A pencil in water looks broken and a swimming pool looks shallower than it is. Both are the same straight line, bent once at a boundary.
What refraction is
DefinitionRefraction is the bending of light at the boundary between two different transparent materials (optical mediums), due to a change in the speed of light as it passes from one material into the other.
Every part of that sentence is doing work. It happens at the boundary, between transparent materials, and it is caused by a change in speed — not by the material being "thicker" in any everyday sense.
Optical density is not the density from Chapter 2
An optical medium has a physical property called optical density, which affects the speed at which light travels through it. The more optically dense a material is, the slower light travels through it.
This is a different property from the mass-per-volume density you met in Chapter 2. Do not use one to argue about the other.
Which way does it bend?
Light travels faster in air than in glass. That single fact settles the direction of every bend.
Crossing the boundary
Angle in the first medium—
Angle in the second—
The light…—
Set the angle to 0° and the ray goes straight through without bending — entering along the normal, there is no sideways difference for the speed change to act on.
Going from
Speed
Bends
Air → glass less dense → denser
slows down
towards the normal
Glass → air denser → less dense
speeds up
away from the normal
The cart on the grass
Roll a cart from a pavement onto a grass patch at an angle. Wheel A reaches the grass first and, because of the increased friction, slows down first. It therefore travels a shorter distance than wheel B while the cart is crossing the boundary — and that difference swings the cart into a new direction. Once both wheels are on the grass they roll straight again, just more slowly.
Coming back the other way, wheel A reaches the pavement first and speeds up first, so it travels further than B and the cart swings the opposite way.
If the wheels cross perpendicularly, both change speed at the same instant, so the cart carries straight on — exactly like a ray entering along the normal.
What refraction makes you see
A pencil or spoon in water looks bent or broken — the bending of light makes the submerged part appear shorter than it is.
Letters under a glass block appear nearer to the surface than they really are.
A swimming pool looks shallower than it is: the perceived depth is less than the actual depth, because light from the object bends away from the normal as it leaves the water, and your brain traces it back along a straight line to a shallower image.
Why that last one matters
A pool always looks shallower than it is. That is a genuine safety point, not just an optical curiosity — and it is a favourite exam context for the perceived-depth diagram.
Dispersion — splitting white light
White light is made up of a spectrum of seven colours: red, orange, yellow, green, blue, indigo and violet — ROYGBIV.
When light crosses into a new medium, each colour slows down to a different extent, so each bends by a different angle. Violet bends the most; red bends the least. The splitting of white light into its component colours is called dispersion.
White light through a prism
Red stays highest because it bends least; violet ends up lowest because it bends most. In a real experiment the spectrum is only visible on a white screen placed far from the prism.
A rainbow is the same process in the sky. After rain there are raindrops in the air; as sunlight crosses the air-water boundary into each drop, each colour slows by a different amount and the sunlight separates into the seven colours.
Two extras worth knowingNewton's colour disc — a disc painted with the seven colours. Turn it quickly and it appears white, recombining the colours.
Double rainbows — the secondary rainbow forms when sunlight is reflected twice inside each water droplet. It is fainter, harder to spot, and its colours appear in the reverse order of the primary rainbow's.