Light & Sound (Years 6–11) — Science

In a thunderstorm you see the lightning flash first, then wait several seconds for the crash of thunder — even though both happen at exactly the same instant. That gap is the clearest everyday proof that light and sound are very different waves.

Why this matters: Everything you see and hear reaches you as one of these two waves. Knowing how each behaves explains echoes, mirrors, why space is silent, and why you can watch fireworks long before you hear them.

Reflection: angle of incidence = angle of reflection

Two waves, two personalities

Light is a transverse wave. It is astonishingly fast (about 300 million metres per second) and it can travel through a vacuum — empty space with no particles. That is why sunlight reaches us across empty space.
Sound is a longitudinal wave made by vibrations — a drum skin, a guitar string, your vocal cords. It is far slower (about 340 m/s in air) and it needs a medium: particles to pass the vibration along. In a vacuum there is nothing to vibrate, so sound cannot travel.

Analogy: sound is like a rumour whispered down a line of people — you need people (particles) standing close enough to pass it on. Take the people away and the rumour dies instantly. Light needs no line at all; it carries itself.

In a vacuum light still escapes, but sound has no particles to travel through.

You see lightning first because light is far faster than sound — the delay is the gap to thunder.

Property	Light	Sound
Type of wave	Transverse	Longitudinal
Speed	Extremely fast	Much slower
Travels through a vacuum?	✓ Yes	✗ No
Caused by	Energy from a source	Vibrations

Bouncing and bending

When light hits a smooth surface like a mirror it reflects. The law of reflection says: the angle of incidence equals the angle of reflection. Both angles are measured from an imaginary line drawn at 90 degrees to the surface, called the normal — never from the surface itself.

When light passes from one material into another (say air into glass or water) it changes speed and bends. This bending is called refraction, and it is why a straw looks broken where it enters a glass of water.

Refraction: light bends towards the normal as it slows entering the denser glass.

Worked reasoning. "Can an astronaut hear an explosion in space?" Ask: "what does sound need?" — a medium of particles to vibrate. "Does space have one?" — no, space is a vacuum. Therefore no sound travels, even though the explosion is real. Light from the explosion, needing no medium, still reaches the astronaut's eyes.

Common mix-up: measuring reflection angles from the mirror surface. Always measure from the normal (the 90-degree line). An answer of "40 degrees from the mirror" is a different number from "40 degrees from the normal" — and only the second is correct.

Memory hook: Light likes a vacuum; sound needs stuff. And reflection is fair: whatever angle goes in, the same angle comes out.