Seeing Red – Teachers’ Notes
Who is it for?
11-14 year olds
How long will it take?
Approx. 45 minutes
Learning outcomes:
Students will learn about colours, the different
frequencies of light and how different coloured light is
absorbed or transmitted by coloured filters. They will also
discover how some animals use bioluminescence and
colour perception to outsmart their prey.
Curriculum requirement:
This activity is designed to support the teaching of Key
stage 3: Physics: Motion and forces: Waves: Light waves:
Colours and the different frequencies of light, white light
and prisms.
What do you need?
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Transparent plastic in a variety of colours - including red,
blue and green. This could be coloured acetate, printed
acetate/OHP sheets, coloured plastic folders etc.
Assorted pictures of different animals (available to
download and print in activity pack)
Colouring pens, pencils or crayons - some matched to
the colours of the acetate and some different
White paper
Black paper
Scissors
There are also some helpful diagrams in the activity pack
that might help you explain additive and subtractive
colour mixing.
Preparation guidelines:
1. Read through the Teachers’ Notes and download the Activity Pack, which
provides resources such as images and diagrams.
2. Gather together the required materials, print out the images or put them into
a presentation.
How to run the session:
1. Find out what the students already know about light and how we see colour.
Make sure that they understand that the colour of an object depends on the
wavelengths of light that are reflected off the object and are detected by
our eyes.
2. Discuss additive colour mixing, and how white light is a mixture of all colours. If
needed, show the image (prism.jpg) of white light being split into its
constituent wavelengths after passing through a prism and the additive and
subtractive colour mixing images.
3. Introduce the first exercise: looking at the different wavelengths of light that
make up colourful pictures.
4. Show different coloured images to the students and discuss the colours that
you can see. Then try putting the differently coloured filters over the images
and discuss how this affects the colours in the picture. This can be done by
handing out copies of the photos and a range of filters to the students, or at
the front of the room on an overhead projector. Alternatively, you could use
the filtered images (supplied in the activity pack) in a PowerPoint
presentation. Explain what is going on using the guidance notes below.
5. Introduce the concept of bioluminescence, and how some animals are able
to produce light. Arkive’s page on bioluminescence may be helpful for this
(www.arkive.org/c/bioluminescence)
6. Using the guidance notes below, discuss how dragonfish are able to hunt
their prey by being able to both produce and see red light at depth.
7. Run the second exercise where the students write coded messages to each
other using different coloured pens and filters.
8. Discuss whether this is a representation of dragonfish predator/prey relations.
Extend the task by putting the same coloured letters onto black paper and
study the difference in being able to read the codes as when compared to
the letters being on white paper.
Notes for Exercise 1: Organisms and their Environments
Objects that are not transparent either absorb or reflect the light that reaches them.
When viewed under white light (which contains all colours), a red beetle reflects red
light and absorbs all the other colours. Similarly, a green leaf reflects green light and
absorbs all the other colours. White objects appear white because they reflect all
colours of light, and black objects appear black as they absorb all colours.
Coloured filters transmit some wavelengths of light and absorb others. Use the filters
to look at the pictures of animals. Alternatively, you can show the prepared images
of how the colour images would look using different filters – these can be
downloaded in the activity pack.
Scarlet macaw:
Full colour
Red filter
Blue filter
Firefly squid:
Full colour
Red filter
Cardinal beetle:
Full colour
Green filter
Red berries:
Full colour
Green filter
Blue filter
Red filter
Red filter
What’s Going On?
An ideal red filter transmits only red light and absorbs all the other colours. When
viewed through a red filter, a picture that contains red, green and blue would
appear red and black. The red filter blocks green light and blue light so that only red
light can get through to your eyes. Any areas of white or yellow reflect some red
light, so these areas look red and lighter than the blue or green areas.
Similarly, a pure blue filter only transmits blue light and absorbs all the other colours.
When viewed through a blue filter, a picture that contains red, green and blue
would appear blue and black. The blue filter blocks red and green light so that only
blue light can get through to your eyes. Any areas of white reflect some blue light, so
these areas look blue and lighter than the red or green areas.
Try looking at the different pictures through the different filters to explore which filters
let through which colours.
Notes for Exercise 2:
Most bioluminescence in the sea produces a blue-green light, as blue light travels
furthest in water and most marine animals are able to detect blue light. Dragonfish
live in the deep sea and have been recorded at depths of over 2,000 metres, in an
area called the ‘midnight zone’ where there is no light at all from the sun.
Dragonfish, and their relatives, are the only fishes that are able to produce red
bioluminescence. They are also unique in the way that they are able to perceive red
light. Most of their prey are not able to detect red light and so they are able to hunt
using a beam of light that is invisible to their prey.
Get into groups of four. Three people should each choose one of the differently
coloured filters, while the other one writes a multi-coloured word or a short message
on the white sheet of paper using the different coloured pens. The person with the
red filter should look at the message through the red filter and say what they can
read (take care not to let them look at the message without the filter!). Repeat this
with the other colours of filter. The person looking through the red filter will notice that
the red letters disappear, but they can still see the blue or green letters. Use the
knowledge of who can read which coloured letters to write a secret message that
can only be decoded by looking through one of the filters. Have fun using the
different coloured pens to write secret messages to your classmates.
What’s Going On?
A red filter blocks green and blue light so that only red light can get through to your
eyes. Red light comes from both red letters and the white paper (as white contains
all colours). The red letters therefore tend to disappear as they blend into the red
light coming from the paper. Letters that do not contain any red look black through
the filter and so they stand out and can be read more easily.
For the dragonfish, if their prey reflected red light, they would look brighter than their
surroundings as there is no other light in the depths of the sea. Try repeating the
above experiment but instead of writing on white paper, try sticking coloured letters
onto a black background. Is it now easier or harder to read the red letters through
the red filter?
Suggestions for extension activities:
Compare additive colour mixing (light) to subtractive colour mixing (pigments) and
discuss how printers are able to reproduce all colours using the CMYK process.
For more information:
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Arkive - Bioluminescence: http://www.arkive.org/c/bioluminescence
NOAA Ocean Explorer: http://oceanexplorer.noaa.gov/facts/red-color.html