Optical illusion using shadows leaves viewers stumped – can you guess what’s behind the ‘simple yet brilliant’ image?

AN optical illusion is stumping viewers, as many struggle to guess what is behind it.

Artist Yves Lappert is behind the image, captioned on Reddit as "a simple, yet brilliant illusion."

Dozens of reddit users commented on the illusion, sharing their insights and how they were duped by the image.

"Legitimately took me a moment. Truly is a simple, but effective illusion," another user chimed in.

"I first thought it was metal work and I was trying to understand how it was held in place. So simple and striking. I love how geometry is mesmerizing, even when it’s simple repeating triangles," one Reddit user wrote.

"Angle of light and lean (against the wall must be) must be 60 degrees for the triangle to be equilateral. Timing - perfect,' wrote another.

"My 10 year old figured it out in about 10 seconds. It took me longer," another person wrote.

In reality, the illusion is a photo of four planks of wood leaning against a wall.

The planks are leaning at the same angle, one under the next.

With the angle of the sun hitting just right, the shadows cast from the wood planks create perfect-looking triangles.

Those who view the photo and immediately see the triangles are left wondering what they're actually seeing in the picture.

But some didn't immediately catch onto the illusion.

"Im confused. Its a cool image, but what is the illusion? What is it taking a second for people to 'get'?," one Reddit user wrote.

"This broke my brain," another wrote.