The traditional spectrum is often listed as seven colors, including red, orange, yellow, green, blue, indigo, and violet.

The longer version

The seven-name list comes from a choice made in the seventeenth century, when Isaac Newton divided the continuous band he had produced into seven steps. His reasons were partly musical: he wanted the colors to correspond to the seven notes of a scale. He was describing his own perception, not measuring anything physical.

A spectrum actually has no natural divisions; hue varies smoothly from end to end, and different languages and textbooks cut it differently. Many modern lists drop indigo and settle on six. Newton originally named only five, then added orange and indigo to reach the number he wanted.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.

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More From Color and Light

Eight more entries

Most people cannot distinguish indigo from blue or violet in an ordinary rainbow.

Color blindness most often involves difficulty telling red from green rather than seeing no color at all.

Some animals, including many birds, see ultraviolet light that human eyes cannot detect.

Mixing red, green, and blue light in equal amounts produces white light on a screen.

Mixing yellow and blue paint produces green, which surprises people taught that mixing light works differently.

Pigments create color by subtracting wavelengths, while lights create color by adding wavelengths together.

A shadow's edges grow blurrier as the light source becomes larger or moves farther away.

The darkest part of a shadow is called the umbra, while the softer edge is the penumbra.

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