Red light is the color of light with the longest wavelengths that people can see. The red light shown here has a wavelength of 750 nanometers, a deep red at the far end of the visible range. A nanometer is a billionth of a meter.

Wavelength

Light travels as a wave. Its wavelength is the distance from one crest of the wave to the next, and each color of light has a different wavelength. Violet has the shortest wavelength people can see, and red has the longest. Orange light has a wavelength of about 620 nanometers, and a typical red about 680 nanometers.

The ends of the visible range are not sharp. NASA gives the range as about 380 to 700 nanometers, and a widely used physics textbook as 400 to 750 nanometers. The International Commission on Illumination, which sets standards for light and lighting, puts the upper limit somewhere between 760 and 830 nanometers. Where it falls depends on how much light reaches the eye, and on the person.

At 750 nanometers, the red shown here sits at the long end of the textbook's range and beyond NASA's. Light travels at about 300,000 kilometers per second. So about 400 trillion crests of this red light pass any point each second. That number is called the light's frequency.

Size comparisons

About 110–130 wavelengths of this red light fit across a human hair, which is 80–100 micrometers wide. A micrometer is a millionth of a meter.
Violet light, at the other end of the rainbow, has a wavelength of about 400 nanometers. This red light's wavelength is almost twice as long.
If one wavelength of this red light were stretched to the width of a tennis ball, about 6.7 centimeters (2.6 inches), a human hair would be 7–9 meters (23–29 feet) wide.
A sheet of paper, about 0.1 millimeters thick, is about 130 wavelengths of this light thick.

How the eye sees red

Color is detected by cells in the eye called cones. Humans have three kinds, called red, green and blue cones, although the names do not exactly match the middle of the range each one detects. Red cones respond to wavelengths of about 500 to 700 nanometers, green cones to 450 to 630, and blue cones to 400 to 500.

A deep red of 750 nanometers lies near the edge of human vision. Other animals see colors differently. Most mammals other than primates have only two kinds of cones. Dogs, for example, see blue and yellow but cannot tell red from green.

Why sunsets are red

Sunlight contains all the colors of the rainbow. As it passes through the air, gas molecules scatter blue light, which travels in shorter waves, more than red light. The scattered blue light, coming from all directions, is why the sky looks blue.

When the Sun is low in the sky, its light passes through more air to reach the ground. Even more of the blue is scattered away, and mostly red and yellow light comes straight through. Dust and other small particles in the air also scatter blue light, which can make a sunset even redder.

Sources

Visible Light. NASA Science, 2010.
The Electromagnetic Spectrum. In College Physics 2e. OpenStax, 2022.
The Electromagnetic Spectrum. In Physics. OpenStax, 2020.
Visible radiation (term 17-21-003). International Lighting Vocabulary, CIE, 2020.
Color and Light. Optical Society of America.
Why Is the Sky Blue?. NASA Space Place.
Speed of light in vacuum. NIST CODATA.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Rules of Tennis 2026. International Tennis Federation, 2026.

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