15.1 The Electromagnetic Spectrum

Properties and Characteristics of Electromagnetic Waves

Electromagnetic waves carry energy through oscillating electric and magnetic fields, and they don't need a medium to travel. Understanding their properties is the foundation for everything else in this unit, from how light bends to why the sky is blue.

more resources to help you study

practice questions

Relationship of electromagnetic wave properties

Every electromagnetic wave can be described by three linked properties:

• Frequency ($f$): the number of wave cycles passing a fixed point per second, measured in hertz (Hz)
• Wavelength ($\lambda$): the distance between corresponding points on adjacent waves, measured in meters (or nanometers for visible light)
• Energy ($E$): the energy carried by each photon of the wave, measured in joules (J) or electron-volts (eV)

Two equations tie these together:

$c = \lambda f$

where $c$ is the speed of light in a vacuum ($3.00 \times 10^8$ m/s).

$E = hf$

where $h$ is Planck's constant ($6.626 \times 10^{-34}$ J·s).

The key takeaway: frequency and wavelength are inversely related (when one goes up, the other goes down), while frequency and energy are directly related (when one goes up, so does the other). That's why gamma rays have extremely short wavelengths and extremely high energy, while radio waves have long wavelengths and low energy.

Electromagnetic spectrum

The electromagnetic spectrum organizes all electromagnetic waves by frequency (or equivalently, by wavelength). From lowest frequency to highest:

Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays

A few things to keep in mind:

• All of these travel at the same speed in a vacuum ($c$). What distinguishes them is frequency and wavelength.
• Visible light is only a tiny sliver of the full spectrum, roughly 380 nm (violet) to 700 nm (red).
• The boundaries between regions aren't sharp cutoffs. They blend into each other.
• Maxwell's equations describe the behavior of electromagnetic waves across the entire spectrum, predicting that changing electric fields produce magnetic fields and vice versa.

Production and Applications of Electromagnetic Radiation

Each region of the spectrum is produced by different physical processes and has distinct practical uses.

• Radio waves are produced by oscillating electrical currents in antennas. They're used for AM/FM broadcasting, cell phone communication, Wi-Fi, and radar. Wavelengths range from about 1 mm to hundreds of kilometers.
• Microwaves are produced by specialized vacuum tubes (magnetrons, klystrons) or solid-state devices. Applications include microwave ovens (which excite water molecules at ~2.45 GHz), satellite communication, and radar systems.
• Infrared (IR) radiation is emitted by the vibration and rotation of atoms and molecules in warm objects. Every object above absolute zero emits some IR. Uses include thermal imaging (night vision cameras), remote controls, and heat lamps.
• Visible light is produced by electronic transitions in atoms and molecules. When an electron drops from a higher energy level to a lower one, it emits a photon in the visible range. Applications include illumination, photography, and fiber optic communication.
• Ultraviolet (UV) radiation is also produced by electronic transitions, as well as by specialized discharge lamps. It's used for sterilization (UV-C destroys bacterial DNA), fluorescence effects (black lights), and triggers vitamin D synthesis in skin.
• X-rays are produced when high-energy electrons are rapidly decelerated (called bremsstrahlung, or "braking radiation") or by inner-shell electronic transitions in heavy atoms. They're used in medical radiography, airport security scanners, and X-ray crystallography for determining molecular structures.
• Gamma rays are produced by nuclear processes: radioactive decay, nuclear reactions, and high-energy astrophysical events like supernovae. Applications include radiation therapy for cancer, food irradiation for preservation, and gamma-ray astronomy.

Interaction of Electromagnetic Waves with Matter

When electromagnetic waves encounter matter, several things can happen depending on the material's properties and the wavelength of the radiation.

Absorption

Matter absorbs electromagnetic radiation and converts its energy into other forms, most commonly heat or chemical energy. Which wavelengths get absorbed depends on the material. For example, glass absorbs UV but transmits visible light.

Biological effects are wavelength-dependent:

1. UV radiation is energetic enough to damage DNA molecules, which can lead to mutations and skin cancer.
2. Infrared radiation is readily absorbed by water molecules in body tissues, causing heating.

Reflection

Electromagnetic waves bounce off surfaces. The law of reflection states that the angle of reflection equals the angle of incidence, measured from the normal (perpendicular) to the surface. How much reflection occurs depends on the material and the wavelength. Mirrors reflect visible light efficiently; sunglasses and sunscreen use reflective or absorptive coatings to protect against UV radiation.

Transmission

Electromagnetic waves can pass through materials that don't absorb or reflect them strongly. Whether a material transmits a given wavelength depends on its molecular structure.

1. Visible light transmits through the eye's cornea and lens, which is what makes vision possible.
2. X-rays pass through soft tissue but are absorbed by dense materials like bone and metal. That contrast is what makes X-ray imaging useful.

Scattering

When electromagnetic waves encounter particles or irregularities in a medium, they scatter in various directions. The amount and pattern of scattering depend on the particle size relative to the wavelength.

Rayleigh scattering occurs when particles are much smaller than the wavelength. It scatters shorter wavelengths (blue/violet) much more strongly than longer ones (red). This is why the sky appears blue: sunlight scatters off tiny air molecules, and the blue component scatters most toward your eyes.

Refraction

Refraction is the bending of electromagnetic waves as they pass from one medium to another (for example, from air into water). The wave changes speed at the boundary, which causes the direction change. This produces phenomena like mirages, the apparent bending of a straw in a glass of water, and the separation of white light into a rainbow by a prism.

Diffraction

Diffraction is the bending of waves around obstacles or through openings. It's most noticeable when the obstacle or opening is comparable in size to the wavelength. Note that the original guide's example of hearing sound around corners is actually a sound wave phenomenon. For electromagnetic waves, diffraction explains patterns like the colored fringes seen when light passes through a narrow slit, and it's the principle behind diffraction gratings used in spectroscopy.

Advanced electromagnetic wave phenomena

• Polarization: Electromagnetic waves oscillate in all orientations perpendicular to the direction of travel. Polarization restricts the oscillation to a single plane. Polarizing filters (like those in certain sunglasses) block all orientations except one, reducing glare.
• Spectroscopy: By analyzing which wavelengths a substance absorbs or emits, scientists can determine its chemical composition and molecular structure. This technique is used in everything from identifying elements in distant stars to detecting pollutants in water.