Visible light spectrum
Also known as: visible spectrum, visible light
The visible light spectrum is the narrow band of electromagnetic radiation the human eye can detect, spanning roughly 400 to 700 nanometers. Violet sits at the short-wavelength, high-energy end and red at the long-wavelength, low-energy end.
Visible light occupies a small slice of the electromagnetic spectrum. Wavelengths just longer than the visible range are infrared; those just shorter are ultraviolet. Within the visible band, wavelength determines perceived colour in the familiar order violet, blue, green, yellow, orange, red, running from about 400 nm to about 700 nm.
Wavelength, frequency, and energy are locked together. All electromagnetic radiation travels at c = 3.00 × 10⁸ m/s in a vacuum, so c = λf means frequency falls as wavelength rises. Photon energy follows E = hf = hc/λ, which makes violet light the most energetic visible light and red the least. A 400 nm violet photon carries about 75% more energy than a 700 nm red photon.
The connection to chemistry runs through electronic transitions. A molecule absorbs a photon when its energy matches the gap between two electronic energy levels, so absorption wavelength reports on molecular structure. Extended conjugation — alternating double bonds sharing delocalized π electrons — narrows that gap, shifting absorption to longer wavelengths. Molecules with enough conjugation absorb within the visible range and therefore appear coloured; the perceived colour is the complement of the wavelength absorbed, which is why a compound that absorbs blue-green light looks red.
This principle underlies UV-visible spectroscopy, in which the absorbance of a sample at a chosen wavelength is used to identify a chromophore or, through Beer's law, to determine concentration.
The MCAT covers the visible spectrum in the chemical and physical foundations section, under how light and sound interact with matter. Useful facts to have memorised are the approximate wavelength boundaries, the ordering of colours by energy, the relationship E = hc/λ, and the rule that greater conjugation shifts absorption toward longer wavelengths.
Key takeaways
- Visible light spans roughly 400 to 700 nm of the electromagnetic spectrum.
- Violet has the shortest wavelength and highest energy; red has the longest wavelength and lowest energy.
- Photon energy follows E = hc/λ, so energy rises as wavelength falls.
- Increased conjugation in a molecule narrows the electronic energy gap and shifts absorption to longer wavelengths.
- A coloured compound appears as the complement of the wavelengths it absorbs.