electromagnetic waves

Meaning: An electromagnetic wave is a transverse wave made of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of travel. A changing electric field produces a changing magnetic field and vice versa, so the wave sustains itself and needs no medium. Accelerating charges produce it. In order of increasing frequency the spectrum runs: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

Equation: c = fλ, where c is the speed of light in vacuum, 3.00 × 10⁸ m/s, f is frequency in Hz (s⁻¹), and λ is wavelength in m.

On the MCAT: Expect rankings of spectrum regions by frequency, wavelength, or photon energy, using E = hf, where E is energy in J and h is Planck's constant, 6.63 × 10⁻³⁴ J·s. Passages may also have light entering a medium: the frequency stays the same, while the speed falls to v = c/n and the wavelength shrinks (n is the unitless refractive index). Because the waves are transverse, they can be polarized.

Common confusion: Students think higher-frequency waves travel faster. In vacuum every electromagnetic wave moves at c; higher frequency means shorter wavelength and greater photon energy. Students also lump these waves with sound, but sound is a mechanical, longitudinal wave that needs a medium.