# Light
**Light** is electromagnetic radiation, a traveling oscillation of electric and magnetic fields, at the frequencies that the human eye can sense; in the broader scientific usage, the word covers the entire electromagnetic spectrum, from radio waves to gamma rays.[1](https://science.nasa.gov/learn/basics-of-space-flight/chapter6-2/) • [2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/) All electromagnetic waves propagate at the speed of light, and all are the same physical phenomenon at different frequencies.[1](https://science.nasa.gov/learn/basics-of-space-flight/chapter6-2/)
| Key fact | Detail |
|---|---|
| Definition | Electromagnetic radiation at frequencies detectable by the human eye; by extension, all electromagnetic radiation[1](https://science.nasa.gov/learn/basics-of-space-flight/chapter6-2/) • [2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/) |
| Speed in vacuum | \( c_0 = 299\,792\,458 \ \mathrm{m/s} \), an exactly defined value; vacuum wavelength and frequency are related by \( \lambda f = c_0 \)[3](https://physics.nist.gov/cuu/pdf/sp330.pdf) |
| Visible band | Roughly 360 nm to 830 nm, the range to which human eyes are sensitive[4](https://www.nist.gov/publications/radiometry-and-photometry-review-vision-optics) |
| Dual character | Behaves as a wave of perpendicular electric and magnetic fields and as a stream of photons, massless packets of energy tied to wavelength[2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/) • [5](https://science.nasa.gov/earth/earth-observatory/remote-sensing/?trk=article-ssr-frontend-pulse_little-text-block) |
| Photon energy | Often expressed in electron volts; 1 eV = \( 1.602\,177\,3 \times 10^{-19} \ \mathrm{J} \), corresponding to a vacuum wavelength of 1 239.842 nm[6](https://physics.nist.gov/Pubs/AtSpec/AtSpec.PDF) |
| Governing laws of direction | The law of reflection and the law of refraction (Snell's law) describe how light changes direction at matter boundaries[7](https://openstax.org/books/college-physics/pages/25-1-the-ray-aspect-of-light) |
| Measurement | Radiometry measures the whole optical spectrum in watts; photometry measures the visible band weighted by the eye's response, in lumens and candelas[4](https://www.nist.gov/publications/radiometry-and-photometry-review-vision-optics) |
## The electromagnetic spectrum
The spectrum is conventionally divided, in order of increasing frequency, into audio frequency, radio frequency, infrared ("below red"), visible light, ultraviolet ("above violet"), X-rays, and gamma rays; these names describe only different frequencies of the same phenomenon.[1](https://science.nasa.gov/learn/basics-of-space-flight/chapter6-2/) [Gamma rays](https://www.edgepedia.org/gamma-rays) have the shortest wavelengths and radio waves the longest, with visible light in the middle.[2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/) The human eye perceives varying wavelengths as varying colors: across the visible spectrum, blue hues correspond to shorter wavelengths, while red hues correspond to longer ones.[2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/)
**Energy follows frequency.** High-frequency electromagnetic waves are more energetic and more able to penetrate matter than low-frequency waves, and they can carry more information per unit time; the shorter the wavelength probing a material, the smaller the detail it can resolve.[8](https://openstax.org/books/college-physics-2e/pages/24-3-the-electromagnetic-spectrum) Gamma rays, at the top of the spectrum, carry the most energy of any electromagnetic wave and are produced by the hottest and most energetic objects in the universe.[9](https://science.nasa.gov/ems/)
The atmosphere shapes which bands reach the ground. Most solar ultraviolet radiation is blocked by the atmosphere; of the UV that reaches Earth's surface, 99% is UV-A (320–400 nm), because most UV-B (290–320 nm) and all UV-C (220–290 nm) is absorbed by ozone in the upper atmosphere.[8](https://openstax.org/books/college-physics-2e/pages/24-3-the-electromagnetic-spectrum) The atmosphere also protects us from X-rays and blocks a majority of ultraviolet, X-ray, and gamma radiation, so scientists observe those bands with satellites.[9](https://science.nasa.gov/ems/)
