Violet is the color of the shortest wavelengths of light that people can see. The explorer shows violet light with a wavelength of 400 nanometers, close to the shortest that human eyes can detect. A nanometer is a billionth of a meter.

Wavelength

Light travels as a wave, and its wavelength is the distance from one crest of the wave to the next. Each color of light has a different wavelength. When white light passes through a prism, the colors spread apart into a rainbow, with red at one end and violet at the other.

Human eyes typically detect wavelengths from about 380 to 700 nanometers. Violet sits at the short end, at around 380 nanometers, and red at the long end, at around 700 nanometers. So 400 nanometers is violet light near the edge of what people can see.

That edge is not sharp. The International Commission on Illumination, which sets standards for measuring light, says there are no precise limits to visible light. That is because very bright sources are sometimes visible at wavelengths shorter than 400 nanometers. The commission puts the short limit somewhere between 360 and 400 nanometers.

Size comparisons

About 200–250 violet waves, laid end to end, would span the width of a human hair, which is 80–100 micrometers wide. A micrometer is a thousandth of a millimeter.
About 19 violet waves would stretch across a red blood cell, which is about 7.5 micrometers wide.
The waves of red light, about 700 nanometers long, are 1.75 times as long as these.
Violet waves are about 6.7 times as long as the 60-nanometer waves of the explorer's ultraviolet light, which human eyes cannot see.
With 400-nanometer light, a good light microscope can separate details about 0.2 micrometers (200 nanometers) apart, about half a wavelength. This link between wavelength and detail sets the limit of optical microscopes.
If one violet wave were as long as a person 1.7 meters (5 feet 7 inches) tall, a red blood cell would be about 32 meters (105 feet) across.

How the eye sees violet

Cone cells in the eye act as receivers tuned to the narrow band of visible wavelengths. The eye responds much less to violet than to the middle of that band. In daylight, light at 400 nanometers looks only about 1/2,500 as bright as the same power of green light at 555 nanometers, the wavelength the eye is most sensitive to. That is why violet light looks dim next to green light of the same power.

Beyond violet

Light with wavelengths shorter than violet is called ultraviolet light. The band next to visible light, UV-A, runs from 315 to 400 nanometers. Some insects, such as bumblebees, can see near-ultraviolet light, which human eyes normally cannot.

In 1801, Johann Ritter showed that this invisible light exists. He knew that photographic paper turned black faster in blue light than in red light. He placed paper beyond the violet end of the spectrum, where no light could be seen, and it still turned black.

Sources

Visible Light. NASA Science, 2023.
Anatomy of an Electromagnetic Wave. NASA Science, 2023.
Ultraviolet Waves. NASA Science, 2023.
Visible radiation. International Commission on Illumination, International Lighting Vocabulary, 2020.
Ultraviolet radiation. International Commission on Illumination, International Lighting Vocabulary, 2020.
CIE spectral luminous efficiency for photopic vision. International Commission on Illumination, 2019.
Resolution. Nikon MicroscopyU.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Erythrocytes. In Anatomy and Physiology, 2nd edition. OpenStax, 2022.

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