Strike a wall straight on and the disc returns the way it came; strike it at a slant and it leaves at the very same angle on the other side of the normal. That mirror-clean bounce is the law of reflection — angle in equals angle out.
With the spin at zero, each rebound is a clean mirror: the disc strikes the wall and leaves at the same angle on the far side of the normal — the line drawn at right angles to the wall. The speed never changes at a bounce; only the direction flips, by the law of reflection. On the flat square wall the normal is fixed; on the circle it points along the radius, so the same equal-angle rule still decides every bounce.
Reflection is what happens when a moving thing — a disc, a ray of light, a wave of sound — meets a surface and bounces back instead of passing through. The path turns at the surface, but in a strict, predictable way: the angle of incidence coming in always equals the angle of reflection going out, and both are measured from the normal, an imaginary line at right angles to the surface. On Kinetica's table, every wall bounce is exactly this.
The rule is the law of reflection: the angle of incidence equals the angle of reflection, measured from the normal. Picture the normal as a line standing straight out of the wall; the incoming and outgoing paths sit at equal angles on either side of it. Strike the wall head-on, along the normal, and the disc comes straight back; graze it at a shallow angle and it leaves just as shallow. The wall changes the direction only — never the speed. This same law steers light in a mirror and an echo off a cliff.
Set the spin to zero and each rebound is a perfect mirror: the disc meets a wall and leaves at the matching angle, so you can read the law of reflection straight off the screen. On the square and rectangle the walls are flat, so the normal points the same way along each wall; on the circle the normal points along the radius toward the centre, yet the equal-angle rule still decides every bounce. Add spin and the rebound tilts off the clean mirror angle — a taste of the spin-phase lesson next door.
Reflection is easiest to believe when you can aim a ray at a mirror and watch the bounce. Change the incoming angle and the outgoing angle matches it every time — both measured from the normal.
However you set it, the angle of reflection always equals the angle of incidence. Slide to 0° (along the normal) and the ray returns straight back; slide toward 85° and it grazes away just as steeply.
A flat mirror is a smooth reflecting surface, so every ray obeys the law of reflection and parallel rays stay parallel. That is why you see a sharp image of yourself — the light reaches your eye at the same angles it left your face.
Shout at a cliff and the sound reflects off the rock and returns a moment later. The farther the wall, the longer the delay — which is exactly how sonar and bats use reflected sound to measure distance in the dark.
A player banking a ball off a cushion is using the law of reflection: the ball leaves the rail at the same angle it struck it. Good players plan the shot by picturing the normal to the cushion — just like the Kinetica disc.
Reflection is the equal-angle bounce that connects mirrors, echoes, and every wall in Kinetica. This FAQ travels from a disc off a wall to mirrors, sonar, optical fibres, retroreflectors, and the everyday seeing of ordinary objects — all through the law of reflection.
Reflection is what happens when a wave or a moving object meets a surface and bounces back instead of passing through or being absorbed. Light reflects off a mirror, sound reflects off a wall as an echo, and a Kinetica disc reflects off the table's edge. In every case the path changes direction at the surface in a strict, predictable way set by the law of reflection. Reflection is one of the things that can happen at a boundary; the others are being absorbed, passing straight through, or bending across it (refraction).
The law of reflection states that the angle of incidence equals the angle of reflection. In symbols, θⁱ = θᵣ. Both angles are measured from the normal, the imaginary line drawn at right angles to the surface where the ray strikes. So if a disc meets a wall at 40° from the normal, it leaves at 40° on the other side. The rule holds for light, sound, and moving objects alike, and it is what lets you predict exactly where any clean bounce will go.
The normal is the line drawn at right angles, 90°, to a surface at the exact point of contact. The angles of incidence and reflection are both measured from this normal, not from the surface itself. Measuring from the normal is what makes the law simple and universal: it gives "angle in equals angle out" for flat walls and curved ones alike. On a flat mirror the normal points straight out; on a curved surface like the circular table, the normal points along the radius, swinging around as the contact point moves.
| Property | Specular | Diffuse |
|---|---|---|
| Surface | Smooth | Rough |
| Parallel rays | Stay parallel | Scatter every way |
| Image formed? | Yes, clear | No |
| Example | Mirror, still water | Paper, brick wall |
A mirror is extremely smooth, so parallel rays of light stay parallel after they reflect, and that organised set of rays carries a clear image to your eye — specular reflection. A painted wall is rough at a microscopic scale, so each tiny patch faces a slightly different way and scatters the rays in many directions — diffuse reflection. The image is broken up, even though every single ray still obeys the law of reflection. Polishing a surface is simply smoothing it until specular reflection takes over.
