Visible Light Definition and Wavelengths

Visible light is a range of electromagnetic radiation that can be detected by the human eye. The wavelengths associated with this range are 380 to 750 nanometers (nm) while the frequency range is approximately 430 to 750 terahertz (THz). The visible spectrum is the portion of the electromagnetic spectrum between infrared and ultraviolet. Infrared radiation, microwaves, and radio waves are lower frequency/longer wavelength than visible light, while ultraviolet light, x-radiation, and gamma radiation are higher frequency/shorter wavelength than visible light.

Units

There are two sets of units used to measure visible light. Radiometry measures all wavelengths of light, while photometry measures light with respect to human perception. SI radiometric units include the joule (J) for radiant energy and watt (W) for radiant flux. SI photometric units include the lumen (lm) for luminous flux, lumen second (lm⋅s) or talbot for luminous energy, candela (cd) for luminous intensity, and lux (lx) for illuminance or luminous flux incident on a surface.

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Colors of the Visible Spectrum Unveiled

By Anne Marie Helmenstine, Ph.D.

Variations in the Range of Visible Light

The human eye perceives light when sufficient energy interacts with the molecule retinal in the eye's retina. The energy changes the molecular conformation, triggering a nerve impulse that registers in the brain. Depending on whether a rod or cone is activated, light/dark or color may be perceived. Humans are active during daylight hours, which means our eyes are exposed to sunlight. Sunlight has a strong ultraviolet component, which damages rods and cones. So, the eye has built-in ultraviolet filters to protect vision. The cornea of the eye absorbs most ultraviolet light (below 360 nm), while the lens absorbs ultraviolet light below 400 nm. However, the human eye can perceive ultraviolet light. People who have a lens removed (called aphakia) or have cataract surgery and get an artificial lens report seeing ultraviolet light. Birds, bees, and many other animals also perceive ultraviolet light. Most animals that see ultraviolet light cannot see red or infrared. Under laboratory conditions, people can often see as far as 1050 nm into the infrared region. After that point, the energy of infrared radiation in too low to produce the molecular conformation change needed to trigger a signal.

Colors of Visible Light

The colors of visible light are called the visible spectrum. The colors of the spectrum correspond to wavelength ranges. Sir Isaac Newton divided the spectrum into red, orange, yellow, green, blue, and violet. He later added indigo, but Newton's "indigo" was closer to modern "blue," while his "blue" more closely resembled modern "cyan." The color names and wavelength ranges are somewhat arbitrary, but they follow a sequence from infrared to ultraviolet of infrared, red, orange, yellow, green, blue, indigo (in some sources), and violet. Modern scientists refer to colors by their wavelength rather than name, to avoid any confusion.

Other Facts

The speed of light in a vacuum is defined to be 299,792,458 meters per second. The value is defined because the meter is defined based on the speed of light. Light is energy rather than matter, but it does exert pressure and it has momentum. Light bent by a medium is refracted. If it bounces off a surface, it is reflected.

Sources

• Cassidy, David; Holton, Gerald; Rutherford, James (2002). Understanding Physics. Birkhäuser. ISBN 978-0-387-98756-9.
• Neumeyer, Christa (2012). "Chapter 2: Color Vision in Goldfish and Other Vertebrates." In Lazareva, Olga; Shimizu, Toru; Wasserman, Edward (eds.). How Animals See the World: Comparative Behavior, Biology, and Evolution of Vision. Oxford Scholarship Online. ISBN 978-0-19-533465-4.
• Starr, Cecie (2005). Biology: Concepts and Applications. Thomson Brooks/Cole. ISBN 978-0-534-46226-0.
• Waldman, Gary (2002). Introduction to Light : The Physics of Light, Vision, and Color. Mineola: Dover Publications. ISBN 978-0-486-42118-6.
• Uzan, J.-P.; Leclercq, B. (2008). The Natural Laws of the Universe: Understanding Fundamental Constants. Springer. doi:10.1007/978-0-387-74081-2 ISBN 978-0-387-73454-5.