Optical Illusions: Refraction

We see what we believe we see. We usually do not doubt our senses when something straightforward happens, but when our eyes see some extraordinary visuals, we begin questioning how our eyes actually work. Most commonly, that is done by optical illusions, and today we are going to see how one of the most interesting optical illusions works – refraction!

To demonstrate this, we don’t have to go far - all we need is a pencil and a transparent glass of water. Once we put the pencil inside the glass, the pencil will seem to bend in water.

We didn’t actually break the pencil, so what changed? Well, we changed the substance in which one part of the pencil resides. Did the water bend the pencil?

To explain refraction, we need to look at the physics of light. In air, light travels almost as fast as it does in a vacuum – about 186 thousand miles per second. In water, however, light travels at a speed of 140 thousand miles per second. The wavelength of light is lower in water, but the frequency stays the same, and that’s why light behaves a bit differently in water.

Let’s say we immerse our pencil fully into the glass of water. The effect will be lost, since there is now no difference in the atmospheres. To identify these differences, we use the refractive index, which quantifies the light bending ability. To calculate this index, we need to know the speed of light in the substance and compare it to the speed of light in vacuum.

The refractive index in a vacuum is 1, and in air it is 1.0003, which makes a very slight difference that is often ignored. We can substitute the vacuum for regular air and retain a high level of accuracy.

That difference is unnoticeable when comparing air to something with a much higher index, like glass with index of 1.33, but when it comes down to comparing cold air and hot air, the difference becomes more clear. Hot air has a refractive index closer to 1.002, and cold air has a refractive index closer to 1.003. That’s why refraction occurs in hot atmospheres, where there are lots of different air temperatures. Cold air generally goes down, and hot air goes up. Gaseous air does not stay still in one spot – it constantly goes up and down, changing its direction by bumping into other molecules. That’s why there is no clear border between cold and hot air, and that’s why we see wavy air in deserts and near beaches on a hot day.

Additional resources:

https://www.youtube.com/watch?v=ll90DP-kOCk – demonstration of another optical illusion caused by refraction.

https://www.olympus-lifescience.com/en/microscope-resource/primer/lightandcolor/speedoflight/ - more information on the speed of light.

https://www.youtube.com/watch?v=nQFhWfmKjms – Another fantastic illusion! This video talks about mirages and how they work in a short animation.