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Experiment : colours of light


To investigate the additive properties of colours and determine the complementary colours of light.


You will need two battery operated torches with flat bulb fronts, a large piece of white paper, and some pieces of cellophane paper of the following colours: red, blue, green, yellow, cyan, magenta. (You should easily be able to get these from a newsagents.)

Make a table in your workbook like the one below:

Colour 1 Colour 2 Final colour prediction Final colour measured
red blue
red green
green blue
magenta green
yellow blue
cyan red

Before you begin your experiment, use what you know about colours of light to write down in the third column "Final colour prediction", what you think the result of adding the two colours of light will be. You will then be able to test your predictions by making the following measurements:


Proceed according to the table above. Put the correct colour of cellophane paper over each torch bulb. e.g. the first test will be to put red cellophane on one torch and blue cellophane on the other. Switch on the torch with the red cellophane over it and shine it onto the piece of white paper.

What colour is the light?

Turn off that torch and turn on the one with blue cellophane and shine it onto the white paper.

What colour is the light?

Now shine both torches with their cellophane coverings onto the same spot on the white paper. What is the colour of the light produced? Write this down in the fourth column of your table.

Repeat the experiment for the other colours of cellophane so that you can complete your table.


  1. How did your predictions match up to your measurements?
  2. Complementary colours of light are defined as the colours of light which, when added to one of the primary colours, produce white light. From your completed table, write down the complementary colours for red, blue and green.

Complementary colours

Complementary colours are two colours of light which add together to give white.

Investigation : complementary colours for red, green and blue

Complementary colours are two colours which add together to give white. Place a tick in the box where the colours in the first column added to the colours in the top row give white.

magenta yellow cyan
(=red+blue) (=red+green) (=blue+green)

You should have found that the complementary colours for red, green and blue are:

  • Red and Cyan
  • Green and Magenta
  • Blue and Yellow

Perception of colour

The light-sensitive lining on the back inside half of the human eye is called the retina. The retina contains two kinds of light sensitive cells or photoreceptors : the rod cells (sensitive to low light) and the cone cells (sensitive to normal daylight) which enable us to see. The rods are not sensitive to colour but work well in dimly lit conditions. This is why it is possible to see in a dark room, but it is hard to see any colours. Only your rods are sensitive to the low light levels and so you can only see in black, white and grey. The cones enable us to see colours.Normally, there are three kinds of cones, each containing a different pigment. The cones are activated when the pigments absorb light. The three types of cones are sensitive to (i.e. absorb) red, blue and green light respectively. Therefore we can perceive all the different colours in the visible spectrum when the different types of cones are stimulated by different amounts since they are just combinations of the three primary colours of light.

The rods and cones have different response times to light. The cones react quickly when bright light falls on them. The rods take a longer time to react. This is why it takes a while (about 10 minutes) for your eyes to adjust when you enter a dark room after being outside on a sunny day.

Interesting fact

Color blindness in humans is the inability to perceive differences between some or all colors that other people can see. Most often it is a genetic problem, but may also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals. The most common forms of human color blindness result from problems with either the middle or long wavelength sensitive cone systems, and involve difficulties in discriminating reds, yellows, and greens from one another. This is called "red-green color blindness". Other forms of color blindness are much rarer. They include problems in discriminating blues from yellows, and the rarest forms of all, complete color blindness or monochromasy, where one cannot distinguish any color from grey, as in a black-and-white movie or photograph.

run demo

When blue and green light fall on an eye, is cyan light being created? Discuss.

  1. Cyan light is not created when blue and green light fall on the eye. The blue and green receptors are stimulated to make the brain believe that cyan light is being created.

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Colours on a television screen

If you look very closely at a colour cathode-ray television screen or cathode-ray computer screen, you will see that there are very many small red, green and blue dots called phosphors on it. These dots are caused to fluoresce (glow brightly) when a beam of electrons from the cathode-ray tube behind the screen hits them. Since different combinations of the three primary colours of light can produce any other colour, only red, green and blue dots are needed to make pictures containing all the colours of the visible spectrum.

Colours of light

  1. List the three primary colours of light.
  2. What is the term for the phenomenon whereby white light is split up into its different colours by a prism?
  3. What is meant by the term “complementary colour” of light?
  4. When white light strikes a prism which colour of light is refracted the most and which is refracted the least? Explain your answer in terms of the speed of light in a medium.

Questions & Answers

ketones's functional group
Moloi Reply
Why does the carboxyl group have acidc properties?
Evi Reply
carboxylic acids
Besides improving appearance what is another reason why medal is electro plated
Elizabeth Reply
it is made of metal
what is different between Dependent variable and independent variables
difference between A.c and D.c
AC has slip rings DC has split rings commutator
are UV rays dangerous?
Khathutshelo Reply
what's this?
if there is extensive exposure, it can lead to skin cancer
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
what is the wave model of atom ?
Trazy Reply
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
the change in frequency of the sound detected by a listener because sound source and listener have different velocity
Newton's first law of motion
Nnuso Reply
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
yes that's true
Always know Newton's second law of motion. It appears to be in every question paper
Is the normal force always 0
Mpilo Reply
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
define the term rate of reaction in word
David Reply
I think it a chemical process in which substance act mutually on each other.
how many bones are in the human body?
it's the rate at which the reactants are able to change to products
206 bones
what is wave lengh
mama Reply
is the de broglie wavelength of the particle
calcium and magnesium, which one can displace aluminum from its compound?
Markia Reply
how is aluminum ion formed?
a substance that has the lower electronegativity which will be in this case the Ca -1.0 this is because the Ca would have a lower ionization energy -needs less energy to fill its outer most shell and therefore will cause a displacement of the Al
an Al ion would form when it has lost electrons and will normally be a cation -positively charged (3+)
molecules with the same molecular formula but different chain
Malaza Reply
what will be the answer
chain isomer
what is structural isomer?
Hey if you define an isomer it will come as an organic molecule with the same number of atoms,same molecule formula ,same molecular mass but different STRUCTURAL FORMULA so what we have is only the chain ,positional and functionally isomer.Never heard of STRUCTURAL isomer
what does the newton's law says?
there are 3 newtons laws which are newton's law of universal gravitation, law of cooling, law of motion
Did the feather or leaf hit the ground first?
Shaloom Reply
both at the same time. air friction ignored

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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