Optional Practical: Helium Spectrum


In this practical you are going to view the line spectrum from a low pressure helium source through a pair of spectral glasses.

The spectrum glasses are actually covered in very fine slits, such an arrangement is called a “diffraction grating”. The spectrum is formed because different wavelengths of light are diffracted by different amounts according to the equation dsinϑ=nλ. where d is the separation of the slits in the diffraction grating and n is the order of the spectrum (note: the spectrum repeats as you look outwards, for the first spectrum n=1). The way the spectrum can be seen on either side of the source is shown below.

Research question

What is the relationship between the angle of diffraction and wavelength?

Independent variable: wavelength
Dependent variable: angle of diffraction
Controlled variables: slit spacing


The angle is going to be measured by holding a vernier calliper in front of your eyes.

  • Put on the glasses and look at the light from a distance of a couple of meters
  • Hold the callipers on a meter rule as shown below then hold the arrangement in front of your eye and look along it like looking through a telescope.

  • Move the calliper along the ruler until the gap is the same as the spacing of the first line in the spectrum.
  • Make a note of the distance D in a suitable table.
  • Keeping the calliper spacing constant find D for all the colours in the spectrum.


If the angles are small sinϑ = x/2D.

Using the values for wavelength given below use a graphical method to find the spacing of the lines on the glasses.

New Helium spectrum 

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