Optical Illusions from Polarisers

[P4 to S4 Science - Light & EM waves]

As an aficionado for science since young, I often wonder where the intrigue for optical illusions one might have, arose from. The fact that they betray our intuition? The fact that they often require some deeper knowledge which, once understood, provides a deep, pleasing feeling within oneself that you've learned something unique?

The video contains a particularly cool optical illusion, where a black disc is formed in the middle of the tube when viewed from the side; yet, when held up towards your face, there is no black disc in the middle at all.

The reason this is, as you might have seen from the video, is that the polarising filter only allows light in a certain direction to pass through it. You might be wondering "Then how does a polarising filter actually work?" Here's a simple illustration for you:

Did you notice how the polarising filter itself looks like it's filled with slots? That's right - If you've helped your mum with cooking foods before, you might remember her using a sieve to drain the food from the soup it's cooking within.

Botht he sieve and the polarising filter work under the same principle : Only things that fit through those gaps (soup, the light in the right direction etc.) will be allowed through, meanwhile block everything else. The polarised light, in the picture above, has passed through a filter which only allows light oriented in the up-down direction.

But the tube in the video, as you might guess by now, is using two different types of filters arranged differently. One that allows the up-down orientation, and one that allows the left-right orientation.

Therefore, when a light that has passed through one of its filters (regardless of which it passed through first) reaches the other filter, they couldn't fit through the slits on the second filter since the ones that could were already blocked by the first filter. That, in turn, means no light from that region enters our eyes, giving us the illusion that a dark disc is inside of the tube.

When you look straight down the tubing, you're looking at light that has not been polarised by either filter. Therefore, where you expect to see a black disc, you see light. It's a wrong concept to say that you're seeing things; It's more appropriate to say the light from other things bounce around and reach your eyes in order for you to see.

Your eyes are a sensor, not a laser projector like Superman's eyes are. Take good care of them, and remember to give them the rest they need. Have a good day!