Live Science Plus
What is visible light?
Visible light is the portion of the electromagnetic spectrum that can be detected by the human eye.
Visible light is a form of electromagnetic (EM) radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Generally, visible light is defined as the wavelengths that are visible to most human eyes.
Electromagnetic spectrum range
The electromagnetic spectrum, from highest to lowest frequency wavelengths.
Visible light is a type of electromagnetic radiation, which is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. That spectrum is typically divided into seven regions in order of decreasing wavelength and increasing energy and frequency. These regions are:
- radio waves (wavelengths greater than 0.4 inch, or 10 millimeters)
- microwaves (wavelengths between 0.004 and 0.4 inch, or 0.1 to 10 mm)
- infrared (IR) (wavelengths between 0.00003 and 0.004 inch, or 740 nanometers to 100 micrometers)
- visible light,(wavelengths between 0.000015 and 0.00003 inch, or 380 to 740 nanometers)
- ultraviolet (UV) (wavelengths between 0.000015 and 0.00003 inch, or 380 to 740 nanometers)
- X-rays (wavelengths between 4 × 10^−7 to 4 × 10^−8 inch, or 100 picometers to 10 nanometers)
- gamma-rays (wavelengths less than 4 × 10^−9 inch, or 100 picometers)
Visible light falls in the range of the EM spectrum between infrared (IR) and ultraviolet (UV). It has frequencies of about 4 × 1014 to 8 × 1014 cycles per second, or hertz (Hz) and wavelengths of about 740 nanometers (nm) or 2.9 × 10−5 inches, to 380 nm (1.5 × 10−5 inches).
Visible light spectrum and color
A diagram showing the visible color spectrum.
Perhaps the most important characteristic of visible light is color. Color is both an inherent property of light and an artifact of the cells in the human eye. Objects don't "have" color, according to The Physics Hypertextbook. Rather, they give off light that "appears" to be a color. In other words, Elert writes, color exists only in the mind of the beholder.
Our eyes contain specialized cells, called cones, that act as receivers tuned to the wavelengths of this narrow band of the EM spectrum, according to NASA's Mission Science website. Humans see light at the lower end of the visible spectrum, having a longer wavelength, about 740 nm, as red; we perceive light in the middle of the spectrum as green; and see light at the upper end of the spectrum, with a wavelength of about 380 nm, as violet. All other colors that we perceive are mixtures of these colors.
For instance, yellow contains light from both the red and green regions of the visible light spectrum; cyan is a mixture of green and blue, and magenta is a blend of red and blue. White light contains all colors in combination. Black is a total absence of light. The first person to realize that white light was made up of the colors of the rainbow was Isaac Newton, who in 1666 passed sunlight through a narrow slit and then a prism to project the colored spectrum onto a wall, according to the website of Michael Fowler, a physics professor at the University of Virginia.
How does heat energy turn into visible light?
As objects grow hotter, they radiate energy dominated by shorter wavelengths, which we perceive as changing colors, according to NASA's Mission Science. For example, the flame of a blowtorch changes from reddish to blue as it is adjusted to burn hotter. This process of turning heat energy into light energy is called incandescence, according to the Institute for Dynamic Educational Advancement (IDEA) website, WebExhibits.org.
Get the world’s most fascinating discoveries delivered straight to your inbox.
Incandescent light is produced when hot matter releases a portion of its thermal vibration energy as photons. At about 1,472 degrees Fahrenheit (800 degrees Celsius), the energy radiated by an object reaches the infrared. As the temperature increases, the energy moves into the visible spectrum and the object appears to have a reddish glow. As the object gets hotter, the color changes to "white hot" and eventually to blue.
Visible light astronomy
This four-panel graphic illustrates how the southern region of the rapidly evolving, bright, red supergiant star Betelgeuse may have suddenly become fainter for several months during late 2019 and early 2020. In the first two panels, as seen in ultraviolet light with the Hubble Space Telescope, a bright, hot blob of plasma is ejected from the emergence of a huge convection cell on the star's surface. In panel three, the outflowing, expelled gas rapidly expands outward. It cools to form an enormous cloud of obscuring dust grains. The final panel reveals the huge dust cloud blocking the light (as seen from Earth) from a quarter of the star's surface.
The color of hot objects, such as stars, can be used to estimate their temperatures, according to IDEA. For example, the sun's surface temperature is about 5,800 kelvin (9,980 F or 5,527 C). The light emitted has a peak wavelength of about 550 nm, which we perceive as visible white light (or slightly yellowish).
According to NASA, if the sun's surface temperature were cooler, about 3,000 C, it would look reddish, like the star Betelgeuse. If it were hotter, about 12,000 C, it would look blue, like the star Rigel.
Astronomers can also determine what objects are made of because each element absorbs light at specific wavelengths, called an absorption spectrum. By knowing the absorption spectra of elements, astronomers can use spectroscopes to determine the chemical composition of stars, dust clouds and other distant objects.
The blue color of the Witch Head Nebula and of the dust surrounding Orion's bright supergiant star Rigel is caused not only by Rigel's intense blue starlight but because the dust grains scatter blue light more efficiently than red. The same physical process causes Earth's daytime sky to appear blue, although the scatterers in Earth's atmosphere are molecules of nitrogen and oxygen.
Additional resources
Learn more about how the human brain perceives light in this video from National Geographic. Walk your way through the electromagnetic spectrum with the stunning book "Light: The Visible Spectrum and Beyond" (Black Dog & Leventhal, 2013), or do your own visible light experiments at home using guidance from Ducksters, a children's educational website.
This article was updated on May 23, 2022 by Live Science managing editor Tia Ghose.
