- 4222 - ISAAC NEWTON - light discovered in rainbows? Isaac Newton (1642-1727) an English mathematician, physicist and astronomer, author of the theory of “terrestrial universal attraction” dispersing light with a glass prism, and engraving colorized result as a property of light.
---------------- 4222 - ISAAC NEWTON - light discovered in rainbows?
- Isaac Newton's
ingenious experiment using prisms helped us understand light. The beauty and
majesty of rainbows have inspired awe in humans for millennia, but it wasn't
until Isaac Newton's groundbreaking work unlocking the secrets of light did we
truly begin to understand how they form.
-
- This puzzle of the
rainbow was resolved in the seventeenth century through the work of the
scientist who some regard as the greatest ever to have lived. In 1666, Isaac
Newton, then a 23-year-old Cambridge graduate, performed an experiment with
light that transformed our understanding.
-
- While it was
thought that the bar of rainbow colors produced when white light (like
sunlight) travels through a glass prism is caused by some property of the prism
that alters the light, Newton showed the colors are already inherent in the
light itself. -
-
- Newton did the
experiment at his family home in Woolsthorpe, Lincolnshire, to which he had
returned to escape the Great Plague that ravaged England in 1665. It did not require any fancy apparatus, just
a few prisms, which could be bought almost as trinkets at markets (although he
needed good-quality ones!).
-
- Newton didn't
report his results until six years later, when he sent an account to the Royal
Society in London, the intellectual center of "experimental
philosophy" in the mid-century.
-
- He was famously
reluctant to disclose the outcomes of his studies, and had to be cajoled into
writing down his celebrated laws of motion and theories about the motions of
the planets in his masterwork the Principia Mathematica in 1687.
-
- In this book in
which he recorded his experiments and theories about light, “Opticks”, was
finally published in 1704.
-
- Newton begins his
1672 account by relating his surprise that the colored spectrum produced by his
prism was rectangular in shape rather than circular, as the received laws of Refraction
would lead one to expect. It seems a rather trifling question, especially to
lead to such profound conclusions. In fact, his "surprise" is hard to
credit, for this effect of a prism was well known, not least to Newton himself,
who had been fascinated with such instruments since he was a boy.
-
- One can imagine
Newton almost literally playing with prisms, screens, and lenses until he found
a configuration that allowed him to formulate and investigate some definite
hypotheses. Newton once famously claimed
that "I feign no hypotheses," but in truth one can hardly do science
at all without them. But only Newton saw
what this implies: that refraction is then all there is to it
-
- Newton closed the
"window-shuts" of his room, admitting a single narrow beam of
sunlight through a hole, which passed into the prism. In the crucial
experiment, Newton investigated the nature of the light after it exited the
prism. If the light became colored because of some transformation produced by
the prism, then a passage through a second prism might be expected to alter the
light again.
-
- Newton used a
board with a hole in it to screen off all the spectrum except for a single
color and then allowed that colored light to pass through the second prism. He
found that this light emerged from the second prism refracted, bent at an
angle, but otherwise unchanged. In other words, a prism seems only to bend
(refract) light, leaving it otherwise unaltered. But it does so to different
degrees (at different angles) for different colors.
-
- This in itself was
nothing new: the Anglo-Irish scientist Robert Boyle had said as much in his
1664 book "Experiments and Considerations Touching Colours," which
Newton had read. But only Newton saw what this implies: that refraction is then
all there is to it.
-
- The colors
themselves are already in the white light, and all the prism does is to
separate them out. As he put it, "Light consists of Rays differently
refrangible". The colors of the spectrum, then, "are not
Qualifications [alterations] of Light …, but Original and connate
properties." That was a bold interpretation: sunlight was not, so to
speak, elemental, but compound.
-
- To test this idea,
Newton used a lens to refocus a many-hued spectrum into a single, merged beam
which, he observed, was white. He also passed this reconstituted beam through
another prism to reveal that it could again be split into a spectrum just as
before.
-
- Newton explained
how his observations could account for the rainbow, produced by the refraction
and reflection of light through raindrops that act as tiny prisms. The colors
of everyday objects, he added, arise because they reflect "one sort of
light in greater plenty than another."
-
- The results
explained the defects of lenses (Newton himself had become adept at making
these by grinding glass), whereby refraction of different colors produces a
defocusing effect called chromatic aberration. The Royal Society's secretary
Henry Oldenburg told Newton that his report was met with "uncommon
applause" when read at a gathering in February 1672. But not everyone
appreciated it.
-
- After the paper
was published in the society's Philosophical Transactions, its in-house curator
of experiments, Robert Hooke, who considered himself an expert on optics,
presented several criticisms (which we can now see were mistaken). Newton
replied with lofty condescension, igniting a long-standing feud between the two
men.
-
- One problem is
that Newton's experiments, despite their apparent simplicity, are not easy to
replicate: some, in England and abroad, tried and failed. But they have stood
the test of time, a testament to the power of experiment to literally
illuminate the unknown that, in the judgment of philosopher of science Robert
Crease, gives Newton's so-called experimentum crucis "a kind of moral
beauty."
-
-
November 14, 2023 ISAAC NEWTON
- light discovered? 4222
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