How do we see Colour?

When you think of colour, you probably think of the colours of the crayons you used in kindergarten… Red Yellow Blue Red, yellow and blue are pigment colours.

A pigment is a chemical that interacts with light, absorbing certain wavelengths of light, and reflecting other wavelengths. The reflected wavelengths are the colours we see. Pigments are what give crayons and paints and inks their characteristic “colours.” Yellow Red Blue

The primary pigment colours are red, yellow and blue. These primary colours can be mixed together in different combinations to make secondary pigment colours: orange, green and purple. Orange Yellow Red Purple Green Blue

When you mix pigment colours together, they become darker and reflect less light. For this reason, pigment colours are called subtractive colours. Orange Yellow Red Purple Green Blue

The primary colours of light are red, green and blue The primary colours of light are red, green and blue. These colours are caued by different wavelengths of visible light. Red has long wavelengths Green has medium wavelengths Blue has short wavelengths Blue Green Red

Yellow The primary colours of light can be mixed together in different combinations to make secondary colours: cyan, magenta and yellow. Red Green Cyan Magenta When you mix all three primary colours together, they make white light. Blue

Yellow The colours of light are called additive colours. When you mix them together, they become brighter as more light is emitted. Red Green Cyan Magenta Blue

Time for a quiz….. Get out your plickers!

Let’s play with coloured lights

How does your eye detect colour?

or Retina remnants Retina

Cone Retina close-up Rod 3 Light 1 2 1 Epithelial Cell 4 5 Blind Spot

Find your blind spot!

The retina is made up of 2 kinds of light-sensing cells:

Rods around the outside of the retina sensitive to light and dark Retina

Cones in the centre of the retina (the fovea) sensitive to different wavelengths (colours) of light Retina

Humans have 3 types of cones: “red” cones - sensitive to long wavelengths “green” cones - sensitive to medium wavelengths “blue” cones - sensitive to short wavelengths

note: the cones themselves are not coloured!!! they are just sensitive to certain colours (wavelength) of light

Note that each type of cone can perceive a wide range of wavelengths (colours) but has a peak of sensitivity

Scientists refer to the cones as long wavelength cones medium wavelength cones short wavelength cones

Note also that “red” cones actually peak in the yellow area of the spectrum!!! What’s with that?

How does your brain know what colour you are seeing?

The brain compares the information it receives from the different types of cones, and uses that to decide what colour you are seeing

if the red cones detect a lot of light but the green and blue cones detect none the brain interprets this as red

if the red and green cones both detect equal amounts of light but the blue cones detect none the brain interprets this as yellow

if the red and green cones detect no light but the blue cones detect a lot of light the brain interprets this as blue

Afterimage

What about the rods?

if the rods detect high intensity (bright) light the brain interprets this as a light colour

if the rods detect low intensity (dim) light the brain interprets this as a dark colour

Yellow So the colour you see is a result of the way your brain interprets the combination of the wavelengths of light detected by the cones the intensities of light detected by the rods Red Green Cyan Magenta Blue

What does this quote mean? “Color vision is an illusion created by the interactions of billions of neurons in our brain. There is no color in the external world; it is created by neural programs and projected onto the outer world we see.” Peter Gouras, WebVision