Refraction · Snell's law (n₁sinθ₁ = n₂sinθ₂) · refractive index · critical angle · total internal reflection
Dip a straw in water and it looks snapped at the surface; aim a torch into glass and the beam kinks. Light refracts — bends — whenever it speeds up or slows down crossing a boundary. One tidy rule, Snell's law, predicts exactly how much: n₁sinθ₁ = n₂sinθ₂. The same bending paints rainbows, traps light inside optical fibres, and floats a mirage on a hot road.
This page covers refraction — the bending of light when it changes speed — together with Snell's law (n₁sinθ₁ = n₂sinθ₂), refractive index (n = c/v), the angles of incidence and refraction, and the critical angle and total internal reflection. By the end you'll be able to:
Every camera, eye, microscope, prism and fibre-optic cable runs on refraction — and "Light" is one of the most-tested chapters everywhere. We go beyond the syllabus, but we never skip it:
Searched as: refraction of light, Snell's law, refractive index, critical angle, total internal reflection, why a straw looks bent in water.
Set the angle of incidence and the second medium's refractive index, then watch the ray bend. Switch to Out of the dense medium and crank the angle until the ray can't escape — total internal reflection. 🟢 real Snell's-law engine
Set the angle and the medium to see how the light bends.
In plain terms: light travels at different speeds in different materials, and changing speed at a boundary makes it bend. Snell's law says exactly how much.
Refraction is the bending of light when it crosses from one transparent medium into another and changes speed. The refractive index n = c/v measures how much a material slows light (air ≈ 1, water 1.33, glass 1.5, diamond 2.42). Snell's law ties the angles together: n₁sinθ₁ = n₂sinθ₂, with both angles measured from the normal. Entering a slower medium the ray bends toward the normal; leaving for a faster one it bends away.
Why does a speed change bend the light? Picture the straight front of a wave reaching the glass at an angle: one edge enters and slows a moment before the other, so the whole wavefront pivots — exactly like a marching band wheeling around when the soldiers on one flank take shorter steps. The bigger the speed change (the higher the index), the sharper the swing. No change in speed — hitting the surface straight on, or entering a matching medium — and the light sails through without bending at all.
The playground above fires a ray at a boundary. The Ray tab shows the incident, refracted and reflected rays, all measured from the dashed normal; the Angle graph tab plots the angle out against the angle in. Going from glass out toward air, raise the angle and the refracted ray bends flatter and flatter until, at the critical angle, it grazes the surface — push past it and the ray vanishes: every bit of light is reflected back inside. That's total internal reflection. 🟡 maths of the picture
Exams want the method, not just the idea. Here is one fully worked, the way you'd set it out in an answer.
Light passes from air (n = 1.0) into glass (n = 1.5), striking the surface at 40° to the normal. Find the angle of refraction.
The ray bent toward the normal (40° → 25°) because it slowed entering the denser glass — exactly what the playground shows.
Exams reward the method, not just the answer. Work it out one step at a time — read the thought, predict the line, then reveal it. Switch to practice to type your own numbers and check them.
The physics is visible in the diagram; the maths usually hides. These little labs make it visible too — drag a slider and watch the angle, the cutoff and the speed answer.
Some wrong ideas about refraction are so common they deserve their own warning label. Tap a card to bust the myth.
A hair-thin glass thread carries light by total internal reflection: the ray strikes the inner wall past the critical angle and bounces entirely back in, zig-zagging along even around bends. Almost no light leaks out, so fibres ferry the internet and phone calls as pulses of light across whole oceans.
On a baking road the air just above the surface is hot and thin, with a slightly lower refractive index. Light from the sky bends gently upward as it dips into that layer, so your eye traces it back to a shimmering 'puddle' on the tarmac — a patch of refracted sky, not water.
Every lens, camera, microscope and eye works by refraction — bending light to bring it to a focus. The same effect breaks a straw at the waterline: light from the submerged part bends as it leaves the water, so your brain, assuming straight lines, sees it shifted and snapped.
Once humans learned how light bends, they built lenses, fibres, cameras, microscopes — and the modern internet.
Every transparent material is a speed bump for light. Drag from vacuum to diamond and watch the same ray bend harder and harder as the refractive index climbs — until, in diamond, light is nearly trapped inside. Higher index means slower light, and slower light means a sharper bend.
