The debate, which started after a photo of the dress was posted on Tumblr, has raised some obvious scientific questions about why people are seeing the dress in different colours. Experts seem to have a few different answers. But it is all related to the way our eyes and brains work.
So here it goes:
A layer of tissue at the back of the eye, called a retina, contains cells called photoreceptors.
The photoreceptors convert light rays into nerve signals, which are then processed by nerve cells in the inner retina, sent to the brain, and translated as images.
The two types of photoreceptor cells are known as rods and cones. Rods are responsible for peripheral and night vision. They detect brightness and shades of gray. Cones are responsible for day vision and colour perception.
Humans have a low concentration of rod receptors and a high concentration of cone receptors, which is why we can't see as well at night but can detect colours better, than say, cats.
We have three types of cones, each tuned to pick up green, red, or blue wavelengths of light. When light hits our eyes, the receptors turn these colours into electrical signals that are sent to the brain. Our brains determine the colour that we see by blending the signals that each receptor senses — like how a TV screen made of millions of different-coloured pixels makes an image.
In person, the dress is c learly blue and black. The lighting of the image, which has a bluish tint, appears to be what is throwing people's brains off. It makes the blue part look white and black part look gold.
Cedar Riener, associate professor of psychology at Randolph-Macon College, explained to Virginia Hughes of BuzzFeed News that the differences in colour perception are probably related to how our brains are interpreting the "quantity of light that comes into our retina."
"Some people are deciding that there is a fair amount of illumination on a blue and black (or less reflective) dress," Riener says. "Other people are deciding that it is less illumination on a white/gold dress (it is in shadow, but more reflective)."
In other words, our individual sensitivity to the blue background lighting of the photo is changing how we see the object in the image.
"What's happening here is your visual system is looking at this thing, and you're trying to discount the chromatic bias of the daylight axis," Bevil Conway, a neuroscientist at Wellesley College, told Wired's Adam Rogers.
"People either discount the blue side, in which case they end up seeing white and gold, or discount the gold side, in which case they end up with blue and black," she added.
Andrew Stockman, a professor of investigative eye research at University College in London, tried to recreate the "dress effect" in the below diagram provided to Business Insider UK. The blue bars are the same at the top, bottom, and middle but appear to change colour (look darker) as your eyes move down the figure.
His analysis of what is going on is below:
If you look at the bottom part of the figure, the overall appearance of the graded background (orange to brown to black bars) looks darker than when you look at the upper part of the figure.
When you look directly at any part of the figure you can resolve the coloured (orange to brown-blue) bars better than bars further away from where you are looking (because your ability to see fine chromatic detail drops quickly away from the centre of vision). The visual system "fills in" the colour of the background (assumed to be behind the blue bars in this case) from where you are looking across the whole background. When you look directly at the upper part of the figure, you can resolve the coloured bars as orange-blue so the visual system tends to fill in the background as more orange. When you look directly at the lower part of the figure it cannot resolve the orange-blue coloured bars at the top very well, but can resolve the brown/black blue bars at the bottom, so the darker brown/black colour at the bottom tends to fill in.