How Can a FLAT Picture Look 3D?

The picture on the screen is flat.

There is no real depth. Nothing is actually sticking out towards you.

Yet when you put on the red and cyan 3D glasses, parts of the picture can suddenly seem to move forwards or backwards.

So how is your brain being fooled?

Each eye is being shown a slightly different picture.

Why Do Your Eyes See Different Images?

Your eyes are a few centimetres apart.

That means your left eye and right eye see the world from slightly different positions.

Hold one finger in front of your face and close one eye, then the other.

Your finger seems to jump sideways because each eye is viewing it from a different angle.

Your brain normally combines these two views to help judge depth and distance.

Scientists call this binocular vision.

What Do the Red and Cyan Lenses Do?

A 3D image contains two slightly shifted pictures layered together.

One is coloured mainly red and the other cyan.

The coloured lenses act like filters.

The red lens allows one part of the image to reach one eye while blocking much of the other. The cyan lens does the opposite.

This means each eye receives a slightly different version of the same scene.

Your brain then tries to combine them into one picture.

Scientists Say…

The small difference between the images seen by each eye is called binocular disparity.

Your brain uses this difference as a clue to depth.

If the two images are shifted in one direction, the object may appear to sit in front of the screen.

Shift them differently, and the object can appear to sit behind the screen.

Nothing has actually moved.

Your brain has interpreted the two flat images as a three-dimensional scene.

Why Is There a 3D Sweet-Spot?

The effect doesn't always look equally strong.

If you move too close, too far away or look from a strange angle, your eyes may have more difficulty combining the two images.

At a certain viewing position, the two pictures line up in a way that your brain can combine more easily.

That is your 3D sweet-spot.

Different images — and different people — may have slightly different sweet-spots.

What Happens If You Swap the Lenses?

Turn the glasses around so the red and cyan filters swap eyes.

Now each eye receives the opposite image.

The depth can become confusing, weaker or even appear reversed.

Something that looked like it was sticking out may suddenly seem to sink backwards.

That is strong evidence that the 3D effect depends on which image reaches which eye.

Where Is This Used in the Real World?

Modern 3D movies use more advanced systems, but the basic idea is similar — deliver different images to each eye and let the brain create the depth.

Virtual reality headsets also show each eye a slightly different view to create the feeling that you are standing inside a 3D world.

Scientists and engineers use stereoscopic imaging in areas such as medicine, robotics, mapping and space exploration.

Curiosity Spark

Can you find the distance where the 3D effect looks strongest?

Measure how far your eyes are from the image and then move closer or farther away.

Does everyone find the same sweet-spot?

Then try changing your viewing angle or swapping the lenses.

Change one thing at a time, collect your evidence and see just how easily you can trick your brain into seeing depth.