# Understand the Visible Spectrum (Wavelengths and Colors)

### Know the Wavelengths of Colors of Visible Light

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

### Colors and Wavelengths of Visible Light

Note 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 get a rainbow on the paper. Mark the edges and compare your rainbow with that of others.

• 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

Note there is no wavelength assigned to indigo. If you want a number, it's around 445 nm, but it doesn't appear on most spectra. There's a reason for this. Sir Isaac Newton coined the word spectrum (Latin for "appearance") in 1671 in his book Opticks. He divided the spectrum into 7 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 solar system objects. So, the spectrum was first described with 7 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 a color we refer to as cyan. Is your blue the same as my blue? Probably, but you and Newton might disagree.

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

The visible spectrum does not encompass all the colors humans perceive because the brain 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 Animals See That Humans Can't

Just because people 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 infrared as humans.

While some people believe goldfish are the only animal that can see both infrared and ultraviolet, this notion is incorrect because goldfish cannot see infrared light.