Push the disc into the glass slab and its path kinks — it bends one way as it slows entering, and back again as it speeds up leaving. Light does exactly this crossing from air into water or glass. That bending as the speed changes is refraction, and it is why a straw looks broken in a glass of water.
The glass slab slows the disc while it is inside, and watch the speed readout drop as it enters and rise as it leaves. Because the speed changes at each boundary, the path kinks — toward the normal going in, away from it coming out. Light does precisely this passing from air into glass and back: the change of speed bends the ray. Cross a boundary head-on (along the normal) and there is no kink at all.
Refraction is the bending of light — or of the Lab disc — when it crosses from one material into another where it travels at a different speed. The path changes direction sharply at the boundary, then runs straight again inside the new material. Light slows in water, glass, or diamond and speeds back up in air, and that change of speed is what bends it. Refraction is why a straw looks broken at the waterline and why lenses can form an image.
At the boundary the part of the beam that enters first changes speed first, swinging the whole beam to a new angle. Crossing into a denser, slower medium, light bends toward the normal; crossing back into a faster one, it bends away. Exactly how much it bends is set by Snell's law and the materials' refractive indices — the higher the index, the slower the light and the sharper the bend. Strike the boundary along the normal and there is no bend at all.
The Lab places a glass slab across the disc's path and makes the disc travel slower while it is inside. Where the disc crosses the boundary its direction kinks — toward the normal entering the slower glass, away from it leaving — a hands-on model of light refracting. The steeper the disc meets the surface, the more its path bends; cross straight on, along the normal, and it sails through unbent. It is Snell's law you can watch happen.
Refraction is easiest to see when you can aim a ray at a boundary and watch it bend. Change the angle, weigh up Snell's law, and find the point where light can no longer escape at all.
Entering the denser glass the ray always bends toward the normal, so the angle inside is smaller than the angle outside. Slide to 0° (along the normal) and the ray passes straight through, unbent.
A straight straw looks broken at the waterline because light from the submerged part bends as it leaves the water, reaching your eye from a shifted direction. The straw is perfectly straight — refraction at the surface is playing the trick.
Each raindrop acts like a tiny prism: sunlight refracts entering, splits into colours by dispersion, reflects off the back, and refracts again leaving. Millions of drops together paint the arc, always opposite the Sun.
A lens is curved glass that refracts light to a focus, forming an enlarged or sharpened image. Magnifiers, cameras, telescopes, and the lens in your own eye all bend light by refraction to let you see.
Refraction is the bending of light that shapes how we see through water, glass, and air. This FAQ travels from the disc kinking through the slab to bent straws, rainbows, lenses, optical fibres, mirages, and the eye itself.
Refraction is the bending of light when it crosses from one material into another where it travels at a different speed. At the boundary the ray changes direction sharply, then carries on straight inside the new material. Light slows in water, glass, or diamond and speeds back up in air, and that change of speed is what bends it. Refraction explains a whole family of everyday sights: a straw that looks broken at the waterline, a pool that looks shallower than it is, rainbows, and the way lenses form images.
Light travels at different speeds in different materials — fastest in a vacuum, slower in glass or water. When a slanted beam reaches a boundary, the edge of the beam that crosses first changes speed first, while the rest is still in the old material. That mismatch swings the whole beam to a new angle, like a marching band wheeling when one side slows. So it is the change of speed across the boundary, not any push or pull, that bends the light.
Snell's law is the rule that gives exactly how much light bends at a boundary. It states n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are the refractive indices of the two materials and θ₁ and θ₂ are the angles of incidence and refraction, both measured from the normal. Knowing three of the four quantities, you can find the fourth. Every lens, prism, camera, and pair of glasses is designed using this single equation.
