The Physics
Opus in profectus

The Nature of Light

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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.


Light is produced by one of two methods…

Types of luminescence with special names
type description example(s) CIE definition
photo­luminescence caused by absorption of optical radiation (IR, light, UV) includes fluorescence and phosphorescence luminescence caused by absorption of optical radiation
fluor­escence radiation is emitted within 10 ns of excitation fluorescent lamps, black light inks emission of optical radiation when a substance is exposed to any type of electromagnetic radiation, where the emitted radiation generally appears within 10 ns after the excitation. Fluorescence is due to an "allowed" transition generally from an excited singlet state to a ground singlet state.
phosphor­escence emission is "delayed" after excitation by more than 10 ns "glow-in-the-dark" posters, etc. delayed emission of optical radiation which appears 10 ns or longer after the excitation. Technically, the term "phosphorescence" should be used only for the delayed emission due to a "forbidden transition" from an excited triplet state to a ground singlet state.
electro­luminescence caused by the action of an electric field some outdoor advertising, some nightlights luminescence caused by the action of an electric field in a gas or in a solid material
radio­luminescence caused by the absorption of x-rays or radioactive radiation mid-century "radium dial" wristwatches luminescence caused by x-rays or radioactive radiation
chemi­luminescence caused by energy released in a chemical reaction glow sticks luminescence caused by energy released by a chemical reaction
bio­luminescence chemiluminescence in living organisms fireflies, anglerfishes chemiluminescence occurring in living organisms
tribo­luminescence occurs when a solid is rubbed or scratched (or ripped or crushed) peeling transparent tape, biting Wint-O-Green Life Savers luminescence caused by the action of mechanical forces
piezo­luminescence occurs when a solid is under changing mechanical stress    
thermo­luminescence occurs when a previously excited material is heated thermoluminescence dating thermally activated luminescence luminescence occurring when a previously excited luminescent material is heated
cathodo­luminescence caused by the impact of electrons CRT displays in 20th century TVs and computers luminescence caused by the impact of electrons on certain types of luminescent materials, such as the coating on a television screen
anti-Stokes luminescence emitted radiation has a shorter wavelength than the absorbed radiation   photoluminescence whose radiation is located in a spectral region of shorter wavelengths than that of the exciting radiation
sonoluminescence caused by imploding bubbles in a liquid formed by sound waves shrimpo­luminescence: pistol shrimp, mantis shrimp  


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, 1638 Galileo 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. 7 December 1676. 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.

other characteristics

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

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

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.

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

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.