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CD Spectrometer

A cereal box with a CD inserted into the top left corner.

Can you do the splits? Well, you can now! With only a few household objects, it’s easy for you to split light.

Australian Curriculum links

  • Science > Physical Sciences > Year 5 > ACSSU080

You'll need

A CD, a pen, a roll of sticky tape, a pair of scissors, a ruler, a roll of aluminium foil, and a cardboard cereal box.

  • empty cereal box with an unopened bottom panel
  • ruler
  • pen
  • scissors
  • CD (or DVD)
  • aluminium foil
  • sticky tape

Try this

  1. Look at the bottom panel of the cereal box (with the long edges at the top and bottom, and the short edges on the sides) and use the ruler to measure 4 cm along the bottom edge from the left-hand corner. Make a mark with the pen. Use the ruler to measure 4 cm along the top edge from the left-hand corner. Make a mark with the pen. Connect the points by drawing a line.
  2. A ruler and a cereal box with a square hole cut out of the top. Create a 4 cm wide rectangular hole in the left side of the bottom panel of the cereal box by cutting along the drawn line, the bottom edge, the left edge and the top edge of the panel.
  3. Look at the front panel of the cereal box (with the new hole on the left-hand side of the bottom panel). On the bottom, left-hand corner of the front panel, draw a 4 cm wide by 7 cm tall rectangle. Draw a diagonal line from the top, right-hand side of the rectangle to the bottom, left-hand side of the rectangle.
  4. Turn the cereal box over and look at the back panel of the cereal box. On the bottom, right-hand corner of the back panel, draw a 4 cm wide by 7 cm tall rectangle. Draw a diagonal line from the top, left-hand side of the rectangle to the bottom, right-hand side of the rectangle.
  5. A cereal box with a CD inserted diagonally into the top. Cut along the diagonal lines drawn on the front and back panels of the cereal box. Slide the CD completely into the groove that has been created by the diagonal cuts.
  6. Turn the box so that you are looking at the side panel furthest away from the previous cuts and the 4 cm wide rectangular hole is at the bottom. Mark 1 cm and 4 cm in height on the left-hand edge of the panel. Mark 1 cm and 4 cm in height from the right-hand edge of the panel. Connect the 1 cm points to each other and the 4 cm points to each other by drawing lines across the width of the panel.
  7. A cereal box with a CD inserted diagonally into the top and a hole cut into the side. Cut along the drawn lines and the two edges of the cereal box to create a 3 cm wide rectangular hole in the side panel of the cereal box.
  8. Cut two, 10 cm squares of aluminium foil and fold the squares in half.
  9. Position the folded edge of one of the pieces of aluminium foil over the middle of the hole created in step 7 to cover the top-half of the hole. Use sticky tape to secure the aluminium foil to the cereal box.
  10. A cereal box with a CD inserted diagonally into the top and aluminium foil over the side. Using the other piece of aluminium foil, repeat step 9 for the bottom-half of the hole, leaving a 0.5 mm to 1.0 mm gap between the two pieces of aluminium foil.
  11. Cover the top-third of the cereal box using one, large piece of aluminium foil and sticky tape. (The purpose of this step is to block out excess light.)
  12. Point the thin gap in the two pieces of aluminium foil (located at the bottom right-hand side of the box) at a lamp or florescent light.
  13. A cereal box with a CD inserted diagonally into the top, and aluminium foil over the bottom and side. Place one eye above the hole on the bottom panel of the box and look at the surface of the CD that is on the inside of the box. What light pattern can you see?

Further investigation

Try pointing your CD spectrometer at a range of different light sources (eg. TVs, computer screens, street lights, etc) and compare the different patterns you see. Which colours appear brighter or darker in your spectrometer image? Do all light sources make white light in the same way?

What's happening?

When light passes through the gap in the aluminium foil and hits the CD, it splits into many different colours. A device that splits light in this way is known as a spectrometer. Depending on the source of the light (halogen globe, fluorescent globe, daylight, etc.), some colours will appear brighter or darker than others.

Visible light is made of a rainbow of different colours, known as the spectrum of visible light. These colours can easily be combined to make white light as well as be split back into separate colours.

The splitting of light by colour is known as diffraction. Diffraction can occur in several ways. Often prisms or other glass objects are used to split light. As light enters and exits glass, the light changes speed causing it to bend and change its path. Because different colours of light bend at different angles, the light that enters the glass is split into a rainbow of colours.

Another technique for splitting light involves using a diffraction grating. A diffraction grating is made up of many tiny little gaps or grooves that are extremely close together. When light hits the grooves, the light bounces off at different angles to show the different colours of the light.

CDs and DVDs are very good diffraction gratings. The data held on a CD is made up of tiny grooves in the disc, much like words written in Braille (text presented for the vision impaired) are made up of small bumps on a page. The size and spacing of the CD grooves are small enough to split visible light into its spectrum of colours.

When we point the CD spectrometer at a halogen globe or at light reflected from the sun, the image on the CD shows a continuous, rainbow pattern. White light from these sources is made up of almost every colour in the spectrum, so we see the whole rainbow in our image.

However, white light from a fluorescent globe is made up of only a handful of colours. Thus the CD spectrometer shows a few bands of colour, with areas of shadow between them. These shadowy gaps represent the colours that are not present in the light source.

Real world links

Humans only have the ability to see three different colours: red, green and blue. All other colours we can see are just combinations of these three 'primary colours of light' (which are different to the primary colours used in painting; red, yellow and blue). Other animals can see different colours. For example, bees and butterflies can see ultraviolet light and snakes can detect infrared light. As a result, these animals could possibly see the world in a much broader rainbow of colours than we can see.