Take a look at the photo above. See anything unusual? Peer closer at the two mountains in the distance, and you may see a trick of the eye: the peaks look transparent.

The shot was taken by vision scientist Thomas Carlson on a visit to the Himalayas – and he was so intrigued by the effect that he and his colleague Susan Wardle wrote a research paper for the journal i-Perception about why it happens.

“We were finishing a long day’s trek, and I was exhausted. I was walking to my room when I first saw the mountain,” he recalls. It was quite a sight. “Then I realised it was the impossibility of seeing a transparent mountain that got my attention.”

The illusion illustrates the ‘good continuation’ principle

Returning to his lab at Macquarie University in Sydney, Australia, he showed it to Wardle. “The thing that immediately struck me about it – besides being a beautiful photo – was how the illusion combines several principles of visual perception at once,” she says.

The strange sight can partly be explained by the way the atmosphere scatters the light, which makes the distant mountains seem much paler – almost the same colour as the sky. But the illusion is compounded by the fact that the two most distant peaks seem to be overlapping to produce a darker region underneath – in the same way two tinted glass sheets look darker if you place one on top of the other.

The darker patch is, in fact, a separate peak, as the video below makes clear. But the brain is biased to connect lines along the simplest possible path – the principle of “good continuation” – leading us to conjure up the illusion that we are seeing through two, overlapping glass mountains.

The good continuation principle also explains why we see two large overlapping triangles in this classic illusion, rather than a jumble of shapes and lines.

“[The mountains are] just a great example of the perceptual inferences our brain uses every day,” says Wardle. “It’s rare to capture such an example in real life, and this one relies on the particular lighting and viewing angle from which Tom was viewing the mountains, so it was a lucky find,” says Wardle.

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