The refractive index of a material is how much it slows light, defined as the speed of light in a vacuum divided by its speed in the material. A vacuum is exactly 1, air is just above 1, water is about 1.33, ordinary glass about 1.5, and diamond about 2.4. The higher the index, the slower the light travels and the more sharply it bends on entering. The refractive index is the single number that, through Snell's law, predicts the bending at any boundary.
| Crossing into… | Speed | Bends |
|---|---|---|
| A denser medium (air → glass) | Slows down | Toward the normal |
| A faster medium (glass → air) | Speeds up | Away from the normal |
| Along the normal (head-on) | Changes | Not at all |
The straw is perfectly straight; only the light is bent. Light from the underwater part of the straw refracts as it leaves the water and enters the air, arriving at your eye from a shifted direction. Your brain assumes light always travels in straight lines, so it places the submerged part where the bent rays seem to point — displaced from the part above the water. The result is an apparent break right at the waterline. The same refraction makes a pool look shallower than it really is.
Optical density describes how much a material slows light — the larger its refractive index, the more optically dense it is and the more it bends light toward the normal. This is not the same as ordinary mass density. For example, some oils are less heavy than water yet slow light more, so they are optically denser despite floating on top. When comparing two media for refraction, what matters is their refractive indices, not how heavy they feel in your hand.
When light inside a denser medium strikes the boundary with a less dense one at a shallow enough angle, it cannot refract out and is reflected entirely back inside — total internal reflection. The threshold angle, beyond which it always happens, is the critical angle, about 42° for glass to air. This effect is enormously useful: optical fibres guide light pulses for kilometres by bouncing them along the inside, and the fire of a cut diamond comes from light bouncing around inside before it escapes.
A rainbow is refraction and dispersion at work in raindrops. Sunlight refracts as it enters each drop, splits into its colours because each bends by a slightly different amount, reflects off the back of the drop, and refracts once more on leaving. Violet bends most and red least, so the colours fan out. Millions of drops each send one colour toward your eye from a particular angle, and together they paint the familiar arc, always centred opposite the Sun on the shadow of your head.
A lens is a piece of glass with curved surfaces, so light passing through different parts is refracted by different amounts. A convex (bulging) lens bends parallel rays inward to meet at a focus, forming an enlarged or sharpened image; a concave lens spreads them out. By shaping the glass, designers control exactly where light converges. Magnifying glasses, cameras, microscopes, telescopes, and spectacles all rely on this, and the lens in your own eye refracts light onto the retina so you can see.
No — refraction does not change a colour into a different colour. What it does is bend each colour by a slightly different amount, because each wavelength travels at a slightly different speed in glass. That is dispersion, and it can spread white light into a spectrum, but every colour keeps its own identity throughout. Red light stays red; it simply bends a little less than violet. So a prism reveals the colours already mixed in white light rather than manufacturing new ones.
A mirage is refraction in air whose density changes with temperature. The air just above a sun-baked road is much hotter, and therefore less optically dense, than the cooler air above it. Light from the sky travelling downward bends as it passes through these layers and curves back up into your eye, so you see a shimmering patch of sky on the road that looks like a puddle. As you approach, the angle changes and the puddle retreats. The same layered air makes stars twinkle.
The Lab lays a glass slab across the disc's path and makes the disc travel more slowly while it is inside the slab. Where the disc crosses the boundary, its direction kinks — toward the normal as it slows entering, and away from it as it speeds up leaving — just as a light ray bends crossing into and out of glass. The steeper the disc meets the surface, the more it bends; cross head-on, along the normal, and it sails straight through. It is Snell's law made visible and physical.
Both happen when light meets a boundary, but they are opposites in what the light does. In reflection, the light bounces back off the surface and stays in the same material, obeying "angle in equals angle out". In refraction, the light passes through into the new material and bends because its speed changes. Often both occur at once — a window shows your faint reflection while letting most light through — but reflection turns light back, while refraction lets it cross over and bends it on the way.
Light leaving the water from the pool floor bends away from the normal as it speeds up entering the air, so it reaches your eye from a steeper direction than it set off. Your brain traces those bent rays back in straight lines, placing the floor higher than it truly is, so the pool looks shallower. The deeper you look and the more slanted your view, the bigger the effect. It is why a submerged object, and a fish, always appear closer to the surface than they are.
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