The Nature of Light – The Physics Hypertextbook (2024)



Light is a transverse, electromagnetic wave that can be seen by the typical human. The wave nature of light was first illustrated through experiments on diffraction and interference. Like all electromagnetic waves, light can travel through a vacuum. The transverse nature of light can be demonstrated through polarization.

  • In 1678, Christiaan Huygens (1629–1695) published Traité de la Lumiere, where he argued in favor of the wave nature of light. Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase.
  • Thomas Young (1773–1829) and Augustin-Jean Fresnel (1788–1827) disproved Newton's corpuscular theory.


Light is produced by one of two methods…

  • Incandescence is the emission of light from "hot" matter (T≳800K).
  • Luminescence is the emission of light when excited electrons fall to lower energy levels
    (in matter that may or may not be "hot").
Types of luminescence with special names
photo­luminescencecaused by absorption of optical radiation (IR, light, UV)includes fluorescence and phosphorescence
fluor­escenceradiation is emitted within 10ns of excitationfluorescent lamps, black light inks
phosphor­escenceemission is "delayed" after excitation by more than 10ns"glow-in-the-dark" posters, etc.
electro­luminescencecaused by the action of an electric fieldsome outdoor advertising, some nightlights
radio­luminescencecaused by the absorption of x-rays or radioactive radiationmid-century "radium dial" wristwatches
chemi­luminescencecaused by energy released in a chemical reactionglow sticks
bio­luminescencechemiluminescence in living organismsfireflies, anglerfishes
tribo­luminescenceoccurs when a solid is rubbed or scratched (or ripped or crushed)peeling transparent tape, biting Wint-O-Green Life Savers
thermo­luminescenceoccurs when a previously excited material is heatedthermoluminescence dating
cathodo­luminescencecaused by the impact of electronsCRT displays in 20th century TVs and computers


Just notes so far. The speed of light in a vacuum is represented by the letter c from the Latin celeritas — swiftness. Measurements of the speed of light.

Veramente non l'ho sperimentata, salvo che in lontananza piccola, cioè manco d'un miglio, dal che non ho potuto assicurarmi se veramente la comparsa del lume opposto sia instantanea; ma ben, se non instantanea, velocissima….In fact I have tried the experiment only at a short distance, less than a mile, from which I have not been able to ascertain with certainty whether the appearance of the opposite light was instantaneous or not; but if not instantaneous it is extraordinarily rapid….
Galileo Galilei, 1638Galileo Galilei, 1638

Ole Rømer (1644–1710) Denmark. "Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des Sciences." Journal des Scavans. 7December1676. Rømer's idea was to use the transits of Jupiter's moon Io to determine the time. Not local time, which was already possible, but a "universal" time that would be the same for all observers on the Earth, Knowing the standard time would allow one to determine one's longitude on the Earth — a handy thing to know when navigating the featureless oceans.

Unfortunately, Io did not turn out to be a good clock. Rømer observed that times between eclipses got shorter as Earth approached Jupiter, and longer as Earth moved farther away. He hypothesized that this variation was due to the time it took for light to travel the lesser or greater distance, and estimated that the time for light to travel the diameter of the Earth's orbit, a distance of two astronomical units, was 22 minutes.

  • The speed of light in a vacuum is a universal constant in all reference frames.
  • The speed of light in a vacuum is fixed at 299,792,458m/s by the current definition of the meter.
  • The speed of light in a medium is always slower the speed of light in a vacuum.
  • The speed of light depends upon the medium through which it travels.The speed of anything with mass is always less than the speed of light in a vacuum.

other characteristics

The amplitude of a light wave is related to its intensity.

  • Intensity is the absolute measure of a light wave's power density.
  • Brightness is the relative intensity as perceived by the average human eye.

The frequency of a light wave is related to its color.

  • Color is such a complex topic that it has its own section in this book.
  • Monochromatic light is described by only one frequency.
    • Laser light is effectively monochromatic.
    • There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
    • Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light"
    • Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light"
  • Polychromatic light is described by many different frequencies.
    • Nearly every light source is polychromatic.
    • White light is polychromatic.

A graph of relative intensity vs. frequency is called a spectrum (plural: spectra).
Although frequently associated with light, the term can be applied to any wave phenomena.

  • A continuous spectrum is one in which every frequency is present within some range.
    • Blackbody radiators emit a continuous spectrum.
  • A discrete spectrum is one in which only a well defined set of isolated frequencies are present.
    (A discrete spectrum is a finite collection of monochromatic light waves.)
    • The excited electrons in a gas emit a discrete spectrum.

The wavelength of a light wave is inversely proportional to its frequency.

  • Light is often described by it's wavelength in a vacuum.
  • Light ranges in wavelength from 400nm on the violet end to 700nm on the red end of the visible spectrum.

Phase differences between light waves can produce visible interference effects.
(There are several sections in this book on interference phenomena and light.)

Leftovers about animals.

  • Falcon can see a 10 cm. object from a distance of 1.5 km.
  • Fly's Eye has a flicker fusion rate of 300/s. Humans have a flicker fusion rate of only 60/s in bright light and 24/s in dim light. The flicker fusion rate is the frequency with which the "flicker" of an image cannot be distinguished as an individual event. Like the frame of a movie… if you slowed it down, you would see individual frames. Speed it up and you see a constantly moving image. Octopus' eye has a flicker fusion frequency of 70/s in bright light.
  • Penguin has a flat cornea that allows for clear vision underwater. Penguins can also see into the ultraviolet range of the electromagnetic spectrum.
  • Sparrow Retina has 400,000 photoreceptors per square. mm.
  • Reindeer can see ultraviolet wavelengths, which may help them view contrasts in their mostly white environment.
The Nature of Light – The Physics Hypertextbook (2024)
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