# Introduction

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

#### Method

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.

Note:

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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