Drop a straw into water and it looks snapped at the surface. Light bends whenever it crosses from one material into another, by an exact amount set by how much each material slows it — and that rule is Snell's law.
Watch the path kink as it crosses into the glass band and kink back as it leaves. Hit the boundary square-on and it barely turns; hit it at a slant and it bends sharply — the exact amount set by n₁sinθ₁ = n₂sinθ₂.
Snell's law is the precise rule for how much light bends as it crosses from one material into another. It links the angle going in, θ₁, to the angle coming out, θ₂, through the two refractive indices: n₁ sin θ₁ = n₂ sin θ₂. The refractive index n simply measures how much a material slows light — air is about 1.0, water 1.33, glass 1.5.
Light travels slower in a denser material. At a slanted boundary, the edge of the wavefront that enters first slows first, swinging the whole front around — like a marching band wheeling when one side hits mud. Entering a denser medium (higher n) light bends toward the normal; leaving into a thinner one, it bends away. Strike the surface head-on and there is nothing to swing, so there is no bend at all.
In The Lab a slab of glass crosses the arena. As the disc enters, its path kinks toward the normal; leaving the far side, it kinks back the other way. The steeper the entry angle, the bigger the kink — and a denser slab (higher n) bends it more. It behaves exactly like a ray of light obeying Snell's law.
Snell's law is easiest to believe when you can swing the incoming ray yourself and watch the refracted ray bend to match. Tilt the beam, change the glass, and read both angles live.
Into glass (n = 1.5) the refracted angle θ₂ is always smaller than the incidence θ₁ — the ray bends toward the straight-out normal. At θ₁ = 0 the two are equal: no bend.
A straw in a glass of water looks snapped at the surface. Light from the underwater part bends as it leaves the water into air, so your eye traces it back to the wrong spot — the straw only looks broken.
Every lens is refraction put to work. A curved piece of glass bends each ray by Snell's law, gathering light to a focus — which is how cameras, telescopes, and your own spectacles sharpen an image.
A swimming pool always looks shallower than it is. Light from the bottom bends away from the normal as it exits the water, lifting the apparent floor — which is why a fish, too, sits deeper than it appears.
Snell's law is the exact arithmetic behind a bent straw, a focusing lens, and a sparkling diamond. This FAQ runs from the everyday surprise to the formula, the speed of light, and the boundary where refraction gives way to total internal reflection.
Snell's law is the rule that tells you exactly how much light bends when it crosses from one material into another. The bigger the difference in how the two materials slow light, the sharper the bend. It connects the angle of the incoming ray and the angle of the bent ray through the materials' refractive indices, so the same simple equation works for water, glass, diamond, or any clear material.
Snell's law is written n₁ sin θ₁ = n₂ sin θ₂. Here n₁ and n₂ are the refractive indices of the first and second materials, θ₁ is the angle of incidence (measured from the normal), and θ₂ is the angle of refraction. Because the indices set how much each material slows light, the equation pins down the exact bend for any pair of materials and any incoming angle.
Light slows down inside a denser material. When a slanted beam reaches the surface, one edge of its wavefront crosses and slows before the other, so the whole front pivots — much like a marching band wheeling when one side hits soft ground. That pivot is the bend. If the materials slowed light equally, there would be no bend at all, no matter the angle.
Entering a denser material like glass, light bends toward the normal, so the refraction angle is smaller than the incidence angle. Going the other way, from glass back into air, it bends away from the normal. The rule of thumb: heading into the slower, denser medium straightens the ray up; heading into the faster, thinner one flattens it out.
| Material | Refractive index n | Bends light |
|---|---|---|
| Vacuum | 1.000 | Not at all (fastest) |
| Air | 1.0003 | Almost none |
| Water | 1.33 | Moderately |
| Glass | 1.5 | Strongly |
| Diamond | 2.42 | Very strongly |
When light strikes a surface exactly along the normal, the angle of incidence is zero, and Snell's law gives a refraction angle of zero too. The whole wavefront crosses the boundary at the same instant, so there is no edge to slow first and nothing to pivot. The ray slows down inside the new material but keeps going perfectly straight — no bend at all.
Light from the underwater part of the straw bends as it leaves the water and enters the air, refracting away from the normal. Your eye and brain assume light travelled in a straight line, so they trace it back to the wrong place, making the submerged part appear shifted. The two halves of the straw no longer line up, and it looks snapped at the waterline even though it is perfectly straight.
Light leaving the water bends away from the normal as it enters the air above. Rays from the pool floor reach your eyes at a steeper angle than they started, so your brain traces them back to a point higher up than the true bottom. The floor looks raised and the water shallower — typically about three-quarters of its real depth. The same effect makes a fish appear closer to the surface than it is.
When light tries to leave a denser medium for a thinner one, it bends away from the normal. Past a certain incidence angle, called the critical angle, Snell's law would demand a refraction angle beyond 90°, which is impossible — so instead all the light reflects back inside. This total internal reflection is what traps light inside optical fibres and makes a cut diamond sparkle, and it has its own Kinetica article.
Yes — the same equation n₁ sin θ₁ = n₂ sin θ₂ governs both directions; you simply swap which index is n₁ and which is n₂. Light paths are reversible, so a ray that bends one way going in retraces exactly the same path coming back out. The only twist is that leaving a denser medium can hit the critical angle and reflect entirely, which has no equivalent going in.
A lens is a curved piece of glass, so different rays strike its surface at different angles and each bends by its own amount under Snell's law. A convex lens is shaped so that all those bends steer parallel rays to meet at a single focal point. This is how a magnifying glass concentrates sunlight, how a camera forms a sharp picture, and how spectacles correct the focus of your own eye.
Yes, slightly. A material's refractive index is a touch higher for blue light than for red, so blue bends a little more. Snell's law still holds for each colour separately; they just refract by different amounts. White light entering a prism therefore fans out into a spectrum — an effect called dispersion, the reason prisms and raindrops make rainbows. It has its own Kinetica article too.
The law is named after the Dutch astronomer Willebrord Snellius, who worked it out around 1621, though the French philosopher René Descartes published it soon after and the Persian scholar Ibn Sahl had described it six centuries earlier. It is one of the oldest quantitative laws of optics, and four hundred years on it still designs every camera, microscope, and pair of glasses made today.
In a vacuum light travels at about 300,000 km/s. Inside glass, with a refractive index near 1.5, it slows to roughly 200,000 km/s — about two-thirds of its vacuum speed. In water it is about three-quarters. That drop in speed is the real engine of refraction: the bigger the slowdown at a boundary, the larger the index difference and the sharper the bend that Snell's law predicts.
No — they are two different things that often happen together. Reflection bounces light off a surface, with the angle out equal to the angle in. Refraction passes light through into a new material and bends it by Snell's law. At a glass window you see both: most light refracts through, while a faint reflection bounces back, which is why you catch a ghostly image of yourself in a lit room at night.
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