visible light
Summary
Visible light is the band of electromagnetic waves the human eye detects, roughly 400 nm (violet) to 700 nm (red). Violet has the shortest wavelength and highest frequency and energy; red has the longest wavelength and lowest. All colors travel at c in a vacuum but at different speeds in a medium.
Detail
Meaning: Visible light is the part of the electromagnetic spectrum the human eye detects, roughly 400 nm (violet) to 700 nm (red). It lies between ultraviolet (shorter wavelength) and infrared (longer wavelength). It is a transverse wave of oscillating electric and magnetic fields. Color is set by frequency, which stays the same when light enters a new medium.
Equation: c = fλ and E = hf, where c = 3.00 × 10⁸ m/s (speed of light in vacuum), f is frequency (Hz), λ is wavelength (m), E is photon energy (J), and h = 6.63 × 10⁻³⁴ J·s (Planck's constant).
On the MCAT: Expect rankings of colors by wavelength, frequency or photon energy (violet highest energy, red lowest), and comparisons with ultraviolet or infrared. In prism or refraction passages, the index of refraction n is slightly larger for violet than for red, so violet bends the most (dispersion).
Common confusion: Students often think red and violet light always travel at different speeds, or that color changes inside glass. In a vacuum all colors share the same speed c. In glass, v = c/n differs slightly by color, and the wavelength shrinks while the frequency, and so the color, stays fixed. The 400 to 700 nm limits are approximate.
Sources
- Physics (Cutnell and Johnson)
- University Physics (Young and Freedman)
- Physics (Giancoli)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.