Demonstrating the production of a spectrum

What the Activity is for

Splitting light with a prism.

This activity is intended to show a really impressive spectrum, made by a prism, from a white light source.

What to Prepare

• quartz-iodine lamp plus 12 V power supply (bright white point source of light)
• high-dispersion prism
• white screen
• a room with good black-out

What Happens During this Activity

Set up the light source and prism as shown, with the screen a good distance away from the prism.

The greater this distance, the larger the spectrum, but you will have to balance this against the brightness of the spectrum, which will be brighter when the screen is closer. How bright you need the spectrum will depend on the darkness in the room. Do not try to do the experiment in a laboratory without blackout. Make sure that the light beam does not go through one of the corners of the prism. Rotate the prism gently so that the direction of the light beam is changed as little as possible. This generally gives the best spectrum.

Enjoy the spectacle. Then draw attention to the ordering of the colours and the path that the light has followed, from source to screen.