## Waves and photons
An electromagnetic wave consists of electric and magnetic fields oscillating perpendicular to each other, and, unlike a mechanical wave, it can travel through completely empty space as well as through matter.[2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/) The wavelength is the distance between successive peaks of these fields, and frequency is expressed in hertz, cycles per second.[1](https://science.nasa.gov/learn/basics-of-space-flight/chapter6-2/) • [2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/)
Light also behaves as a particle. A particle of light is a photon, the smallest possible amount of electromagnetic energy of a particular wavelength; each photon carries a specific energy corresponding to its wavelength, and blue photons carry more energy than red photons.[2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/) • [5](https://science.nasa.gov/earth/earth-observatory/remote-sensing/?trk=article-ssr-frontend-pulse_little-text-block) Photons are massless and move at the speed of light, about 300,000 km/s.[5](https://science.nasa.gov/earth/earth-observatory/remote-sensing/?trk=article-ssr-frontend-pulse_little-text-block) In practice, physicists tend to speak of waves at lower frequencies, such as radio waves, and of photons at higher frequencies and energies.[1](https://science.nasa.gov/learn/basics-of-space-flight/chapter6-2/)
Because frequency, wavenumber, and wavelength in vacuum are tied together by the exactly defined speed of light, a measurement of any one of them is an equally accurate determination of the others.[6](https://physics.nist.gov/Pubs/AtSpec/AtSpec.PDF)
## Speed
The speed of light in vacuum is so precisely known, and so fundamental, that its value is fixed by definition: \( c_0 = 299\,792\,458 \ \mathrm{m/s} \).[3](https://physics.nist.gov/cuu/pdf/sp330.pdf) In a medium, light travels more slowly, and this change of speed is what makes refraction happen.[10](https://openstax.org/books/physics/pages/16-2-refraction)
## Interaction with matter: rays, reflection, and refraction
When light interacts with objects several times larger than its wavelength, it travels in straight lines and acts like a ray; since the wavelength of visible light is less than a micron, it behaves as a ray in most everyday situations, from mirrors to lenses.[7](https://openstax.org/books/college-physics/pages/25-1-the-ray-aspect-of-light) This ray-based description is called geometric optics, and it is governed by two laws: the law of reflection, for light bouncing off matter, and the law of refraction, for light passing into matter.[7](https://openstax.org/books/college-physics/pages/25-1-the-ray-aspect-of-light) Reflection and refraction are the basic properties behind all optical instruments, from eyeglasses to giant astronomical telescopes.[11](https://openstax.org/books/astronomy/pages/5-3-spectroscopy-in-astronomy)
**Refraction.** When light passes a boundary between materials of different composition, its direction changes, a bending called refraction; the cause is the change in the light's speed between the media.[10](https://openstax.org/books/physics/pages/16-2-refraction) [Snell's law](https://www.edgepedia.org/snells-law) relates the angles and the indices of refraction: \( n_1 \sin\theta_1 = n_2 \sin\theta_2 \).[10](https://openstax.org/books/physics/pages/16-2-refraction) Because the index of refraction varies with wavelength, different wavelengths refract differently, which spreads white light into a spectrum in a prism or a raindrop; the frequency of the light, however, is not affected by refraction and remains constant.[10](https://openstax.org/books/physics/pages/16-2-refraction) [Isaac Newton](https://www.edgepedia.org/isaac-newton) demonstrated the underlying fact in 1672, showing in his first paper to the Royal Society that sunlight, which looks white, is actually a mixture of all the colors of the rainbow.[11](https://openstax.org/books/astronomy/pages/5-3-spectroscopy-in-astronomy)
When light passes from a medium into one with a lower index of refraction, the ray bends away from the perpendicular; beyond a certain incident angle, the critical angle, all the light reflects back into the first medium, a condition called total internal reflection.[10](https://openstax.org/books/physics/pages/16-2-refraction) For a diamond-to-air surface the critical angle is only 24.4°, so light that enters a diamond has trouble getting back out, which is why diamonds sparkle.[10](https://openstax.org/books/physics/pages/16-2-refraction)
