
Aim: to estimate the Wavelength of the Light given
by a Sodium Lamp 
See The Diffraction Grating, Spectra 

Method 
Use a diffraction grating as shown in the diagram
below. 

One person looks through the diffraction grating
while the other person places a thin rod so that its
position coincides with the observer's view of one
of the diffracted images of the slit. 
Mark the positions of as many images as can be seen. 
From the diagram it can be seen that the angle to be
measured is given by 

When θ is known,
the diffraction grating equation can be used to find the
wavelength of the light. 

Calculate the maximum number of diffracted images theoretically produced by the diffraction
gratings available. 
When you do the experiment, you will not be able to
see as many images as your calculations suggest because
the intensity of the light in those images is too low to
observe with the naked eye. 