A simple spectrum

Class practical

Producing a simple spectrum by dispersion of light.

Apparatus and Materials

• Ray box (see below)
• +7D cylindrical lens
• Single slit
• Multiple slit
• L.T. variable voltage supply
• 60-degree prism
• Pieces of coloured filter

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Direct a single streak of light at the prism and observe the emerging ray carefully. Twist the prism and observe the effect on the emerging ray.

2. Direct a fan of rays (from lamp and multiple slit) at a +7D cylindrical lens so that the emerging rays pass through an image-point 40 to 50 cm away. Place the prism just beyond the lens and look at the effect. If you hold a small piece of paper or card upright to catch the rays above the table, the spectrum will show clearly.
3. Try placing small pieces of colour filter in the path of the light, before it strikes the prism.

Teaching Notes

• This should be as simple as possible, without complicated optics, as an introductory experiment.

• The spectrum will be the most pure if the screen is held at the same optical distance from the lens as the image was before the prism was inserted, and the prism is turned to minimum deviation. However, turning the prism to a greater deviation will show a wider spectrum.
• Different shaped prisms could be used, and the way rays pass through them compared. Looking along the ray towards the source will show that the ray appears to be straight.
• Using a coloured filter next to the ray box, the emergent ray will also be coloured, and as the prism is rotated the colours will become clearer.

This experiment was safety-tested in January 2007