Visible light is the narrow band of the electromagnetic spectrum the human eye can see, from violet at about 380 nm to red at about 750 nm. This reference maps each colour to its wavelength and frequency range and includes a lookup that names the colour for any wavelength and computes its frequency.
How it works
Each colour corresponds to a band of wavelengths, measured in nanometres (billionths of a metre). To find the frequency, use the wave equation:
f = c / λwhere c ≈ 2.998×10⁸ m/s and λ is the wavelength in metres. Because the speed
of light is fixed, frequency rises as wavelength falls. Violet light, with the
shortest visible wavelength, has the highest frequency (around 670 THz), while
red light, with the longest wavelength, has the lowest (around 430 THz).
Reading the spectrum
The order from short to long wavelength is violet, blue, cyan, green, yellow, orange, red — the familiar rainbow sequence reversed depends only on whether you list by wavelength or by the order you see them refracted. Green sits in the middle near 550 nm, where human eyes are most sensitive in daylight, which is why safety and high-visibility gear often uses yellow-green.
Wavelength-to-colour reference table
| Colour | Wavelength range | Frequency range |
|---|---|---|
| Violet | ~380–450 nm | ~665–790 THz |
| Blue | ~450–495 nm | ~606–665 THz |
| Cyan | ~495–520 nm | ~576–606 THz |
| Green | ~520–565 nm | ~530–576 THz |
| Yellow | ~565–590 nm | ~508–530 THz |
| Orange | ~590–625 nm | ~480–508 THz |
| Red | ~625–750 nm | ~400–480 THz |
Example and real-world wavelengths
A 633 nm helium-neon laser glows red: the tool places it in the red band and computes a frequency of about 474 THz. Here are some other well-known wavelengths to put the spectrum in context:
- 405 nm — violet laser (common in Blu-ray players), near the UV edge
- 450 nm — royal blue LEDs, used in white LED phosphor excitation
- 532 nm — green laser pointer (frequency-doubled Nd:YAG)
- 589 nm — sodium doublet, the yellow-orange of street lamps
- 633 nm — helium-neon laser, classic optical lab tool
- 660 nm — standard deep-red LED, common in phototherapy devices
- 700 nm — near the edge of visible red; some people can’t see this clearly
Why green is the peak of human vision
Human photoreceptors (specifically the M-cone for medium wavelengths) peak in sensitivity around 555 nm — right in the yellow-green range. This is why emergency exit signs in some countries are green rather than red, and why high-visibility safety vests are yellow-green. It is also why CRT and early LCD monitors use green phosphors that glow near 520–560 nm: the eye is most efficient at detecting photons in this range, so less power is needed to produce a perceived given brightness.
Red and violet, at either end of the visible spectrum, require much more light energy for the eye to detect at the same perceived brightness. This is why a 1-milliwatt red laser appears dimmer than a 1-milliwatt green laser, even though the power is identical.
Beyond the visible edges
Just outside the visible range on each side are the first bands of the invisible spectrum:
- Near-UV (300–380 nm): Can cause sunburn and fluorescence (blacklight effects). Visible to some insects and birds.
- Near-IR (750–1000 nm): Used in remote controls (around 850–950 nm), fiber optic communication, and night-vision devices. Digital camera sensors often detect near-IR; that’s why a TV remote looks bright on your phone camera even though you can’t see it.
The colour boundaries in this reference are approximate conventions — colour perception is continuous and varies between people. Treat the ranges as guides rather than exact cut-offs. Values just outside 380–750 nm are flagged as ultraviolet or infrared.