From the earliest times it
has been clear that light originates from glowing matter. First and foremost was the sun, then fire, either
natural or man made. In short,
something burning. Clearly, light
cannot be completely understood without understanding the nature of matter. Some steps along the way:
Antiquity
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(c 582 – c 500 B.C.) Pythagoras and his school believed that light is a
stream of particles emitted by glowing objects. The particles impinge on the eye triggering the sensation of
sight.
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(c 427 – c 347 B.C.) Plato and his school believed that light originated,
quite literally, “in the eye of the
beholder”. He thought that rays came
from the eye and bounced back to the observer.
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(384 – 322 B.C.) Aristotle proposed a wave theory of light. Postulating an analogy with acoustics, he proposed that colors could be understood
as harmonics of fundamental wavelengths, just like musical sounds.
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(c 300 B.C.) Some optical properties of lenses such as
magnification were known in ancient times.
The earliest “lenses” were glass spheres filled with water. However,
true glass lenses were not common until the middle ages.
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(287 B.C.) The Greek scientist-philosopher Archimedes is said
to have arranged a series of concave mirrors to focus the rays of the sun on
Roman troops besieging Syracuse, a Greek city in Sicily. Although this is almost certainly not a true
story, it does suggest that the optical properties of curved mirrors were known
in ancient times.
Middle Ages
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(965 – 1039) Alhazen (Ibn Al Haitam) was a Persian scientist who
wrote extensively on optical theory. He
is credited with inventing the camera obscura. The earliest
versions of this device consisted of a darkened room with a small opening which
projected an image on the opposite wall.
Smaller versions were used by Renaissance artists, essentially a camera
without film.
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(c 1214 – c 1294) Roger Bacon, and English Monk, is credited with inventing spectacles
(eyeglasses), suggesting the telescope and developing the camera obscura
described by Alhazen. In his writings
he envisions a time when “…pictures could be projected into space, into air
where it could become visible for the multitudes.”
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(1608) Hans Lippershey, a Dutch optician, applies for a
patent on the telescope.
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(1609) Galileo uses the telescope for astronomical
observations.
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(1676) Ole Romer measured the speed of light by noting that
the predicted positions of Jupiter’s moons differed from the observed positions
by an amount dependent on the distance between Jupiter and the Earth.
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(1690) Christian Huygens, a Dutch scientist proposed a wave
theory of light. His view was that
light propagated much like sound wave through the “ether”, a mysterious
substance permeating all space.
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(1704) Isaac Newton proposes a “corpuscular” theory of
light. Today this is described as a
“particle” theory . Newton also
used a prism to demonstrate that white light is composed of colors (the spectrum). His basic idea was that the particles of
light excited vibrations in the optic nerve when the entered the eye. Particles corresponding to different colors
would excite different modes of vibration in the optic nerve. So Newton’s model was a sort of wave theory,
it was just that the waves were in the eye, rather than external.
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(1727) Johann Heinrich Schultz showed that certain
silver compounds are darkened when exposed to light. This is the basic mechanism underlying all photographic
processes.
19th Century
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(1800) William Herschel discovers that the solar spectrum
delivers heat in a region beyond visible red (“infrared”)
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(1801) J.W. Ritter discovers that the spectrum also extends
beyond visible violet (“ultraviolet”) by observing a chemical reaction
occurring when reactive substances were placed in a position just beyond the
violet end of the spectrum.
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(1819) John Herschel showed that images produced on silver
salts exposed to sunlight could be made permanent if treated with certain
chemicals containing sulfur. Herschel
coined the word “photography” for this process.
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(1827) John Herschel notes that heated salts give off
distinctive spectra, allowing their identification, even in small quantities.
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(~1828) Josef Fraunhofer had published a number of
significant finding by this date. Among
the most important were:
o The discovery of dark lines in the solar spectrum
o The discovery of dark lines in some stars, similar,
though not exactly the same as those in the sun
o The demonstration that the spectra obtained from the
moon and planets were essentially the same as the sun.
o Descriptions of bright line spectra (isolated lines
of color produced by some substances) and the possible correlation between them
and the dark lines in the solar spectrum.
o the development of the diffraction grating to obtain
spectra
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(1839) Louis Daguerre announces the first practical
photographic process, combining the camera obscura with a panel treated with
light-sensitive salts. Photography will
play a major role in astronomy toward the end of the 19th century
and beyond.
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(1859) First basic understand of spectra given by Wilheim
Bunsen and Gustav Kirchhoff. These principles, today known as Kirchhoff’s Laws
are:
o A hot solid or liquid produces a continuous spectrum
(i.e., a broad band of color from red to violet with no colors missing)
o A hot gas produces a bright-line (also
called emission) spectrum (i.e., isolated lines of color with
many colors missing).
o A continuous spectrum, when passed through a gas
becomes a dark-line (now called absorption)
spectrum (i.e., a continuous spectrum with superimposed dark lines).
o The chemical element producing either bright lines or
dark lines can be identified by noting the positions these lines occupy in the
spectrum. In other words, each chemical
element produces a unique spectrum (a “fingerprint”) which can be used to identify
that element wherever it occurs.
o
The University of Oregon has a great website illustrating absorption and emission spectra for all the chemical elements: Elemental Spectra
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(1859) By analyzing the dark lines in the solar spectrum,
Kirchhoff was able to identify some of the chemical elements present in the
sun.
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(1864) By using a spectroscope at the telescope, William
Huggins was able to show that at least some of the nebulae must be glowing
gasses because they produced bright-line spectra.