Cyberphysics - Colour

Colour

COLOUR

Dispersion through a prism

Filters

Pigments

Care with video 2 - red and blue light travel at the same speed - Bill confuses frequency with speed - I would expect you NOT to do that!

All of the colours that we see are due to the stimulation of cells in our eyes called cones. These are cone shaped light sensitive cells on the retina that change light energy into an electrical impulse that passes along the optic nerve to the brain. (See where the retina is by jumping to the diagram of the human eye). Human beings usually have three types of cones - animals sometimes have more or less than three!

Visible light is made up of photons of electromagnetic energy. Photons of different energy ranges stimulate the cells on the retina in different ways and our brains interpret this by calling that band of energies a 'colour'. The part of the electromagnetic spectrum that has wavelengths between 380nm and 740nm can be detected by the human eye and is therefore called 'visible'. Each band of wavelengths of light stimulate the cones in a different way and we say that there are seven distinct colours in the visible spectrum.

(Note that the 'blue' in this 'rainbow spectrum' is a light blue (it's nearer to 'cyan' than 'blue') and the indigo is nearer to the colour we class as 'blue' in colour addition - a deep royal blue colour - the differences come from the first naming of the colours in the rainbow as ROYGBIV).

Click here for an interactive page that allows you to look at this in more detail - remember each light source has its own distribution of colours.

Single cone-type stimulation is acheived by three 'pure' colours red, blue and green. These are called the PRIMARY COLOURS.

All other colours are seen as the result of two or three of the cone types being stimulated.

It is therefore possible to simulate ANY colour on a TV screen by varying the intensity of one of these colours on the screen and therefore the amount of stimulation of the cones in our eyes. That is why your colour TV screen only needs to give out three colours.

Click here to go to a Java applet that allows you to vary the intensity of the three colours individually and to make up any colour you wish!

Colour Addition

Coloured light sources overlapping stimulate cones of both of the colours that overlap - that is why we then see the secondary colours of light at the overlap - two sets are stimulated - secondary. Where there is no overlap we see a primary colour - one set of cones stimulated.

Filters

Filters only allow certain wavelengths of light through them, they absorb the others.

A red filter will only allow red light to pass through it - it will absorb blue and green.
A magenta filter will allow the blue and red light through but will absorb the green.

So if we pass white light through a yellow and then a cyan filter what colour will we see passing through the combination of filters?

So if we pass white light through a magenta and then a green filter what colour will we see passing through the combination of filters?

Click here for filters used with prisms

Colour Subtraction

When light shines onto an object some of it is absorbed and some of it is scattered back into the eye of the observer.

White objects send all of the colours of the spectrum back into your eye - all three cones in your eye are stimulated equally so the object is perceived as white.

Red objects only send red light into your eye. Therefore only the red cones are stimulated and you say the object 'is red'. The red object absorbs all of the other colours.

Yellow objects scatter both red and green light. Therefore they stimulate the red and green cones of the eye and the brain recognizes the object as being yellow. The blue light in absorbed by the yellow object.

Using this information we can work out what objects look like under different lighting.

Pigments

The primary coloured objects can only scatter their own colour of light. If it is not there to be scattered they are seen as black. So if the light shining onto them does contain that colour they will be seen as it if it does not contain it they appear black. Therefore red materials always look red or black (no other colour!). If illuminated by cyan light red will look black.

The secondary coloured objects can scatter any combination of the two colours that make them up - therefore under some light they look as if they are of a primary colour. Only when illuminated by light of the colour they totally absorb do they look black. So, cyan objects look black when seen red light.

Pigments appear the colors they are because they selectively reflect and absorb certain wavelengths of light. White light stimulates each of the three colour cones on the retina. When this light encounters a pigment, some wavelengths are absorbed by the chemical bonds and substituents of the pigment, and others are reflected or scattered into our eyes. This creates the appearance of a colour for the pigment.

Blue paint pigment (usually nearer to cyan than the deep pure blue) reflects light that stimulates the blue and green cones and absorbs most of the light that would stimulate the red.

Yellow paint pigment reflects light that stimulates the red and green cones and absorbs most of the light that would stimulate the blue.

The eye therefore gets a double dose of stimulation for the green cones and only a single dose for the other two. The brain therefore registers the mixture of pigments as green.

When you use a colour computer printer the ink colours are yellow, cyan and magenta (and black). If you print a green object the tiny dots on the page will be of cyan and yellow. Both of these dots viewed in white light stimulate the green cones in your eyes but only the magenta ones stimulate the red and only the cyan ones stimulate the blue. You therefore get double stimulation of the green cones and see the object as green on the page of print. Look at the red object - do you understand why the dots are magenta and yellow?

See here for how cameras work