What it shows
A CD acts as a diffraction grating, splitting light into colours. Different sources give different patterns of colour, and astronomers use the same idea to find what stars are made of.
The science
The tiny grooves on a CD scatter light in different directions depending on its colour, so a white beam splits into a rainbow. A narrow slit makes a thin line of light and so a clean spectrum. A hot solid glows with a continuous rainbow, whereas a gas lamp gives a few bright lines, which are like fingerprints of its elements.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cardboard box with a lid, about 20 cm long
- A piece of an old CD with its silver layer on
- Two razor-blade-free straight edges, such as thick card, to make a narrow slit
- Black paper and tape
- A phone to take photographs
- A CFL tube light, an LED torch and a white card for daylight
Safety
For this project: An adult must cut the CD with scissors, and handle sharp CD edges. Never look at the Sun through the slit; use light from the sky or a sheet of white paper in sunlight.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Make a narrow slit about 1 mm wide at one end of the box using two card edges.
Cut a viewing hole on the side near the other end and cover the inside with black paper.
Fix a CD piece inside the far end at about 45 degrees with the shiny side facing the slit.
Aim the slit at a white card lit by daylight and look through the hole to find the rainbow band.
Adjust the CD angle until the spectrum is clear and tape it in place.
Look at an LED torch and a CFL tube and sketch the spectrum of each with colour pencils.
Photograph each with a phone placed at the viewing hole.
Compare the sketches and describe the bright lines in the CFL.
What you should see
A continuous rainbow band for daylight and the LED, but a spectrum with a few bright lines for the CFL, showing different sources emit different colours.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does a diffraction grating do?
It splits light into colours by bending each colour by a different angle.
2Why use a narrow slit?
It makes a thin line of light, so the colours do not overlap.
3What do bright lines in a spectrum tell us?
Which elements are present in the source.
4How do astronomers use spectra?
To find what stars are made of and how fast they move.
Take it further
Mark a scale using a known source, and estimate the wavelength of the green line in a CFL.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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