Light · Process
Refraction
A straw in a glass of water is straight and looks broken. Every part of that illusion is doing exactly what it should — including your brain, which is making the only reasonable assumption available to it.
Start here
The straw that is not broken.
A straw standing in a glass of water looks snapped in two at the surface, and the part under the water looks shorter and shifted sideways. Lift it out and it is perfectly straight. Nothing has been done to the straw at all.
What has happened between the straw and your eye?
Light leaving the submerged part speeds up as it crosses back into the air, and it changes direction as it does so. Your brain then does the only sensible thing and assumes the light came straight to you, which puts the lower half of the straw somewhere it is not. The straw is straight; the ray is the thing that bent.
Light travels fastest in a vacuum, and almost as fast in air. Send it into water, perspex or glass and it slows down — in glass to about two thirds of its speed in air. That slowing is the whole cause of what follows.
If the light arrives along the normal, straight on, it slows and carries on in the same direction. If it arrives at an angle to the normal, one side of the beam reaches the slower material before the other, and the beam swings round: it bends towards the normal on the way in. Coming out of the other side it speeds up again and bends away from the normal by the same amount, so the ray leaving a parallel-sided block travels in the original direction but shifted sideways. Bending light like this on going from one material to another is called refraction.
Your eye and brain assume light has come in a straight line, because it almost always has. Refracted light breaks that assumption, and the straw appears where the straight line would have started rather than where it is.
At the bench · a ray box and a rectangular block
Send one ray in. Watch where it goes.
Change a control to begin
A narrow ray from a ray box enters a parallel-sided block through its flat face, with the normal drawn in. Set the angle it arrives at, and set what the block is made of.
Commit first. A ray is aimed straight at the block, exactly along the normal. What does it do on entering?
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What the block is
Angle in the air
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from the normal
Angle inside the block
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from the normal
Speed of light inside
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against 300 000 000 m/s in a vacuum
What the ray does
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The figure
Why the straw looks broken
Light leaves the submerged end of the straw, speeds up as it leaves the water and bends away from the normal. Your eye and brain trace it back in a straight line, because light almost always has come in a straight line — and the straight line starts higher up and closer in than the real end. The straw is not bent. The rule your brain is using is.
Key fact
Light slows down when it enters a denser transparent material and speeds up when it leaves. Arriving along the normal it carries straight on; arriving at an angle it bends towards the normal on the way in and away from the normal on the way out. That bending is refraction, and it is why a straw in water looks broken: the eye traces the light back in a straight line it did not take.
Think again
“The straw really does bend in water.”
Lift it out and it is straight; put a ruler in and the ruler is straight when you take it out too. Nothing mechanical has happened. What changes is the path the light takes from the submerged part to your eye — it bends as it leaves the water — and your brain, which has no way of knowing that, follows the ray backwards in a straight line to a place the straw is not. Every trick of this kind is the same trick: the light bent, and the assumption that it did not is what produces the illusion.
“Light bends because water is thicker and pushes it sideways.”
Nothing pushes it. Light slows in water because of how it interacts with the material, and the bending follows from the slowing plus the angle: one edge of the beam reaches the slower material before the other, so the whole beam swings round — the same way a trolley with one stiff wheel veers. Send the ray in exactly along the normal and both edges arrive together, so it slows just as much and does not bend at all. If thickness pushed light aside, the straight-on ray would bend too.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Apply the rule
A ray of light passes from air into a glass block, arriving at 40° to the normal. Which describes what happens?
Rung 2 · The one that catches people
A ray is aimed at a glass block exactly along the normal, arriving straight on. What happens?
Rung 3 · Explain
Explain why a straw standing in a glass of water looks broken at the surface, using the words refraction, normal and straight line.
Rung 4 · Take it somewhere new
A swimming pool with a flat bottom always looks shallower than it is, and the effect is stronger when you look at the far end of the pool than when you look straight down at your feet. Explain both parts.
Key note
Light slows down on entering a denser transparent material and speeds up on leaving it. Arriving along the normal it carries straight on; arriving at an angle it bends towards the normal going in and away from the normal coming out. That change of direction is refraction. Because the eye traces light back in a straight line, refracted light makes objects appear where they are not: a straw looks broken and a pool looks shallower than it is.
Going further
Push the angle far enough going the other way — from glass or water out into air — and the bending runs out of room. Past a certain angle the ray cannot escape at all and is reflected back inside instead, which is called total internal reflection. It is what makes an optical fibre work: light fired into a thin glass thread keeps striking the inside of the wall at a steep angle and keeps bouncing back in, all the way along, even round bends. Almost all long-distance internet traffic is doing that at this moment.
Refraction also depends slightly on colour, because the different colours in white light travel at slightly different speeds in glass. Blue is slowed a little more than red and so bends a little more, and a triangular block turns that tiny difference into a spread of colours across a wall.
Before this lesson
Connects to
At GCSE this becomes
- Ray diagrams for refraction at a boundary, the refractive index, total internal reflection and the critical angle, and optical fibres.
Where to next
Ask Mr Badmus AI
Got a ray, an angle and a material, and want to know where it goes?
The bench is a teaching model. The angles are calculated and drawn to scale for a typical sample of each material with light of an average visible colour; real values shift slightly with the colour of the light and with the exact composition of the glass or perspex. Speeds are rounded. Some light is reflected at every surface as well as refracted, and the drawing leaves that out to keep one ray to follow. The block is treated as having exactly parallel faces, which is what makes the emerging ray parallel to the original. In the straw figure the angles are not to scale: the ray leaving the water is drawn much flatter than water really bends it, so that the bend and the line your brain draws back can both be seen on one small picture. The bench above is the one to measure.
Lesson content © MrBadmusAI.