light

Meaning: Light is electromagnetic radiation: a transverse wave of oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel. It needs no medium. Visible light is roughly 400 to 700 nm, but the same rules hold across the electromagnetic spectrum. Light also behaves as photons, packets of energy; interference and diffraction show wave behavior, while the photoelectric effect shows particle behavior.

Equation: c = fλ, and E = hf for one photon, where c = 3.00 × 10⁸ m/s in vacuum, f is frequency (Hz), λ is wavelength (m), E is photon energy (J), and h is Planck's constant (J·s).

On the MCAT: Expect comparisons of colors: violet has a shorter wavelength, higher frequency and greater photon energy than red. In a medium, v = c/n, where n is the index of refraction (unitless), so light entering glass from air slows, its wavelength shortens, and it bends toward the normal.

Common confusion: Students often say wavelength stays fixed at a boundary. The source sets the frequency, so frequency stays constant; speed and wavelength change together. Also, a brighter beam usually means more photons (greater intensity), not more energy per photon.