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.
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.