**Transmission, reflection, absorption.** Whether a material transmits, reflects, or absorbs a wave depends on the frequency: glass is transparent to visible light but largely opaque to ultraviolet radiation, while human skin is opaque to visible light but transparent to X-rays.[8](https://openstax.org/books/college-physics-2e/pages/24-3-the-electromagnetic-spectrum) Across the spectrum, light waves can be transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the object they encounter.[9](https://science.nasa.gov/ems/)
## Measuring light: radiometry and photometry
Two parallel systems of measurement apply to light. Radiometry measures optical radiation, the electromagnetic radiation between \( 3 \times 10^{11} \ \mathrm{Hz} \) and \( 3 \times 10^{16} \ \mathrm{Hz} \), corresponding to wavelengths from 10 nm to 1000 m and covering the ultraviolet, visible, and infrared regions; its units are purely physical, such as the watt for radiant flux and watt per square meter for irradiance.[4](https://www.nist.gov/publications/radiometry-and-photometry-review-vision-optics) Photometry measures light in the way the human visual system experiences it: it is restricted to the visible region, from 360 nm to 830 nm, and every quantity is weighted by the spectral response of the eye.[4](https://www.nist.gov/publications/radiometry-and-photometry-review-vision-optics) Its typical units are the lumen (luminous flux), the candela (luminous intensity, an [SI base unit](https://www.edgepedia.org/si-base-unit)), the lux (illuminance), and candela per square meter (luminance).[4](https://www.nist.gov/publications/radiometry-and-photometry-review-vision-optics) • [12](https://www.nist.gov/programs-projects/photometry) Photometry is therefore the appropriate system for evaluating light sources intended to be seen by humans, such as lighting, signaling, and displays.[12](https://www.nist.gov/programs-projects/photometry)
## Light in astronomy
Astronomy is built on decoding light. Because each chemical interaction leaves a predictable imprint on the light emitted or absorbed, spectroscopy can identify materials on Earth and in space from color alone; wavelength and energy are effectively the same thing, and eyes and detectors alike read differences in them as differences in color.[2](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/) Astronomers now combine light with other cosmic messengers, such as particles and gravitational waves, in multimessenger astronomy, but light remains the messenger humans sense directly.[13](https://science.nasa.gov/universe/sensing-the-universe/)
## References
1. [Chapter 6: Electromagnetics, NASA Science (Basics of Space Flight)](https://science.nasa.gov/learn/basics-of-space-flight/chapter6-2/)
2. [Spectroscopy 101 – Light and Matter, NASA Science](https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-light-and-matter/)
3. [The SI unit of length: practical realization of the meter (SP 330), NIST](https://physics.nist.gov/cuu/pdf/sp330.pdf)
4. [Ohno, Y. (2000). Radiometry and Photometry: Review for Vision Optics, OSA Handbook of Optics, NIST](https://www.nist.gov/publications/radiometry-and-photometry-review-vision-optics)
5. [Remote Sensing, NASA Earth Observatory](https://science.nasa.gov/earth/earth-observatory/remote-sensing/?trk=article-ssr-frontend-pulse_little-text-block)
6. [Atomic Spectroscopy: An Introduction, NIST](https://physics.nist.gov/Pubs/AtSpec/AtSpec.PDF)
7. [The Ray Aspect of Light, College Physics, OpenStax](https://openstax.org/books/college-physics/pages/25-1-the-ray-aspect-of-light)
8. [The Electromagnetic Spectrum, College Physics 2e, OpenStax](https://openstax.org/books/college-physics-2e/pages/24-3-the-electromagnetic-spectrum)
9. [The Electromagnetic Spectrum Video Series & Companion Book, NASA Science](https://science.nasa.gov/ems/)
10. [Refraction, Physics, OpenStax](https://openstax.org/books/physics/pages/16-2-refraction)
11. [Spectroscopy in Astronomy, Astronomy, OpenStax](https://openstax.org/books/astronomy/pages/5-3-spectroscopy-in-astronomy)
12. [Photometry, NIST](https://www.nist.gov/programs-projects/photometry)
13. [Sensing the Universe, NASA Science](https://science.nasa.gov/universe/sensing-the-universe/)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Waves and optics › Physical and wave optics › Scattering, absorption, and radiative transfer › Absorption, transmittance, and opacity*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license