Because it's directly accessible to human senses,
visible light has been an object of meditation
ever since human beings became capable of meditating.
Light was once thought to propagate
from the eyes of the observer to
the observed object. This
theory was due to the founder of rhetoric,
(c. 492-432 BC) who is also responsible for the long-held
belief that Nature could be explained in terms of
four elements (Earth, Water, Air and Fire).
He preached that human vision was primarily due to the Fire
that the goddess Aphrodite had put into the human eye.
Other obvious components were deemed only of secondary importance...
also argued (correctly) that light should propagate
at a finite speed, which is extremely difficult to reconcile with
the wrong direction of propagation which he advocated.
(This contradiction was noted in Euclid's
Optica, around 300 BC.)
Aristotle (384-322 BC),
most scholars thought that light was an instantaneous phenomenon.
This helped mask the inadequacy of their religious belief
in the weird causation of light described above (thus delaying its demise).
Epicurean physics is best known from its presentation by
Lucretius (c. 99-55 BC)
in De Rerum Natura
Its atomistic take on light
anticipates modern developments but was largely ignored at the time.
In Catoptrica (c. AD 60)
Hero of Alexandria
(c. AD 10-75) remarked that the law of reflection
used by Euclid (the angle of reflection is equal to the angle of incidence)
can be derived from a Principle of Least Length,
according to which light should travel along a
The propagation of light along straight lines in free space
can also be deduced from that principle, which would later be generalized by
In AD 1015,
finally put an end to centuries of confusion by using his
camera obscura to dismiss the need for an
hypothetical beam emanating from the eye.
He concluded that sight must be entirely
due to light coming into the eye from outside sources.
(Alhazen's use of Occam's razor
directly inspired Newton.)
attempted to determine the speed of light experimentally,
but his method was too crude to produce definite results because
it involved human reaction times.
Galileo was simply sending light signals to a distant assistant,
who was responding with his own lantern as fast as he could.
The finiteness of the speed of light was first established in
by the Danish astronomer
Ole Rømer (1644-1710).
By observing the motion of Io around Jupiter,
Roemer deduced it should take about 22 minutes
for light to travel a distance equal to the mean diameter of the orbit of the Earth
(this time is now known to be 24% less: 16 minutes and 38.01 seconds).
Another issue, which took a couple centuries to settle, is that light
travels slower in a denser medium.
This was correctly proposed by
Fermat in 1655 as part of his
Principle of Least Time
to provide a unified explanation for Hero's law of reflection
and Snell's law of refraction
(which had been discovered independently by Harriot in July 1601,
by Snell in 1621 and by Descartes in 1637).
However, the issue remained controversial among scientists
until 1850, when the celerities of light in water and air were actually
compared directly by
Fizeau and Foucault,
using a protocol which Arago
had suggested in 1838.
The corpuscular nature of light was championed by
Isaac Newton (1643-1727)
Christiaan Huygens (1629-1695)
who argued it was made of waves (subject to diffraction).
Newton also established that white light is the superposition of different colors
of light and that the index of refraction of light in glass or in
depends on its color (that's called dispersion,
it explains rainbows).
In 1802, Thomas Young (1773-1829)
demonstrated interferences of light waves, which seemed to settle the issue in favor
of Huygens... for a while.
Young thought that light-waves were mostly longitudinal but
Augustin Fresnel (1788-1827; X1804) and
François Arago (1786-1853; X1803)
showed, in 1821, that light-waves are entirely transversal.
They consist of a superposition of two orthogonal states of
polarization which do not interfere with each other and are refracted
differentlty (Fresnel equations, 1821).
In 1861, James Clerk Maxwell (1831-1879)
found that a
dynamic generalization of Ampère's law
makes the speed of light appear in the laws of electromagnetism,
providing a decisive clue to the electromagnetic nature of light anticipated by
Michael Faraday (1791-1867).
In 1883, the Irish physicist George FitzGerald (1851-1901) remarked that an oscillating current
ought to generate electromagnetic radiation.
This was demonstrated experimentally by
Heinrich Hertz (1857-1894) in 1888.
At this point, visible light could simply be thought of as an electromagnetic
wave of very high frequency.
However, in 1887, Hertz himself had discovered the
which could not be fully described in those terms...
In 1900, Max Planck (1858-1947)
explained the experimental shape of the
blackbody spectrum by postulating that
matter could only exchange energy with the electromagnetic field in discrete
lumps, called quanta, whose energy was
proportional to the frequency of radiation.
In 1905, Albert Einstein (1879-1955)
showed how the features of the photoelectric effect (Hertz, 1887)
imply that light consists of those quanta of energy
(which we now call photons).
Photons are the particles of light envisioned by Newton.
Light is thus both wavelike and corpuscular.
In 1923, Louis de Broglie
(1892-1987) suggested that all corpuscules,
massless or not, are wavelike. This motivated the development of
modern Quantum Theory.
Light through the Ages
All luminous quantities correspond to a physical radiant
measurement, involving net exchanges of energy at all frequencies.
However, luminous quantities are weighted according to the spectral
response of the normal human eye. (Radiant quantities aren't
dependent on human perception.)
The basic calibration between radiant and luminous units is determined for
a specific monochromatic light. In principle,
any frequency could have been used for that purpose, but
540 THz was chosen as good approximation to the peak sensitivity
of the human retina to bright light.
The conversion factor between the luminous and radiant
units of power (the lumen and the watt,
respectively) at that frequency was defined to be 683 lm/W
to match historical definitions of luminous quantities (based on standard candles).
The translation of a radiant quantity into a luminous one, or vice-versa,
is ultimately based on subjective determinations of what constitutes the same brightness
for light sources of different colors.
This was standardized scientifically so that most of the population will
be in rough agreement over the result.
Universal agreement is not possible because of genetic differences between individuals:
About 5% of the population (mostly males) suffer from
some kind of color blindness of genetic origin.
A tiny fraction (exclusively females) are genetically endowed
with the capacity to perceive an extra dimension
of color, like birds do.
Standard luminous unis are based on the average spectral response of the
retina in the majority of human beings, not affected by any of the above.
Because the different classes of receptors in the human eye behave differently, the human
eye has a different spectral reponse in bright light (photopic conditions)
and low light (scotopic conditions) or anything inbetween
(mesotopic conditions, twilight).
For standardization purposes, the average photopic spectral sensitivity of the human eye
was determined in 1931 by the International Commission on Illumination
(Commission Internationale de l'Eclairage, abbreviated CIE,
now based in Vienna, Austria).
In 1951, the CIE adopted the curve corresponding
conditions (which is of lesser importance for color vision, since
low-light is almost entirely monochromatic.
Commission on Illumination
CIE 1931 color space