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
extramission
theory was due to the founder of rhetoric,
Empedocles
(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.
Logical inconsistencies with that theological viewpoint were summarily dismissed.
Curiously, Empedocles
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.)
Following
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
(55 BC).
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
minimal route.
The propagation of light along straight lines in free space
can also be deduced from that principle, which would later be generalized by
Fermat.
In AD 1015,
Alhazen (965-1039)
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.)
Galileo (1564-1642)
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
1676
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 few centuries to settle, is that light
travels more slowly 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 Arago
had suggested in 1838.
The corpuscular nature of light was championed by
Isaac Newton (1643-1727)
against
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
water
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 longitudinal vibrations
analogous to sound pressure in a gas. However, this was disproved in 1809
by one of the first Polytechniciens,
Etienne Louis Malus (1775-1812, X1794) who discovered that light
can be polarized, (which is to say that it's at least partly
transverse, possibly wholly so).
Indeed in 1821,
François Arago (1786-1853; X1803) and
Augustin Fresnel (1788-1827; X1804)
duly showed light-waves to be entirely transverse:
They're just superpositions of two orthogonal polarization states
which don't interfere with each other and are reflected or refracted
differently (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,
a decisive clue to the electromagnetic nature of light, which
Michael Faraday (1791-1867) had anticipated.
In 1883, the Irish physicist George Fitzgerald (1851-1901)
remarked that an oscillating current ought to generate electromagnetic radiation.
This was first demonstrated experimentally by
Heinrich Hertz (1857-1894) in 1888.
At this point, visible light seemed to be just an electromagnetic
wave of very high frequency.
However, Hertz had already discovered in 1887 the
photoelectric effect
which couldn't be described in those terms. New thinking was needed...
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 photoelectric effect (Hertz, 1887)
implies that light consists of those quanta of energy,
which we now call photons.
For this, he received the Nobel Prize in 1921.
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 corpuscles,
massless or not, are wavelike. This motivated the development of
modern Quantum Theory.
Light through the Ages
How the speed of light was determined (16:32)
by Paul (PhysicsHigh, 2019-04-19).
Most defining quantities correspond to physical radiant
measurements, 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 a 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 response 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
to scotopic
conditions (which is of lesser importance for color vision, since
low-light is almost entirely perceived monochromatically).
Luminosity function
|
International
Commission on Illumination
(CIE)
|
CIE 1931 color space
Here, we'll equate color-blindness with its more common form:
Daltonism, the ailment which the
famous chemist John Dalton (1766-1844)
suffered from and was first to describe in sufficient details to initiate ongoing studies.
It is a genetic disease caused by a recessive gene located on the human
X chromosome. Women have two of these and men only one.
The frequency of occurence of that gene is usually quoted to be 8%
(another way of rounding the same result is to state that
that one in 12 men is affected, which would correspond to a prevalence of about 8.33%).
For simplicity hereafter, we'll take the former number of 8% at face value and
quote only the precise numbers that would ensue if this number was rigorously exact:
8% of men are affected, since they have the disease
whenever they carry the bad gene on their only X chromosome. Only
0.64% of the women suffer from the disease because they do so only if
both of their X chromosomes carry the wrong gene (which happens in 8% of 8% of cases).
That simple genetic arithmetic implies that 84.64% of women (92% of 92%)
have two dominant genes and can't have a color-bling daughter.
The heterozygote women (14.72% of women) can't have a color-blind daughter unless
the father is color-blind (in which case 50% of their female offsprings will be color blind).
For some obscure reason, it seems that a small fraction of those heterozygote women
develop a fourth type of functional cones which give them the rare superhuman
tetrachromatic vision described below:
They experience permanently in broad daylight the four-color vision that most of us can only perceive
fleetingly in twilight condition (which can be a mixed blessing).
Photopigments
|
Color blindness
