5.2 Waves and Wavelengths

Properties of Waves and Sensory Perception

Characteristics of Wave Forms

Every wave has a few core properties, and each one maps directly onto something you experience through your senses.

• Amplitude is the maximum displacement of a wave from its resting position. It determines the intensity of a stimulus. Higher amplitude means a stronger sensory experience: louder sounds, brighter lights.
• Wavelength is the distance between two corresponding points on adjacent waves (peak to peak, for example). It determines the quality of what you perceive. Different wavelengths of light produce different colors, and different wavelengths of sound produce different pitches.
• Frequency is the number of complete wave cycles that pass a fixed point per second, measured in Hertz (Hz). Frequency and wavelength are inversely related: as one goes up, the other goes down. Higher frequencies correspond to higher-pitched sounds and shorter-wavelength, more energetic light (like ultraviolet).

One detail that trips people up: wavelength and frequency describe the same underlying property from two different angles. A sound wave with a short wavelength is a high-frequency wave. You don't need to think of them as separate things.

Light Waves and Visual Perception

The visible light spectrum is the narrow band of the electromagnetic spectrum that human eyes can detect, ranging from about 380 to 700 nanometers (nm) in wavelength. Everything outside that range (infrared, ultraviolet, radio waves) is invisible to us, even though it's the same type of energy.

Within that range, wavelength determines color:

• Shorter wavelengths (~380–450 nm): violet and blue
• Medium wavelengths (~500–575 nm): green and yellow
• Longer wavelengths (~630–700 nm): orange and red

Brightness is a separate dimension from color. It's determined by amplitude, not wavelength. Sunlight and moonlight can contain the same wavelengths (colors), but sunlight has much higher amplitude, so it looks brighter.

Sound Waves and Auditory Experience

Sound waves work on the same principles as light waves, but they travel through a physical medium (like air or water) rather than through empty space.

• Pitch is the perceptual experience of frequency. A soprano voice produces high-frequency sound waves, while a bass guitar produces low-frequency waves. The human hearing range spans roughly 20 Hz to 20,000 Hz.
• Loudness is the perceptual experience of amplitude. A jet engine produces high-amplitude sound waves; a whisper produces low-amplitude ones.
• Timbre is what makes two instruments sound different even when they play the same note at the same loudness. It comes from the unique combination of additional frequencies (called overtones or harmonics) layered on top of the main frequency. That's why a violin and a trumpet playing middle C sound nothing alike.

Wave Interactions and Sensory Experiences

Waves don't always travel in isolation. When multiple waves overlap, a few things can happen:

• Interference occurs when waves combine. If their peaks line up, they reinforce each other (constructive interference), making a stimulus stronger. If a peak meets a trough, they cancel out (destructive interference), weakening the stimulus. Noise-canceling headphones use destructive interference to reduce sound.
• Diffraction is the bending of waves around obstacles or through openings. This is why you can hear someone talking around a corner even when you can't see them: sound waves have long enough wavelengths to bend around barriers.
• Resonance happens when an object naturally vibrates at a particular frequency and an incoming wave matches it, amplifying the sound. The body of an acoustic guitar resonates to make the strings' vibrations louder.