Refraction
We've heard of reflection, as in mirror-mirror on the wall.
But reflection has a close cousin called refraction. Reflection is when light bounces off a shiny surface, and refraction occurs when light goes from one material to another.
So, let's learn all about it.
All bent out of shape
We know and love light.
But understanding light is a lot harder than one might think. But for our purpose, think of light as a wave rippling along like ripples in water.
But when light ripples from one material to another, its speed can change.
How does this affect the light? Imagine each bump in the ripple of light as a line. The change in speed changes the distance between these ripples (it's called a wavelength to be technical). This is huge when understanding how light goes between different materials. To see how, view the picture below of light going from one material to another with a new, shorter spacing.
Light ripples going from one material to another – the white line being the surface. The arrow is the direction the wave move.
Do you see the problem?
The wave ripples don't match up where the two materials meet. There is a break in the wave. The second wave must go in a different direction to properly connect the two, as shown below.
Here's the bottom line – when light (or any wave) goes from one material to another, the light's direction must change.
This is refraction.
You can see refraction in the picture below. A ray of light goes from the air through a plastic block and back into the air.
A plastic block shows how a refracted, or bent, beam of light as it goes through.
Refraction also explains why a pencil in a glass of water looks broken (try it!)
And of course, it provides for some fun photography.
Out of thin air
Refraction can happen anywhere, even in space. Imagine you're an astronaut in space, and you're looking at the moon. Sometimes only part of it will be seen through the atmosphere. In this case, part of the moon will be seen properly, while the other part will have its light refracted by the air. The result is a half swished-shaped moon.
Moon as seen halfway through the atmosphere. Notice that the part seen through the air is squished, while the part seen through space looks normal.
But there are more down-to-earth examples as well. Hot air is less dense than cool air and, as light passes through, hot air can be refracted or bent.
You see this when looking over a hot surface or hot exhaust.
On a hot day when conditions are right, refraction can bend light so one sees the sky on the ground.
It's a mirage!
The eye of the beholder
Refraction is not just a novelty or something that gives us fun images or mirages. It can be handy. Consider some examples:
You're out camping and want to start a campfire but, sadly for you, the matches were left at home. Fortunately for you, you remember from your wilderness survival training that you can use a magnifying glass to focus the sun's light and start a fire.
How does this work?
The trick is in the shape of the magnifying glass lens. The lens shape refracts the light in such a way so that it all goes to one point – the focal point. Here all the energy of the light gets focused in one spot.
Later on in your camping trip, you see a Rainbow Lorikeet. Sadly, it is too far away to get a good look. You need a telescope or binoculars—the Lenses inside bend the incoming light to make the bird look closer – and easier to see.
After coming home from camping you realize that you cannot see as well as you would like. What's happening?
In the front of the eye is a lens that bends the light to produce an image at the back of the eye. It is here where the seeing happens. When we don't see properly, it's because our eyes get out of shape, and so its lens can't do its job right.
You need glasses. The lens of glasses helps to correct this problem, allowing you to see well.
Diagram of an eye that cannot focus on its own, then with a lens to correct this.
Mirror, mirror
So far, we have seen how light moves between different materials. But to see the strangest part imagine light rays leaving a light bulb and traveling through a material (water, for example) and going into something less dense (let's say air). The diagram below shows how several rays of light would go.
Some things to see:
The light bends towards the more dense material (water in this case).
Light passing directly out does not bend. However, the more the light hits the air going to one side, the more it gets bent.
The last ray barely makes it into the air. Another beam at a little more angle would not be able to make it out at all.
The is important.
There is an angle, called the "critical angle," beyond which the light cannot go out of the material.
But if it can't go out, what would happen? Logically, light can only do two possible things:
Bounce or reflect back – like a mirror.
Go through and be refracted.
Since #2 is out, all that is left is #1. The light that hits a border beyond the critical angle must be completely reflected, just like a mirror.
This is called "internal reflection."
You can see it in the picture below.
Because of internal reflection, the surface of the pool acts as a mirror.
Internal reflection shows up in many different places.
It allows a single light bulb to set a whole swimming pool aglow.
It is the reason for the sparkle, called the “fire,” in a diamond.
Laser light can be sent through glass fiber for miles for communication. It's called fiber optics and had made communication much better.
It also helps explain rainbows.
The key here is that the critical angle is different for different colors. So light comes in, but different colors of light come out in different directions.
Refraction teaches us that light does not simply move in boring straight lines, but can bend and turn. Because of this, many of the beautiful things of this world, such as rainbows, come to pass. But just as important, it is because of refraction that we can enjoy seeing them.
Great books
Opticks
Book by Sir Isaac Newton in which he describes his views on light. A bit of a heavy read.
On the web
Refraction of Light Experiment: Easy Science for Kids
This site shows different experiments suitable for children.
Refraction of light-Science Learning Hub
This site from Science Learning Hub has a lesson on refraction.
Fiber optic cables: How they work
In this video, an engineer shows how a fiber optics cable can guide a laser beam along its path. He does a really neat demonstration where you can see the beam being guided along a curved path.
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