14.4 Electromagnetic Waves

An electromagnetic (EM) wave is a traveling transverse wave made of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of travel (so E, B, and k are mutually perpendicular). In vacuum EM waves don’t need a medium and travel at c ≈ 3.0×10^8 m/s. We usually treat them as plane waves (flat wavefronts) for problems. Different wavelengths give the electromagnetic spectrum (radio → microwave → infrared → visible → ultraviolet → X-ray → gamma), and visible light is just the small band we call colors (red → … → violet). Polarization describes the direction of the E-field oscillation. On the AP exam you should be able to describe these properties and order the spectrum (CED 14.4.A). For a concise review, see the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo). For broader unit review and lots of practice Qs, check the unit page (https://library.fiveable.me/ap-physics-2-revised/unit-14) and practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

In an electromagnetic wave the electric field (E) and magnetic field (B) oscillate sinusoidally, are perpendicular to each other, and both are perpendicular to the direction the wave travels—so the wave is transverse (E ⟂ B ⟂ k). For a plane wave the E and B oscillations are in phase (they reach maxima and zeros at the same points). Their magnitudes are related by B = E/c in vacuum, and E, B, and the propagation vector k follow the right-hand rule (E × B points in the direction of travel). EM waves don’t need a material medium to propagate and can be polarized (orientation of E). On the AP exam you may be asked to describe these properties (CED 14.4.A). Review this topic on Fiveable’s study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and practice lots of problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Because electromagnetic (EM) waves are made of changing electric and magnetic fields, they don’t need matter to wiggle the way sound does. Maxwell’s equations show that a time-varying electric field creates a magnetic field and a time-varying magnetic field creates an electric field, so the fields sustain each other and carry energy through space. That’s why EM waves are transverse (E and B oscillations perpendicular to the direction of travel) and can propagate in a vacuum—there’s no medium required (CED Essential Knowledge 14.4.A.1 and 14.4.A.2). For the AP exam, remember to describe EM waves as oscillating, mutually perpendicular E and B fields and to note vacuum propagation when asked (Topic 14.4). For a quick refresher, check the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and more unit resources (https://library.fiveable.me/ap-physics-2-revised/unit-14). For extra practice, see the 1000+ problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Electromagnetic waves vs mechanical waves—short version: electromagnetic (EM) waves are oscillating electric and magnetic fields that travel through space without needing any material medium; mechanical waves need a medium (air, water, solid) to carry their oscillations. Key AP points: EM waves are transverse (E and B fields oscillate perpendicular to the direction of propagation and to each other) and are commonly treated as plane waves (CED 14.4.A.1). Mechanical waves can be transverse or longitudinal (sound is longitudinal). EM waves can propagate in a vacuum and come in an ordered spectrum (radio → gamma)—visible light is just a small part (CED 14.4.A.2–3). For exam practice, know these definitions, transverse behavior, and the spectrum ordering (you don’t need exact wavelengths). Review Topic 14.4 on Fiveable (study guide: https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and try practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Think of an electromagnetic wave as two perpendicular fields (electric E and magnetic B) that wiggle while the wave moves forward. In an EM wave the E and B oscillations are both perpendicular to the direction of travel (and to each other)—that’s exactly why the CED calls them transverse waves (14.4.A.1 and 14.4.A.1.i). Mathematically Maxwell’s equations show a changing E makes B and vice versa, and the resulting E, B, and propagation vector k are mutually perpendicular for a plane wave. Because the fields themselves carry the energy, no material medium is needed (14.4.A.2). The transverse nature is also why light can be polarized: you can pick out one direction of the E-field oscillation. Review the CED wording and examples in the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and try practice problems (https://library.fiveable.me/practice/ap-physics-2-revised) to see this applied on the exam.

From longest to shortest wavelength: radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays. For visible light (inside “visible” on the spectrum) the colors go, from longest to shortest wavelength: red → orange → yellow → green → blue → violet. This ordering is exactly what the AP CED expects you to know for Topic 14.4 (you don’t need exact wavelength ranges, just the order). Electromagnetic waves are transverse, made of perpendicular oscillating E and B fields, and don’t need a medium to propagate (CED 14.4.A.1–A.3). For a quick refresher, check the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and use the unit review (https://library.fiveable.me/ap-physics-2-revised/unit-14) or practice problems (https://library.fiveable.me/practice/ap-physics-2-revised) to drill this ordering.

The visible colors, in order of decreasing wavelength (red → violet), are red, orange, yellow, green, blue, violet—memorized easily as “ROY G. BIV” (Richard Of York Gave Battle In Vain) or a shorter “ROYGBV.” Remember the physics: as you go from red to violet the wavelength decreases, frequency and photon energy increase (CED essential knowledge 14.4.A.3.ii). That relationship helps on the AP: if a question asks about wavelength vs. frequency or energy, tie the color order to “↓ wavelength → ↑ frequency/energy.” If you want extra practice spotting these on spectrum or blackbody questions, check the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and try practice sets (https://library.fiveable.me/practice/ap-physics-2-revised).

They’re both electromagnetic because they share the same physical nature: oscillating electric and magnetic fields perpendicular to each other and to the direction of travel (i.e., transverse EM waves) and they don’t need a medium to propagate—that’s the core CED point (14.4.A.1–14.4.A.2). What’s different is wavelength/frequency (14.4.A.3): radio waves have very long wavelengths and low photon energy; gamma rays have extremely short wavelengths and very high photon energy. Those differences change how they’re produced and how they interact with matter (antennas for radio, nuclear processes for gammas), but not the underlying Maxwell-wave behavior (polarization, plane-wave idea, vacuum propagation). For the AP exam you should be able to describe these shared properties and order the spectrum (you don’t need exact ranges). For a quick review, check the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and more unit resources (https://library.fiveable.me/ap-physics-2-revised/unit-14). Practice problems are at (https://library.fiveable.me/practice/ap-physics-2-revised).

