This is the dress that's got the internet in a spin. But what colour is it?

This dress has got the internet in a spin - over what colour it is.

The picture was posted on Tumblr yesterday by 21-year-old student Caitlin McNeill from Scotland after her friends saw it in different shades.

In the blog post, Caitlin wrote: "Guys please help me - is this dress white and gold or blue and black? Me and my friends can't agree and we are freaking out."

And in just a few hours she had the whole internet talking about whether it was blue or black or white and gold.

It has now become the top trending thing on Twitter under the hashtag #TheDress and even celebrities are wading in on the debate.

Experts claim the reason people see the dress in different colours is all down to optical illusion, and the key is in how we see the background colour.

Those who see a dark background are more likely to see the dress as blue and black, while those who think it is white and gold are seeing a lighter background.

This is because of the way light enters our eyes and sends a message to our brains.

Optician Ali Waugh explained: "We perceive colours according to ambient light and surrounding background colours, and what colours we have been focussed on previously.

"That's why it may look one colour to you initially and then the other at a later stage. It all depends on what you have been looking at prior."