Non-spectral colours and iridescence
PERCEIVING COLOURS:
The Rainbow and beyond
Discover non-spectral colours when you experiment with iridescent materials.
Details:
All ages
Time: 20 minutes
Learning Outcome: By examining iridescent and related objects, notice and describe the hue families which are not part of the visible spectrum or rainbow.
Colour Concept: We can see a range of hues which are not seen in the visible spectrum or rainbow.
Materials:
Background - Rainbows, Newton and the colours of the spectrum
Background - What is the relationship between light, wavelength and colour?
Diffraction grating from Colour paddle set
Bottle of bubbles with bubble wand (or make your own bubble solution with 3 parts Dawn dishwashing soap and 1 part water)
Iridescent paper (Download instructions on preparing Iridescent paper)
Instructions:
Diffraction grating: Look at the X shown below with your diffraction grating (can look at X on a screen, or print Big X Handout and look at a printed page). What do you notice? Describe the colours you see.
Bubbles: Blow bubbles. What do you notice? Describe the colours you see.
Iridescent paper: Prepare the iridescent paper in advance. What do you notice? Describe the colours you see.
Vocabulary:
Iridescent colours, Non-spectral colours, Hue, Hue family, Magenta, Visible light spectrum, Diffraction grating
Questions & Observations:
Do you see any colours with the diffraction grating, in the bubbles or iridescent paper which are not present in the visible spectrum from the Examining the Rainbow exercise?
Sometimes sources refer to these non-spectral hues as not being ‘real’. This is a common misconception. Why can non-spectral hues be considered as ‘real’ as any other hue?
What is the difference between magenta and pink?
What is the difference between purple and violet?
How does the visual quality of the colours you see in this exercise compare to the colours you see in coloured papers or paints?
Is the order of colours you see in this exercise the same as you see in the spectrum?
What’s going on?
Magentas and purples are examples of non-spectral hues (or hue families). This means they are not part of the visible light spectrum, produced when a beam of white light passes through a prism, yet they are visible to our human eyes. We see magenta and purple-coloured objects and lights when light from both ends of the visible light spectrum enters our eyes at the same time, and activates our visual system. Colour is a perceptual experience. When we see magenta, purple or any other colour, our visual system interprets the balance or relative amounts of different wavelength ranges of light entering our eyes.
When we look at the Big X through a diffraction grating, it creates a situation where both ends of the visible light spectrum can overlap, allowing us to perceive magenta. Iridescent objects like bubbles or iridescent paper can also direct light from both ends of the visible light spectrum to our eyes. When this happens we see magenta or purple coloured bands in these objects.
Find out more:
See the discussion on non-spectral hues, which corrects the common misconception that magenta is not a ‘real’ colour.
See the discussion on how light activates our visual system in So how do we see all the colours in the world?
Check out Peter Donahue discussing his fascination with soap bubbles, and how they are not rainbow-coloured
Watch Peter Donahue find magenta, with a prism and a piece of tape!
