Light, radio, microwaves, and sound are all called "waves." But they travel through different things at different speeds. Light and radio are electromagnetic; sound is mechanical. Here's the full comparison.
Two fundamental wave types
Mechanical waves: require a medium (something to vibrate). Sound, water waves, earthquake waves.
Electromagnetic waves: oscillating electric and magnetic fields. Travel through vacuum. Light, radio, X-rays, gamma rays — all the same kind of wave at different frequencies.
Speeds compared
| Wave | Speed in vacuum | Speed in air |
|---|---|---|
| Light, radio, microwaves | 299,792,458 m/s | ~99.97% of vacuum speed |
| Sound | 0 (can't travel) | 343 m/s at 20°C |
Light is about 874,000× faster than sound in air. This is why you see lightning before you hear thunder.
The electromagnetic spectrum
All electromagnetic waves travel at the speed of light. They differ only by frequency (and wavelength, which is inversely proportional):
| Type | Frequency (Hz) | Wavelength |
|---|---|---|
| AM radio | 5×10⁵–1.7×10⁶ | 176-600 m |
| FM radio | 8.8×10⁷–1.1×10⁸ | 2.7-3.4 m |
| TV (UHF) | 3×10⁸–3×10⁹ | 10 cm – 1 m |
| Wi-Fi (2.4 GHz) | 2.4×10⁹ | 12.5 cm |
| Wi-Fi (5 GHz) | 5×10⁹ | 6 cm |
| 5G (mid-band) | 2.5×10⁹ | 12 cm |
| Microwaves | 3×10⁹–3×10¹¹ | 1 mm – 10 cm |
| Infrared | 3×10¹¹–4×10¹⁴ | 700 nm – 1 mm |
| Visible light | 4×10¹⁴–8×10¹⁴ | 380-700 nm |
| Ultraviolet | 8×10¹⁴–3×10¹⁶ | 10-380 nm |
| X-rays | 3×10¹⁶–3×10¹⁹ | 0.01-10 nm |
| Gamma rays | >3×10¹⁹ | <0.01 nm |
The full spectrum spans 14+ orders of magnitude in frequency.
Sound's much smaller spectrum
Audible sound: 20 Hz to 20,000 Hz (20 kHz). That's only 3 orders of magnitude — far less variety than electromagnetic waves.
- Below 20 Hz: infrasound (whales, earthquakes, sometimes felt as vibration).
- Above 20 kHz: ultrasound (medical imaging, dog whistles, bat echolocation).
Different animals hear different ranges. Dogs to 45 kHz. Bats to 200 kHz. Elephants down to 5 Hz.
What each wave can do
Light/visible spectrum: we see by it. The most useful frequency range for biological organisms.
Radio: long wavelengths penetrate buildings, weather, distances. AM goes around obstacles; FM is line-of-sight.
Microwaves: heat water and food. Used by Wi-Fi, cell phones, satellite communication.
Infrared: heat radiation. Night vision uses it.
Ultraviolet: sunburn, vitamin D production, sterilization.
X-rays: medical imaging — different absorption by bone vs soft tissue.
Gamma rays: nuclear medicine, sterilization, astronomy of energetic events.
Sound: communication, audio entertainment, sonar.
How sound and light differ
Speed: light is ~10⁶× faster than sound.
Vacuum: light works; sound doesn't.
Frequency range: light spans 4-8×10¹⁴ Hz (visible). Sound spans 20-20,000 Hz. Light frequencies are billions of times higher.
Wavelengths: visible light wavelengths are 400-700 nm. Audible sound wavelengths are 1.7 cm to 17 m.
Reflection/refraction: both reflect off surfaces. Both refract through different media (light through glass, sound through hot vs cold air).
Doppler effect: both shift frequency for moving sources. Cosmic redshift uses Doppler on light from distant galaxies.
Why we see and hear different ranges
Human eyes evolved to see the spectrum where the Sun emits the most energy: 400-700 nm. Our eyes are perfectly matched to sunlight.
Human ears evolved to hear the range where natural sounds occur: 20 Hz to 20 kHz covers speech (200-3000 Hz), music, environmental sounds.
Other species see/hear different ranges adapted to their environments. Bees see UV; bats hear ultrasound; whales hear infrasound.
Wave properties common to all
All waves share characteristics:
- Reflection: bouncing off surfaces.
- Refraction: bending when entering different medium.
- Diffraction: spreading around obstacles.
- Interference: waves combine; can amplify or cancel.
- Polarization: for transverse waves (light, water, but not sound which is longitudinal).
This unified wave physics is why understanding light helps you understand sound and vice versa.
Why microwaves heat food
Water molecules have a dipole (positive on H side, negative on O side). Microwaves at 2.45 GHz cause water dipoles to rotate billions of times per second, generating heat through molecular friction.
Food with low water content heats less efficiently in microwaves. Pure ice barely heats (water molecules are immobilized).
Why Wi-Fi uses 2.4 GHz
2.4 GHz is in the "ISM" (Industrial, Scientific, Medical) band — license-free for low-power use. Wavelength of 12.5 cm is short enough for compact antennas, long enough to penetrate walls.
5 GHz Wi-Fi has more bandwidth (faster) but shorter range and worse wall penetration.
Why X-rays see through skin
X-rays have very high energy and very short wavelengths. They penetrate soft tissue (which is mostly water) but are blocked by bone (mostly calcium phosphate).
The shadow created shows internal structures.
Higher frequency = higher energy = more penetration. Gamma rays go through almost everything.
The speed of light is universal
Special relativity: nothing with mass can reach the speed of light. The universe has a fundamental speed limit.
For practical purposes, this means radio signals to Mars take ~10-20 minutes (one way). Communication with Voyager 1 takes 22 hours each direction.
The cosmic microwave background — the oldest light we can see — is from 380,000 years after the Big Bang, 13.8 billion years ago.
Wave equations
For all waves: v = f × λ. Speed equals frequency times wavelength.
Different waves have different speeds (343 m/s for sound in air, 3×10⁸ m/s for light in vacuum) but the same fundamental relationship.
Calculate wave properties
Our wave frequency calculator handles v = f × λ for any wave. Use 343 m/s for sound in air, 3×10⁸ for light in vacuum, or any custom speed for waves in other media.