You will need
- A high quality printout of this template
- A pencil
What to do
- Download this pdf and print it out in colour OR view it on a computer monitor.
- How many different shades of green can you see?
- Place a pencil directly over the line where the first two shades touch.
- Do the first two shades still look different?
- Repeat for the next two shades.
Think of your eyeball as a tiny cinema. Light shines through the pupil and the lens like a projector, casting an image on the back wall. This movie screen – or ‘retina’ – is actually made up of thousands of nerve cells capable of detecting light.
There are two kinds of cell, each named for their shape. Rod cells are quite sensitive and are good at detecting low levels of light, but can’t tell the difference between colours. Cones, on the other hand, come in three varieties – one can detect colours around the blue part of the spectrum, another detects colours in the green part of the spectrum and a third works best around the yellow part of the spectrum. With all three types of cone working together, your brain can form a good guess at the colour it is seeing.
It’s far from a perfect system. One thing your eyes do to improve the chances of seeing shapes amongst objects of the same colour is called ‘lateral inhibition’. This describes the behaviour of retinal cells stopping their neighbours from working. For example, a cone cell that detects green light will stop its neighbour from also seeing green light. The effect is like increasing the contrast on your television, making the dark greens darker and the light greens lighter.
This also means your brain can easily see the difference between colours of similar shades when they are right next to one another. However, if there is a space between them, lateral inhibition can’t work and your brain no longer receives any clues that two shades of a colour are actually different.
Optical illusions are typically described as the brain messing up or making mistakes. In fact, they say more about how the brain gets things right with a minimal amount of effort.
Considering that the average adult’s brain accounts for two per cent of their total weight, it’s surprising to know it uses up to 25 per cent of the body’s energy. Being such a demanding organ, it pays for it to take shortcuts. It often does this by identifying situations where something is usually true and then making assumptions.
This guessing can occasionally be wrong, which we see as our eyes tricking us. Yet the fact it is usually right, and happens to be so useful, is simply one of the many marvellous features of our brain.
Download this template and print it, or view it on the screen.
Cover the join between two shades.