Waves, Light, and Sound

Waves are one of the most useful ideas in science because they connect phenomena that seem unrelated at first glance: the ripples on a pond, the pitch of a violin, the colors in a rainbow, and the signals that carry radio and Wi-Fi. Whether a wave travels through a material medium (like air or water) or through empty space (as light does), the same core properties describe how it behaves and how we can measure it.

This article introduces the fundamentals of wave phenomena across mechanical and electromagnetic domains, with a focus on frequency, wavelength, amplitude, the electromagnetic spectrum, basic light behavior, and sound.

What a Wave Is and Why It Matters

A wave is a repeating disturbance that transfers energy from one place to another. In many cases, waves also carry information. A spoken word is a pattern of pressure changes in air. A fiber-optic cable transmits data by sending controlled pulses of light.

Waves come in two broad categories:

• Mechanical waves, which require a medium. Sound waves need air, water, or solids to travel.
• Electromagnetic waves, which do not require a medium. Light can travel through a vacuum, which is why sunlight reaches Earth.

Within mechanical waves, we often distinguish between:

• Transverse waves, where the disturbance is perpendicular to the direction of travel (like waves on a string).
• Longitudinal waves, where the disturbance is parallel to the direction of travel (like sound in air, where air compresses and expands along the direction of motion).

Core Wave Properties: Frequency, Wavelength, and Amplitude

Several key quantities describe nearly every wave, regardless of its type.

Frequency

Frequency is how many cycles of a wave pass a point each second. It is measured in hertz (Hz), where $1\text{Hz}=1$ cycle per second.

• Higher frequency means more cycles per second.
• In sound, higher frequency corresponds to higher pitch.
• In light, higher frequency corresponds to higher photon energy and shifts toward the blue and ultraviolet end of the spectrum.

Wavelength

Wavelength ($\lambda$) is the distance between repeating points on a wave, such as crest-to-crest in a transverse wave or compression-to-compression in a longitudinal wave. It is typically measured in meters.

Longer wavelengths mean the wave repeats more slowly in space. Shorter wavelengths mean the wave cycles more quickly.

Amplitude

Amplitude is the size of the disturbance. In a water wave, it relates to the height of the wave. In sound, it relates to pressure variation, which affects loudness (though perceived loudness also depends on frequency and human hearing sensitivity). In light, amplitude relates to intensity or brightness.

Importantly, amplitude does not determine wave speed. It primarily affects how much energy the wave carries.

The Wave Speed Relationship

Wave speed connects frequency and wavelength through a simple relationship:

$$v=f\lambda$$

Where:

• $v$ is wave speed
• $f$ is frequency
• $\lambda$ is wavelength

This relationship is central to both sound and light. If a wave travels faster in a medium but the source frequency stays the same, the wavelength must increase.

Light as an Electromagnetic Wave

Light is an electromagnetic wave: coupled oscillating electric and magnetic fields that propagate through space. What we call “visible light” is only a small part of a much broader range of electromagnetic radiation.

The Electromagnetic Spectrum

The electromagnetic spectrum is organized by wavelength or frequency. As frequency increases, wavelength decreases, and the energy carried per photon increases.

Broadly, the spectrum includes:

• Radio waves (long wavelengths, low frequencies)
• Microwaves
• Infrared
• Visible light
• Ultraviolet
• X-rays
• Gamma rays (short wavelengths, high frequencies)

Everyday technologies map directly onto this spectrum: radio communication uses radio waves, microwave ovens operate in the microwave region, thermal cameras detect infrared, and medical imaging can use X-rays.

How Light Behaves: Reflection and Refraction

Light interacts with matter in ways that are both predictable and practical. Two of the most important behaviors are reflection and refraction.

Reflection

Reflection occurs when light bounces off a surface. The basic rule is the law of reflection:

• The angle of incidence equals the angle of reflection (measured relative to the normal line perpendicular to the surface).

Smooth surfaces, like a calm lake or a mirror, produce specular reflection, which preserves images. Rough surfaces scatter light in many directions, producing diffuse reflection, which is why most objects are visible from many viewing angles.

Reflection is central to periscopes, mirrors in telescopes, and many optical instruments.

Refraction

Refraction is the bending of light as it passes from one medium to another, such as air to glass or air to water. It occurs because light travels at different speeds in different materials. When the speed changes, the direction generally changes too.

Refraction explains:

• Why a straw in a glass of water looks “bent”
• How lenses focus light to form images
• Why prisms spread white light into colors (different wavelengths refract by different amounts)

A practical example is eyeglasses: a lens shape is chosen to refract incoming light so it focuses correctly on the retina, compensating for near-sightedness or far-sightedness.

Optics Fundamentals: Lenses and Image Formation

Optics is the study of how light forms images. Two major components of basic optics are lenses and mirrors.

Lenses

Lenses use refraction to converge or diverge light:

• Convex (converging) lenses bend parallel rays inward toward a focal point. They are used in magnifying glasses and cameras.
• Concave (diverging) lenses spread rays outward. They are common in glasses for correcting myopia.

The focal length describes how strongly a lens bends light. Short focal length lenses bend light more strongly and can focus over a shorter distance.

Mirrors

Mirrors use reflection rather than refraction:

• Concave mirrors can focus light, similar to converging lenses.
• Convex mirrors spread light and increase the field of view, which is why they appear in vehicle side mirrors and security mirrors.

Sound Waves: Mechanical Waves in Motion

Sound is a mechanical, longitudinal wave that travels through vibrations of a material. In air, the wave consists of alternating compressions and rarefactions of air pressure.

Sound and Wave Properties

The same wave properties show up clearly in sound:

• Frequency determines pitch. A 440 Hz tone is the standard tuning “A” in music.
• Amplitude relates to intensity and perceived loudness.
• Wavelength depends on the speed of sound in the medium and the frequency, via $v=f\lambda$.

Speed of Sound and the Role of the Medium

Unlike light in a vacuum, the speed of sound depends strongly on the medium:

• Sound generally travels faster in solids than in liquids, and faster in liquids than in gases.
• Temperature and material properties affect sound speed in air.

This is why you can sometimes hear a train through the rails before you hear it through the air. The solid metal transmits the vibrations efficiently.

Connecting the Concepts: One Framework, Many Phenomena

A powerful takeaway is that waves, light, and sound share a common framework. Frequency, wavelength, and amplitude are not just vocabulary words. They are tools for prediction.

• If you know the frequency of a sound and the speed of sound in air, you can find its wavelength.
• If you shift light into a medium where its speed changes, its wavelength changes while its frequency remains tied to the source.
• If you increase a wave’s amplitude, you increase its energy transfer, whether it is a louder sound or a brighter beam.

From musical instruments to medical imaging and from eyeglasses to wireless communication, wave behavior underlies the technologies and experiences that shape daily life. Understanding the fundamentals of waves makes light and sound less mysterious, and it offers a unified way to interpret the physical world.