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Activity

Rainbows from light with a CD/prism/scratched plastic

Summary: 
Separate the colours in sunlight, or in bulbs, with prisms, cut glass, CD, scratched plastic. Discuss where the colours come from.
Science content (2016 curriculum): 
Physics: Light and Sound (1)
Materials: 
  • a sunny day, room lights, white holiday lights
  • optional: coloured holiday lights
  • one ore more of:

  • glass/plastic prism, or other cut glass objects (e.g. paper weight, drinking glass) and a white board
  • CD
  • scratched plastic, diffraction grating, or rainbow glasses
Procedure: 

Hand out prisms, CDs, other cut glass objects. Ask students to tip the objects towards one or more light source(s), including the sun, and make a spectrum - the colours in the light spread into a rainbow, or some colours within it.

Hand out scratched plastic to see spectra emanating from objects or lights.
With objects or light sources that only emit some colours e.g. holiday lights or various coloured objects, a fraction of the colour spectrum is seen.

Using marker pens, the scratched plastic can be used to find out the primary colours of light (cyan (a light blue), magenta (pinky red) and yellow).
Give students purple or dark blue, green and red markers. Ask them to make a blob of colour on white paper, then look at the blob to see what colours "bleed" out of the sides. They should find that cyan, magenta and yellow appear around many colours. This is because the scratched plastic is splitting up the light coming from the marker pen and splitting it into its component colours - these are the colours that make up light (cyan, magenta and yellow). Just like paint there are also secondary colours, which might be seen where primary colours overlap.

Discuss where the colours come from (with youngest grades and older grades if necessary: while repeating the experience with the whole class looking at each light source in turn):
White light is a mixture of many colours. When white light hits the groves in the CD, the edges of cut glass or the surfaces of bubbles or oil, the light is separated into its component colours, which line up into a rainbow.
Coloured lights, or coloured objects are only emitting a portion of the whole colour spectrum. So when their light is separated, a reduced range of colours are seen.
(Astronomy connection lesson plan on star spectra.)

With older students, discussion can include the method of making the colours:
The edge of a glass prism, or the cut glass of a gem or drinking glass, bends each of the wavelengths (colours) of light slightly differently, so that they are separated out. The bending of light is called refraction.
The colours in a CD or scratched plastic, in bubbles or an oil slick are formed by interference. When light is reflected from the grooves of the CD or the top and bottom surfaces of a bubble or oil, the light waves interact with each other. Interference of the waves enhances some wavelengths and cancels others in different places, resulting in a rainbow.

Grades taught: 
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Teacher: 
Becky Evermon
Ingrid
Julia Szampias
Mari Matsuo
Monika Pilat
Teaching site: 
General Gordon Elementary Science Club
Science Workshop for Destination Imagination
Fraser Elementary
Tyee Elementary