11-05-2025
What colour do YOU see? This fire truck isn't actually red - how mind-boggling optical illusion tricks your brain
From the cigar nestled in the brickwork to 'The Dress', many optical illusions have left viewers around the world baffled over the years.
But the latest illusion is arguably one of the most bizarre yet.
Dr Dean Jackson, a biologist and BBC presenter, has shared a strange illusion on TikTok, which tricks your brain into thinking a fire truck is red.
At the start of the video, Dr Jackson shows a picture of a red fire truck on a road.
He then adds a cyan filter, before asking what colour you think the fire truck is.
While your inital reaction is likely 'red', Dr Jackson explains that the fire truck is actually now grey.
'Red light cannot pass through a cyan filter, it just can't,' he explained.
'So now there is no red light in that picture, I can promise you. And yet your brain is still telling you that it's red.'
@beatonthebeeb
There is no red in this picture! #opticalillusion #mindgame
♬ original sound - Dean Jackson
The strange optical illusion occurs because our brains are confused by what our eyes see.
The back of the human eye contains two types of photoreceptor which allow us to respond to light shining in.
While 'rods' are sensitive to motion, 'cones' are sensitive to light, with each responding to a different colour.
In Dr Jackson's video, the cyan filter only lets through cyan-coloured light, meaning anything else should appear grey.
But when our brain recognises that a fire truck is usually red, it can interpret the grey light to appear this way.
'[Your brain] is overcompensating for the filter. The parts that you're being told are red are actually this colour,' Dr Jackson explains, as a grey square flashes up on screen.
To prove this is the case, Dr Jackson then moves the grey square over the top of the picture, confirming that it's the same colour as the truck.
'There's no red in that picture anymore, it's all gone,' he said.
The video has garnered huge attention on TikTok, with hundreds of amazed viewers flocking to the comments.
'That square turned red when you moved it in front of the photo,' one baffled viewer commented.
Another added: 'The block and the truck are fading grey and red, grey, red. It keeps going.'
And one wrote: 'The grey turned to a red/brown colour as soon as it was in place. stayed grey when it first went through the cyan filter but changed when in place.'
In repsonse to these comments, Dr Jackson reassured that the square was not changing colour.
'I promise you it didn't change colour,' he replied to one user.
'It's your brain doing it. The brains are remarkable organ.'
WHAT IS THE CAFÉ WALL OPTICAL ILLUSION?
The café wall optical illusion was first described by Richard Gregory, professor of neuropsychology at the University of Bristol, in 1979.
When alternating columns of dark and light tiles are placed out of line vertically, they can create the illusion that the rows of horizontal lines taper at one end.
The effect depends on the presence of a visible line of gray mortar between the tiles.
The illusion was first observed when a member of Professor Gregory's lab noticed an unusual visual effect created by the tiling pattern on the wall of a café at the bottom of St Michael's Hill in Bristol.
The café, close to the university, was tiled with alternate rows of offset black and white tiles, with visible mortar lines in between.
Diagonal lines are perceived because of the way neurons in the brain interact.
Different types of neurons react to the perception of dark and light colours, and because of the placement of the dark and light tiles, different parts of the grout lines are dimmed or brightened in the retina.
Where there is a brightness contrast across the grout line, a small scale asymmetry occurs whereby half the dark and light tiles move toward each other forming small wedges.
These little wedges are then integrated into long wedges with the brain interpreting the grout line as a sloping line.
Professor Gregory's findings surrounding the café wall illusion were first published in a 1979 edition of the journal Perception.
The café wall illusion has helped neuropsychologists study the way in which visual information is processed by the brain.
The illusion has also been used in graphic design and art applications, as well as architectural applications.
The effect is also known as the Munsterberg illusion, as it was previously reported in 1897 by Hugo Munsterberg who referred to it as the 'shifted chequerboard figure.'
It has also been called the 'illusion of kindergarten patterns', because it was often seen in the weaving of kindergarten students.
