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Diffraction Grating with White Light Source

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Physics Laboratory Report

The number and the name of the experiment:

Experiment C3- Diffraction Grating with White Light Source
The date of experiment:19 / 5 / 08image00.jpg


  • Estimate the wavelength of the light using a diffraction grating
  • Measure the angle of diffraction by simple geometry
  • Measure the angle of diffraction by simple spectrometer
  • Learn the techniques in using a spectrometer



meter rule


set square(woodblock)




Diffraction grating(300 lines per mm)


ray box with vertical filament


12 V low voltage power supply


long pin




laboratory jack


color filter – red, green, blue

1 each

sodium lamp(optional)



In the visible wavelengths of the electromagnetic spectrum, red, with the longest wavelength, is diffracted most; and violet, with the shortest wavelength, is diffracted least. Because each color is diffracted a different amount, each color bends at a different angle. The result is a separation of white light into the seven major colors of the spectrum or rainbow. The diffraction grating consists of many narrow slits provided for the diffraction of the light.image02.jpg

Constructive interference occurs between waves from A traveling in the direction AA1 and waves from B traveling in the direction BB1.

...read more.


1  can be obtained using the formula tanΘ =

Finally, by substituting the values of Θ1and d

into the equation d sinθ1 = 1λ, the required

 wavelength can be calculated.


  1. Use the ray box with vertical filament. Place the lamp about 2 m away.
  1. Place two meter rules at right angles to each other as shown. Set the diffraction grating vertically at one end of the meter rule. Observe the horizontal diffraction pattern (color spectrum) of the filament by viewing through the diffraction grating.
  1. Ask the partner to move a long pin to the observer’s left hand side along the other meter rule until it lines up with the middle of the red band of the first order spectrum. Place the red filter in front of the lamp to helping locating the position.
  1. Repeat step 3 for the position on the observer’s right hand side with another pin. Find the separation, 2x, of the pins. Record the results in the table provided.
  1. Repeat step 3 and 4 for green and then blue.
  1. Determine the angles of diffraction for red, green and blue light.
...read more.



  • Whether it is better to use pins with the addition of laser bean rather than just pins to locate the position of the light to be found.

Explanation﹕Absolutely, it is better to accept this replacement. The principle of using the pins is to move the pins along the either side of the meter rule until it lines up with the middle of the red band of the first order spectrum in order in order to find out the exact position of the required light. As using the naked eyes to judge the middle point of the red region within its wide range is already really difficult, if we just use the mechanical work (moving the pins) to match up with the observer’s observation, tremendous error will exist due to the inexact human sensate system. Thus, by using the laser bean, we can first point out the relative middle position of the light, then moving the pins along the meter rule until it coincides with the infra-red light. It is no doubt that it will increases the accuracy of the results taken.

...read more.

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