Bouncing Beams

AIM:

To introduce the idea of how we see objects and that white light comprises a spectrum of coloured light.

CURRICULUM LINKS:

KS2: Sc4 Physical Processes, Light and Sound. 3(a) and (d)
KS1: Sc4 Physical Processes, Light and Sound. 3(a)
KS3: Sc4 Physical Processes, Light and Sound. 3(e)

TIME:

15 minutes. 5 minutes explanation, 10 minutes investigation.

MATERIALS REQUIRED:

Per group:

• A compact disc
• 4 discs of stiff card (17 cm diameter)
• 4 circles of paper, same size as the card, which have been divided into equal sections. Sections should be alternately coloured with the following colours:
1. Green and Blue [PDF version] [Word version]
2. Red and Blue [PDF version] [Word version]
3. Red and Green [PDF version] [Word version]
4. Red, Yellow, Green, Cyan, Blue, and Magenta
   [PDF version] [Word version]
• 4 pointed wooden sticks (or pencils)
• blu-tack, plasticine, or glue gun

METHOD:

• Prepare spinning discs in advance
• Explain to the kids that we need a source of light to see
• Get them to list some light sources
• Discuss briefly that Newton discovered that white light was made of a spectrum of colours by splitting sunlight with a prism
• Explain that a CD can split the light and tilt the CD in the light to see the spectrum
• Explain that coloured light seen does not mix in the same way as paint would
• In groups let the children spin the discs after predicting the colour mixing that will occur
• Look at the spectrum of light made using the CD
• Explain that for red objects only red light is reflected, for cyan objects both blue and green light is reflected, for white objects all colours are reflected and for black all light is absorbed and non-reflected.

Spinning Discs:

• Download coloured discs and print onto A4 paper
• Stick the paper discs onto the card and pierce a hole in the centre, large enough for the wooden stick to fit through
• Push the stick through the card, point-side down, and fasten in place on the underside with blu-tack, plasticine, or using the glue gun

THEORY:

• We can see the object since the light bounces (or reflects) off it into the eye and the brain then processes the image.
• Light travels as little packets of energy - photons
• The CD has tiny ridges off which the light can reflect. The different colours in the white light reflect at different angles and so the colours are seen to split apart.
• The coloured patterns on the spinning discs should mix as follows:
1. Green and Blue - cyan (turquiose)
2. Red and Blue - magenta (purple)
3. Red and Green - yellow
4. Red, Yellow, Green, Cyan, Purple, and Magenta - white
• When the discs are spinning rapidly we see the colours mix together as our eyes are not quick enough to distinguish each one
• In most cases the colours will be a little muddy due to the colours from the printer not being completely true
• If this experiment is done in a room with artificial light then the light from the bulbs is not truly white light which will also affect the colours seen for the discs

WEB LINKS:

back to Light