No. A reflection changes only the direction of travel, not the speed. A Kinetica disc leaves a frictionless wall at the same speed it arrived; light reflecting off a mirror keeps the same speed it had in the air. The law of reflection is purely about angles. Speed only changes when a wave passes into a different material — for light that is refraction, a separate effect from the bounce described here.
With the spin set to zero, every wall bounce in Kinetica is a pure reflection. The disc arrives at some angle from the normal and leaves at the same angle on the other side, so the rebounds look like clean mirrors. On the square and rectangle the walls are flat and the normal is fixed; on the circle the normal points along the radius, but the equal-angle rule still decides each bounce. Adding spin tilts the rebound off the mirror angle — the subject of the spin-phase lesson.
A mirror is a sheet of glass backed by a thin, smooth layer of metal such as aluminium or silver. When light waves reach the dense cloud of electrons in the metal, the electrons cannot let the wave pass, so they send it straight back out — and they do so at the equal angle the law of reflection demands. The glass simply protects the metal and keeps it flat. Because the metal layer is smooth, the reflection is specular and a sharp image forms.
An echo is sound reflecting off a distant surface and returning to your ears after a delay. Because the sound travels to the surface and back, the distance is found from distance = speed × time ÷ 2, where the time is the round trip. Bats and submarines use exactly this: they emit a pulse, time how long the reflection takes to return, and calculate how far away an object is. Medical ultrasound scanners and ships' sonar work on the same reflected-sound timing.
When light inside a dense material like glass strikes the boundary with a lighter material at a shallow enough angle, it does not pass out at all — it is reflected completely back inside. This is total internal reflection. Optical fibres exploit it: a thin glass thread guides light by bouncing it down the inside surface again and again, so signals travel for kilometres with almost no loss. The same effect makes a diamond sparkle, trapping light to bounce around inside before it escapes.
A flat mirror reverses the direction pointing toward and away from it, which we read as a left-right swap; up stays up and down stays down, so the image is not turned upside down. That is why your right hand appears to be the reflection's left hand, and why text held to a mirror reads backwards. Ambulances print the word AMBULANCE mirror-reversed on the front so that, seen in a driver's rear-view mirror, it reads the right way round.
A retroreflector sends light back toward wherever it came from, whatever the angle it arrives at. It does this with corners: three mirrors at right angles, or tiny glass beads, reflect the ray two or three times so it leaves parallel to its incoming path. Road cat's-eyes, bicycle reflectors, and safety vests use this so a car's headlights bounce straight back to the driver. Astronauts even left a retroreflector on the Moon, so a laser fired from Earth returns and measures the distance precisely.
Yes — the equal-angle law of reflection is a property of waves in general, not just light. Sound reflects off a wall as an echo at equal angles, water waves bounce off a harbour wall the same way, and light reflects off a mirror. Each obeys "angle of incidence equals angle of reflection, measured from the normal." This is why one set of ideas explains mirrors, echoes, sonar, and the ripples in a bathtub, and why a Kinetica disc is a fair stand-in for them all.
Reflection is everywhere in technology. A periscope uses two angled mirrors to bend your line of sight over an obstacle. A reflecting telescope gathers faint starlight with a large curved mirror and focuses it to a point. Radar bounces radio waves off aircraft and times the reflection to find their range and speed, just as sonar does with sound underwater. Even solar furnaces use fields of mirrors to reflect sunlight onto one spot. All of these rest on the same equal-angle law.
You see almost everything around you by diffuse reflection. Light from the Sun or a lamp lands on an object's rough surface and scatters in all directions; some of those scattered rays reach your eyes, and your brain builds an image. A shiny object reflects specularly and shows a bright highlight or mirror image, while a matte object reflects diffusely and looks evenly lit from every angle. Either way, what reaches your eye is reflected light — without reflection, you would see nothing but direct light sources.
Seven question formats, the way Beyond Dictionary serves them. Every question has layered hints — a quick nudge, the reasoning, then a deeper connection — so a wrong answer opens a door, never a dead end. 32 questions across all seven formats — multiple choice, multiple-correct, fill-in-the-blank, match, sequence, read-think-connect, and write-your-own.