Every transparent thing is a speed bump for light — and light always bends toward the slow lane.
Physics you can hold. Each project below demonstrates the law you just met — and the measuring is what turns a demo into a science-fair winner. Pick one, build it from things at home, and graph something.
Build: stand a pencil in a glass of water and look from the side — it appears snapped at the waterline.
Measure: how the apparent break grows as you tilt your view, and as you swap water for oil (higher index).
Build: put a coin at the bottom of an opaque cup, step back until the rim just hides it, then pour in water.
Measure: the coin rises into view — refraction makes the water look shallower than it is.
Build: tilt a shallow tray of water with a mirror in it toward the sun, or use a glass prism.
Measure: the spread of colours — different colours refract by slightly different amounts, splitting white light.
Build: shine a laser pointer into a block of set jelly or agar at different angles (with an adult).
Measure: the bend angle against the incidence angle — and find where it flips to total internal reflection.
Build: submerge a small glass object in cooking oil whose index nearly matches the glass.
Measure: the object almost vanishes — with no index step, light barely bends, so you can't see the edges.
Build: punch a hole low in a bottle, shine a laser through the opposite side into the pouring stream (in the dark).
Measure: the light follows the curving water by total internal reflection — a fibre you can pour.
Light bends its way through every lens and every drop of water. Here are the questions that come up most — each answer reads on its own, lifted clean off the page.
Refraction is the bending of light as it passes from one transparent medium into another and changes speed. Light travels fastest in a vacuum and slower in materials like water or glass; crossing a boundary at an angle, the change of speed swings its direction. It's why a straw looks broken at the waterline and how lenses focus. The exact bending is set by Snell's law.
| Feature | Reflection | Refraction |
|---|---|---|
| What happens | Light bounces off the surface | Light passes through and bends |
| Cause | The surface acts like a mirror | The light changes speed |
| Angle rule | Angle out = angle in | n₁sinθ₁ = n₂sinθ₂ |
| Example | Your face in a mirror | A straw bent in water |
Snell's law links the angles either side of a boundary: n₁sinθ₁ = n₂sinθ₂, where n₁ and n₂ are the refractive indices and θ₁, θ₂ are the angles of incidence and refraction, both measured from the normal. Knowing any three of these, you can find the fourth — which is how lens and prism designers predict exactly where light will go.
The refractive index n measures how much a material slows light: n = c/v, the speed of light in vacuum divided by its speed in the material. It has no units and is at least 1 — air ≈ 1.0003, water 1.33, glass ≈ 1.5, diamond 2.42. A higher index means slower light and a sharper bend on entering.
Light bends because it changes speed. Picture a wave's straight edge reaching the glass at an angle: one side enters and slows before the other, so the whole wavefront pivots — like a marching band wheeling when one flank takes shorter steps. Entering a slower medium it bends toward the normal; leaving for a faster one it bends away. Straight on, it only slows, with no bending.
Going from a denser medium toward a rarer one, light bends away from the normal. Raise the angle and the refracted ray flattens until, at the critical angle, it grazes the surface. Beyond that angle no light escapes — it is all reflected back inside, called total internal reflection. The critical angle comes from sinθc = n₂/n₁, and it is what keeps light trapped in optical fibres.
Light from the submerged part of the straw slows as it leaves the water and bends away from the normal on the way to your eye. Your brain assumes light travelled in straight lines, so it traces the bent rays back to a shifted spot, making the underwater part look raised and offset. The straw is perfectly straight — refraction just moves where its image appears.
An optical fibre is a thin glass thread that carries light by total internal reflection. Light enters at a shallow angle and hits the inner wall beyond the critical angle, so it reflects entirely back in and zig-zags along the fibre instead of leaking out — even around bends. With almost no loss, fibres carry internet and phone signals as pulses of light across oceans.
Yes. Light travels at about 3×10⁸ m/s in vacuum but slower in any material — roughly three-quarters of that in water and two-thirds in glass. The refractive index n = c/v measures exactly how much it slows, and this slowing is the whole cause of refraction. On leaving the material, light speeds straight back up to its full vacuum speed.
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Light keeps no straight allegiance — it bends to whatever slows it, and finds its way regardless.