Build a CD spectroscope

The question

Is all "white" light the same? If I split the light from a filament bulb, an LED, a street lamp and daylight into colours, what's different about each one?

Learn just enough first

• Watch a short explainer on spectra and spectral lines
• Read up on how each chemical element gives off its own pattern of colours — a "barcode"

Kit

• An old CD or DVD (DVDs give a wider rainbow)
• A cardboard tube or small box
• Two razor blades or bits of card to make a thin slit
• Tape, scissors, a phone camera

My prediction

Write this before looking. Which light sources will show a smooth rainbow, and which will show separate bright lines?

Method

1. Make a very thin slit (about 1 mm or less) at one end of the tube.
2. Fix a piece of the CD at the other end, angled at roughly 60°, shiny side facing in.
3. Point the slit at a light source and look through the CD end at the spectrum.
4. Photograph each spectrum through the eyepiece with your phone.

Safety: Never point the spectroscope at the Sun. For daylight, aim at a bright white cloud or a white wall in daylight.

Data

Result

What patterns did I find? Fluorescent tubes and some street lamps should show sharp lines — those are the fingerprints of mercury or sodium atoms.

Explain it to a friend

Atoms can only give out light at particular energies, so each element has its own barcode of colours. Astronomers spread starlight out the same way and read off which elements are there — this is also one of the first clues that energy comes in quantum steps.

New questions

• Why do some sources give smooth rainbows and others separate lines?
• Could I spot dark lines in sunlight reflected off a cloud?