How do we See Colour?

The world around us is a vibrant tapestry of colour, yet the experience of seeing a brilliant red rose or a calm blue sky isn't as simple as it seems. It's a fascinating and complex process involving physics, biology, and neurology. Seeing colour is essentially our brain's interpretation of different wavelengths of light.

The Role of Light: The Starting Point

Colour perception begins with light.  Light is a form of electromagnetic radiation, and the portion visible to the human eye is called the visible spectrum. This spectrum ranges from approximately 380 nanometres (nm) to 740 nm.

• Wavelengths and Colour: Different wavelengths correspond to different colours:
  

  • Shortest Wavelengths (approx. 380–450 nm) are perceived as violet and blue.
    

  • Medium Wavelengths (approx. 500–570 nm) are perceived as green.
    

  • Longest Wavelengths (approx. 620–740 nm) are perceived as red.
    

• The Object's Interaction: When light hits an object, the object's surface selectively absorbs and reflects certain wavelengths. We only see the wavelengths that are reflected back to our eyes. For example, a banana appears yellow because its surface absorbs the blue and violet wavelengths but reflects the yellow ones. If an object reflects all wavelengths, it appears white; if it absorbs all wavelengths, it appears black..

Anatomy of the Eye: Capturing the Light

The reflected light enters the eye through the pupil and is focused onto the retina at the back of the eye by the lens. The retina contains millions of specialised light-sensitive cells called photoreceptors.

There are two main types of photoreceptors, named for their shape:

1. Rods

• Function: Rods are extremely sensitive and primarily responsible for vision in low light (scotopic vision). They allow us to see in shades of grey.
  

• Colour Blindness: Rods are not involved in colour vision.

2. Cones

• Function: Cones require bright light to work and are responsible for high resolution detail and colour vision (photopic vision). They are concentrated in the fovea, the centre of the retina.

Humans typically have three types of cones, each sensitive to a different range of wavelengths. This is known as trichromatic vision:

The colour we perceive is determined by the relative strength of the signals from these three cone types. For instance, when we look at something yellow, it stimulates the L-cones and M-cones almost equally, while the S-cones are minimally stimulated.

Neural Processing: From Light to Perception

Once the photoreceptors are stimulated, the light energy is converted into electrical nerve impulses (a process called phototransduction). These signals are then transmitted along the optic nerve to the brain.

The Opponent Process Theory

Before reaching the visual cortex, the signals are processed by nerve cells in the retina and a brain structure called the lateral geniculate nucleus (LGN), according to the Opponent Process Theory.

This theory suggests that colour vision is controlled by three opposing receptor complexes:

1. Red vs. Green

2. Blue vs. Yellow

3. Black vs. White (Lightness)

For example, a cell in the visual pathway might be excited by red light and inhibited by green light. This explains why we never see "reddish green" or "bluish yellow." It also accounts for afterimages—staring at a red object for a long time fatigues the 'red' receptors, and when you look away, the unopposed 'green' signal briefly dominates, making you see a green afterimage.

Interpretation in the Visual Cortex

Finally, the complex nerve impulses travel to the primary visual cortex at the back of the brain, and then on to other areas for further processing. This is where the electrical signals are finally interpreted and translated into our conscious perception of colour, form, and motion.