Refraction Of Light
Have you ever noticed if you look through a glass of water, the image behind the glass sometimes looks funny or distorted?
It’s like the glass of water is playing tricks on your eyes.
The trick is actually created by the refraction of light.
Light travels at different speed through different materials.
As a result, the light “turns” when it passes from one medium to another.
There are so many good refraction of light experiments we want to do.
Let’s start with these 2 simples ones.
- a drinking straw or a pen
- clear water
- a long clear glass
- adult supervision
- Put a straw into the glass of water vertically and watch from the side.
- Slowly tilt the straw and watch how the straw appears to be broken at the water surface.
- Raise the glass or lower your eyes to look at the straw from under the water surface. The straw appears to be bent instead of broken.
- Move to different angles and watch how the straw seems to change in shape at the water surface as you move.
Why: Experiment 1
Light bends when it passes from one medium (air) into another medium of a different density (water). This bending of light, called refraction, causes the straw to look broken1. The portion of the straw that is submerged in water also appears to be wider than the portion of the straw above the water.
Since the glass is relatively thin, we can assume not much refraction takes place there. So the optical illusions are mainly caused by the water inside.
However, even though the light bends at the water/air interface, our brain does not know that or account for that. Our brains believe that light always travels in a straight line and extend the light rays backward to a location different from the original straw. That is why the submerged part of the straw appears to be shifted in water.
When I was doing this experiment with my 4 year old daughter, she kept saying that the straw must be broken. But I kept pulling it out of the water to show her that it wasn’t broken at all. She was fascinated.
Why: Experiment 2
Light bends when it passes from water into the air. When the light comes from a location close enough to the surface (or the angle of incidence is larger than water’s critical angle) the light bends so much that instead of passing out into the air, all of it reflects back into the water as if the water surface were a mirror. This is called total internal reflection.
Diagram below: When there is total internal reflection, your brain thinks that the underwater fish is at above the water (and upside down).
This is what happened when you looked at the straw from below the water surface. The reflected straw seems to be connecting between the above-water portion and the under-water distortion. That is why the straw looked crooked.
What did you see when you moved your head to different positions? Can you think of some fun experiments to do using this refraction property of light?
- 1.Jiang W, Chen RT, Lu X. Theory of light refraction at the surface of a photonic crystal. Phys Rev B. June 2005. doi:10.1103/physrevb.71.245115