speed of light

Meaning: The speed of light, c, is the speed of every electromagnetic wave in a vacuum: 3.00 × 10⁸ m/s. Radio waves, visible light and X-rays differ in frequency and wavelength but share this speed in a vacuum. In a material, light travels more slowly. The ratio of c to the speed in the material is the index of refraction. When light crosses into another medium, its frequency stays the same (the source sets it), so its wavelength changes.

Equation: c = fλ and E = hf = hc/λ, where c is the speed of light (m/s), f is frequency (Hz, or s⁻¹), λ is wavelength (m), E is photon energy (J), and h is Planck's constant (6.63 × 10⁻³⁴ J·s). In a medium, v = c/n, where v is the speed in the medium (m/s) and n is the index of refraction (unitless, 1 for a vacuum and larger than 1 in matter).

On the MCAT: You are given a wavelength and asked for frequency or photon energy, often in spectroscopy, the photoelectric effect or atomic emission. The trend to predict: shorter wavelength means higher frequency and higher energy. Another setup is light entering glass or water, where speed and wavelength drop and the ray bends toward the normal.

Common confusion: Students think red and violet light travel at different speeds in a vacuum. They do not; only f and λ differ, and they change inversely. In a medium, speed falls and wavelength shrinks, but frequency does not change.