an interference pattern is formed on the wall. Sketch the interference pattern below.

Physics, Wavelength of Light, Figure 2

2. Using a ruler, measure the width of the central antinode, from the centers of the nodes on either side of the bright central antinode. This is the distance x which was measured in Part I of this lab.

3. Record your values for x, d, and L below, and use them to calculate the wavelength of the laser light.


  • x = _________
  • d = _________
  • L = _________


1. List your colors from lowest frequency to highest frequency.

  1. ________
  2. ________
  3. ________
  4. ________
  5. ________
  6. ________

2. What is the difference between laser light and regular light?

3. Compare the wavelength of the laser to the wavelengths of the colors you measured. What color (wavelength) does the laser light most resemble?

4. List two reasonable sources of error in this lab.