# The Visible Spectrum: Wavelengths and Colors

The human eye sees color over wavelengths ranging roughly from 400 nanometers (violet) to 700 nanometers (red). Light from 400–700 nanometers (nm) is called visible light, or the visible spectrum because humans can see it. Light outside of this range may be visible to other organisms but cannot be perceived by the human eye. Colors of light that correspond to narrow wavelength bands (monochromatic light) are the pure spectral colors learned using the ROYGBIV acronym: red, orange, yellow, green, blue, indigo, and violet.

## Wavelengths of Visible Light

Some people can see further into the ultraviolet and infrared ranges than others, so the "visible light" edges of red and violet are not well-defined. Also, seeing well into one end of the spectrum doesn't necessarily mean you can see well into the other end of the spectrum. You can test yourself using a prism and a sheet of paper. Shine a bright white light through the prism to produce a rainbow on the paper. Mark the edges and compare the size of your rainbow with that of others.

The wavelengths of visible light are:

• Violet: 380–450 nm (688–789 THz frequency)
• Blue: 450–495 nm
• Green: 495–570 nm
• Yellow: 570–590 nm
• Orange: 590–620 nm
• Red: 620–750 nm (400–484 THz frequency)

Violet light has the shortest wavelength, which means it has the highest frequency and energy. Red has the longest wavelength, the shortest frequency, and the lowest energy.

## The Special Case of Indigo

There is no wavelength assigned to indigo. If you want a number, it's around 445 nanometers, but it doesn't appear on most spectra. There's a reason for this. English mathematician Isaac Newton (1643–1727) coined the word spectrum (Latin for "appearance") in his 1671 book "Opticks." He divided the spectrum into seven sections—red, orange, yellow, green, blue, indigo, and violet—in keeping with the Greek sophists, to connect the colors to days of the week, musical notes, and the known objects of the solar system.

So, the spectrum was first described with seven colors, but most people, even if they see color well, can't actually distinguish indigo from blue or violet. The modern spectrum typically omits indigo. In fact, there is evidence Newton's division of the spectrum doesn't even correspond to the colors we define by wavelengths. For example, Newton's indigo is the modern blue, while his blue corresponds to the color we refer to as cyan. Is your blue the same as my blue? Probably, but it may not be the same as Newton's.

## Colors People See That Aren't on the Spectrum

The visible spectrum does not encompass all the colors humans perceive because the brain also perceives unsaturated colors (e.g., pink is an unsaturated form of red) and colors that are a mixture of wavelengths (e.g., magenta). Mixing colors on a palette produces tints and hues not seen as spectral colors.

## Colors Only Animals Can See

Just because humans can't see beyond the visible spectrum doesn't mean animals are similarly restricted. Bees and other insects can see ultraviolet light, which is commonly reflected by flowers. Birds can see into the ultraviolet range (300–400 nm) and have plumage visible in UV.

Humans see further into the red range than most animals. Bees can see color up to about 590 nm, which is just before orange starts. Birds can see red, but not as far toward the infrared range as humans.

Some people believe the goldfish is the only animal that can see both infrared and ultraviolet light, but this notion is incorrect. Goldfish cannot see infrared light.

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