Refractive index - an amazing fish tank

 I spotted this fish tank today - look how narrow it looks from the front.

 This is how wide it actually is end on.

 I measured its width as 30cm.

Then I used the ruler from the front to measure how narrow it looked to me. In this view you can see that the 0cm mark seems to be lined up with the back of the fish tank but that the width only seems to be 16.5cm. The reason for this illusion is that light travels slower in the water and glass than it does in the air. This means that the light bends as it goes through the tank, a phenomenon called refraction. Our brains cannot cope with the fact that the light is not going straight so our brains compute where a straight ray of light would appear to have come from. The result is this optical illusion. The bending power of the water and glass is called the refractive index. It can be calculated by doing real width divided by apparent width. So 30 divided by 16.5 = 1.8.