Waves, Sound & Light
Waves transfer energy without transferring matter. Sound and light are both waves, but they travel very differently.
Wave Properties
Wavelength (λ): distance between peaks.
Frequency (f): waves per second (Hz).
Amplitude: height of wave (related to energy).
Wave speed: v = fλ
Transverse vs Longitudinal
Transverse: vibration perpendicular to wave direction (light, water waves).
Longitudinal: vibration parallel to wave direction (sound — compressions and rarefactions).
Sound
Sound needs a medium (can't travel in vacuum). Speed in air ≈ 343 m/s. Higher frequency = higher pitch. Higher amplitude = louder. Humans hear 20 Hz–20,000 Hz.
The Electromagnetic Spectrum
EM waves need no medium. All travel at speed of light (3×10⁸ m/s). From lowest to highest frequency:
Radio → Microwave → Infrared → Visible light → UV → X-rays → Gamma rays
Reflection & Refraction
Reflection: wave bounces off surface (angle in = angle out).
Refraction: wave bends when entering new medium (why straws look bent in water).
Using the Wave Equation
The relationship v = fλ ties the three main wave properties together. Know any two and you can find the third.
Find the wavelength of a 440 Hz sound in air
- Sound travels about 343 m/s in air, and
f = 440 Hz. - Rearrange:
λ = v / f λ = 343 / 440 ≈ 0.78 m
That is concert-pitch A, and its wavelength is roughly the length of your arm.
Why Sound Speed Changes
Sound is a vibration passed between particles, so it moves faster when particles are closer together and can pass the vibration along quickly.
- In air: about 343 m/s.
- In water: about 1,480 m/s — more than four times faster.
- In steel: about 5,000 m/s.
- In a vacuum: no sound at all, because there are no particles to vibrate.
Light behaves in the opposite way. It travels fastest through a vacuum and slows down in denser materials, which is exactly what causes refraction.
The Doppler Effect
Why a passing siren changes pitch
As an ambulance approaches, each sound wave is emitted a little closer to you than the last. The waves bunch up, the wavelength shortens, and the pitch rises. Once it passes, the waves stretch out and the pitch drops.
The siren itself never changes. Only the motion between you and the source does.
Astronomers use the same effect with light: galaxies moving away from us have their light shifted toward red, which is how we know the universe is expanding.
Colour and the Visible Spectrum
Visible light is a narrow band of the electromagnetic spectrum, running from red at the lowest frequency to violet at the highest.
- Red has the longest wavelength and the least energy of visible light.
- Violet has the shortest wavelength and the most energy.
- A prism separates white light because each colour refracts by a slightly different amount.
- An object looks red because it absorbs other colours and reflects red toward your eye.
Common Mistakes
- Thinking waves carry matter along. Waves transfer energy; a floating cork bobs in place rather than travelling with the wave.
- Mixing up amplitude and frequency. Amplitude controls loudness and brightness; frequency controls pitch and colour.
- Assuming light needs a medium. Electromagnetic waves cross empty space, which is how sunlight reaches Earth.
- Believing all waves slow down in denser materials. Sound speeds up; light slows down.
Summary
- Waves move energy, not matter, and obey
v = fλ. - Sound is longitudinal and needs a medium; light is transverse and does not.
- Frequency sets pitch and colour; amplitude sets loudness and brightness.
- Reflection bounces a wave; refraction bends it as its speed changes.
- The Doppler effect shifts observed frequency when source and observer move relative to each other.
FAQ
Why is there no sound in space? Sound needs a medium to travel. Space is nearly a vacuum.
What color has the highest energy in visible light? Violet — highest frequency.
Why does a straw look bent in a glass of water? Light slows as it enters water and changes direction at the surface. Your eye assumes light travelled in a straight line, so the submerged part appears displaced.
What is an echo? Sound reflecting off a hard surface and returning to you. If the reflection arrives more than about a tenth of a second later, your ear hears it as a separate sound.
Are radio waves and light really the same thing? Yes, both are electromagnetic waves travelling at the same speed. They differ only in frequency and wavelength.
Why can we not see ultraviolet or infrared? Our eyes evolved receptors tuned to the narrow band the Sun emits most strongly. Some animals, such as bees, do see into the ultraviolet.
Quick Quiz
Test what you just learned. Choose the best answer for each question.