A “planar wave front” means the surfaces of constant phase (places where the oscillating E and B fields have the same value at the same time) are flat planes instead of curved surfaces. For a plane electromagnetic wave the electric and magnetic fields oscillate perpendicular to each other and to the direction of propagation, and every point on a given plane wave front is in phase—so the fields look identical anywhere across that plane. Practically this is a simplifying assumption in the CED (14.4.A.1.ii): it makes math easier (sinusoids that depend only on one spatial coordinate) and models light that’s approximately uniform across a beam much wider than a wavelength. On the AP exam you’ll be expected to know that EM waves are transverse and are often treated as plane waves with planar wave fronts (Topic 14.4). For a quick refresher, check the Topic 14.4 study guide on Fiveable (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo).

Yes—electromagnetic waves can travel through a vacuum. Per the CED, an EM wave is a transverse wave made of oscillating electric and magnetic fields that are mutually perpendicular and perpendicular to the direction of propagation (14.4.A.1). In a vacuum, a changing electric field creates a changing magnetic field and vice versa (Maxwell’s equations), so the fields sustain each other and the wave moves outward without any material medium. Those waves travel at the speed of light c in vacuum and are commonly treated as plane waves with planar wavefronts (14.4.A.1.ii, 14.4.A.2). This is exactly why light, radio waves, X-rays, etc., can get to us from space—different categories are just different wavelengths (14.4.A.3). For AP exam review, focus on the transverse nature, vacuum propagation, and spectrum ordering (see the Topic 14.4 study guide on Fiveable: https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo). For more practice, check the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-14) and the practice problem bank (https://library.fiveable.me/practice/ap-physics-2-revised).

Wavelength basically tells you what kind of electromagnetic radiation you have: longer wavelengths = lower frequency and lower photon energy (radio → microwaves → infrared → visible → ultraviolet → X-rays → gamma rays). AP expects you to know that ordering (radio to gamma) and, for visible light, the color order from longest to shortest wavelength: red, orange, yellow, green, blue, violet (all of these are transverse EM waves with perpendicular E and B fields and don’t need a medium). You don’t need to memorize exact wavelength ranges for the exam—just the order and the idea that shorter wavelength = higher frequency and higher photon energy. If you want a quick review, check the Topic 14.4 study guide on Fiveable (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-14). For more practice (1000+ questions), use Fiveable practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Think of an electromagnetic wave as two linked oscillations, not little arrows pushing a medium. The electric field E oscillates in one direction (say up/down, the y-axis) while the magnetic field B oscillates at right angles to that (say in/out of the page, the z-axis). Maxwell’s equations say a changing E creates a changing B and vice versa; those changes travel together. The energy and information move in the direction given by E × B (the cross product), which is perpendicular to both fields (here, the x-direction). That’s why EM waves are transverse and often modeled as plane waves with orthogonal E and B oscillations perpendicular to propagation (CED 14.4.A.1.i). Nobody needs a medium because the fields sustain each other as they propagate (CED 14.4.A.2). If you want a quick diagram and AP-aligned notes, check the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo). For lots of practice problems, see (https://library.fiveable.me/practice/ap-physics-2-revised).

Short answer: historically “light” meant what our eyes detect (visible colors), but in physics “light” often means all electromagnetic (EM) radiation—the whole spectrum—because visible light is just one wavelength range of EM waves. Why: AP Topic 14.4 defines EM waves as oscillating electric and magnetic fields, transverse, that don’t need a medium. The EM spectrum categories (radio → gamma) differ only by wavelength; visible (red → violet) is the slice our eyes respond to, so everyday speech uses “light” for visible. In physics and the CED language, it’s common to call all those wavelengths “electromagnetic radiation” or sometimes “light” because they’re the same kind of wave, just different λ and frequency. For AP review, focus on the CED keywords (oscillating E and B fields, transverse, spectrum). See the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-14). For extra practice, try the Fiveable practice set (https://library.fiveable.me/practice/ap-physics-2-revised).

They’re both electromagnetic waves (oscillating E and B fields, transverse, don’t need a medium), so physically they obey the same laws. The difference is wavelength (and therefore frequency and photon energy): radio waves have longer wavelengths than microwaves. AP only expects you to know the ordering on the spectrum, not exact ranges (CED 14.4.A.3.i). That change in wavelength/frequency causes practical differences: antennas for radio waves are much larger, radio waves pass through some materials that microwaves don’t, and microwaves interact strongly with polar molecules (that’s why they heat food). On the exam, focus on the spectrum ordering and how wavelength/frequency relate to energy and behavior at boundaries (reflection, transmission, absorption). For a quick review, check the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo) and practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

They’re both electromagnetic waves, but X-rays have much shorter wavelengths and therefore much higher photon energy (E = hf). That higher energy lets individual X-ray photons ionize atoms and break chemical bonds in cells and DNA, causing biological damage and increasing cancer risk. X-rays also penetrate tissues more deeply than visible light, so they can deposit energy inside your body rather than just at the surface. For the AP exam you should remember the spectrum order (visible vs. X-rays) and that frequency/wavelength determine photon energy (Topic 14.4 keywords: electromagnetic spectrum, wavelengths, oscillating fields). Exposure risk depends on dose, energy, and time; heavy shielding (e.g., lead) reduces X-ray penetration. For a quick review see the Topic 14.4 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/4-electromagnetic-waves/study-guide/hZjkcwjYXeKC1jmo), the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-14), and practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).