Light and Sound Waves (Intro)

Look around the room. You can SEE this page because of light, and you can HEAR a clock tick or a friend speak because of sound. Both light and sound carry information from one place to another, and they both do it in the same basic way: as WAVES. A wave is a regular pattern that moves through space and carries energy with it. Once you know the parts of a wave, you can describe almost anything from a ripple in a pond to a beam of sunlight.

What is a Wave?

Drop a pebble in a still puddle and watch the ripples spread out. The water itself does not travel from the splash to the edge of the puddle — only the SHAPE of the wave moves. The water just bobs up and down in place. That is the big idea of a wave: a pattern that travels and carries energy, while the stuff it passes through mostly stays put. Light and sound work the same way.

The Four Parts of a Wave

1. Crest — the high point at the top of a wave (the peak).
2. Trough — the low point at the bottom of a wave (the dip).
3. Wavelength — the distance from one crest to the next crest. Long wavelengths look stretched out; short wavelengths look squashed together.
4. Amplitude — the height of the wave from rest to crest. Tall waves carry more energy; short waves carry less.

There is one more property worth knowing: FREQUENCY. Frequency is how many full waves pass a point each second. A high frequency means lots of short waves zipping by every second. A low frequency means fewer, longer waves. Frequency, wavelength, and speed are all linked: speed = wavelength x frequency.

Light Waves — Fast and Need No Medium

Light is one kind of wave called an electromagnetic wave. It can travel through empty space — that is why sunlight reaches us across millions of kilometers (1 km ≈ 0.62 miles) of vacuum. Light is unbelievably fast: about 300,000,000 meters per second (about 186,000 miles per second), or 300 thousand kilometers EVERY second. Light bounces off mirrors (reflection) and bends as it passes from air into water (refraction). Different wavelengths of light look like different colors: long waves are red, short waves are blue and violet.

Sound Waves — Slower and Need a Medium

Sound is also a wave, but it works differently. Sound is made by something vibrating — a guitar string, a drum skin, your vocal cords — and those vibrations push air molecules. Sound NEEDS a medium (air, water, or a solid) to travel. In empty space there is no air, so there is no sound — that is why explosions in space are silent in real life, even though movies make them loud. Sound in air travels at about 343 meters per second (about 767 miles per hour), which is much slower than light. That is why you see lightning flash BEFORE you hear the thunder, even though they happen at the same moment.

Loudness, Pitch, and Brightness

• Bigger amplitude in a sound wave -> louder sound. Bigger amplitude in a light wave -> brighter light.
• Higher frequency in a sound wave -> higher pitch (a whistle vs a bass drum).
• Higher frequency in a light wave -> bluer / more violet color. Lower frequency -> redder color.
• Wavelength and frequency are opposites: short wavelength = high frequency, long wavelength = low frequency.