Optics

Replica of Newton's second reflecting telescope, which he presented to the Royal Society in 1672[39]

In 1666, Newton observed that the spectrum of colours exiting a prism in the position of minimum
deviation is oblong, even when the light ray entering the prism is circular, which is to say, the prism
refracts different colours by different angles.[40][41] This led him to conclude that colour is a property
intrinsic to light – a point which had, until then, been a matter of debate.
From 1670 to 1672, Newton lectured on optics.[42] During this period he investigated the refraction of
light, demonstrating that the multicoloured spectrum produced by a prism could be recomposed into
white light by a lens and a second prism.[43] Modern scholarship has revealed that Newton's analysis
and resynthesis of white light owes a debt to corpuscular alchemy.[44]
He showed that coloured light does not change its properties by separating out a coloured beam and
shining it on various objects, and that regardless of whether reflected, scattered, or transmitted, the
light remains the same colour. Thus, he observed that colour is the result of objects interacting with
already-coloured light rather than objects generating the colour themselves. This is known
as Newton's theory of colour.[45]

Illustration of a dispersive prism separating white light into the colours of the spectrum, as discovered by
Newton

From this work, he concluded that the lens of any refracting telescope would suffer from
the dispersion of light into colours (chromatic aberration). As a proof of the concept, he constructed a
telescope using reflective mirrors instead of lenses as the objective to bypass that problem.[46]
[47]
 Building the design, the first known functional reflecting telescope, today known as a Newtonian
telescope,[47] involved solving the problem of a suitable mirror material and shaping technique.
Newton ground his own mirrors out of a custom composition of highly reflective speculum metal,
using Newton's rings to judge the quality of the optics for his telescopes. In late 1668,[48] he was able
to produce this first reflecting telescope. It was about eight inches long and it gave a clearer and
larger image. In 1671, the Royal Society asked for a demonstration of his reflecting telescope.
  Their interest encouraged him to publish his notes, Of Colours,[50] which he later expanded into the
[49]

work Opticks. When Robert Hooke criticised some of Newton's ideas, Newton was so offended that
he withdrew from public debate. Newton and Hooke had brief exchanges in 1679–80, when Hooke,
appointed to manage the Royal Society's correspondence, opened up a correspondence intended to
elicit contributions from Newton to Royal Society transactions, [51] which had the effect of stimulating
Newton to work out a proof that the elliptical form of planetary orbits would result from a centripetal
force inversely proportional to the square of the radius vector. But the two men remained generally
on poor terms until Hooke's death. [52]

Facsimile of a 1682 letter from Isaac Newton to Dr William Briggs, commenting on Briggs' A New Theory of
Vision.

Newton argued that light is composed of particles or corpuscles, which were refracted by
accelerating into a denser medium. He verged on soundlike waves to explain the repeated pattern of
reflection and transmission by thin films (Opticks Bk.II, Props. 12), but still retained his theory of 'fits'
that disposed corpuscles to be reflected or transmitted (Props.13). However, later physicists
favoured a purely wavelike explanation of light to account for the interference patterns and the
general phenomenon of diffraction. Today's quantum mechanics, photons, and the idea of wave–
particle duality bear only a minor resemblance to Newton's understanding of light.
In his Hypothesis of Light of 1675, Newton posited the existence of the ether to transmit forces
between particles. The contact with the Cambridge Platonist philosopher Henry More revived his
interest in alchemy.[53] He replaced the ether with occult forces based on Hermetic ideas of attraction
and repulsion between particles. John Maynard Keynes, who acquired many of Newton's writings on
alchemy, stated that "Newton was not the first of the age of reason: He was the last of the
magicians."[54] Newton's interest in alchemy cannot be isolated from his contributions to science.
[53]
 This was at a time when there was no clear distinction between alchemy and science. Had he not
relied on the occult idea of action at a distance, across a vacuum, he might not have developed his
theory of gravity.
In 1704, Newton published Opticks, in which he expounded his corpuscular theory of light. He
considered light to be made up of extremely subtle corpuscles, that ordinary matter was made of
grosser corpuscles and speculated that through a kind of alchemical transmutation "Are not gross
Bodies and Light convertible into one another, ... and may not Bodies receive much of their Activity
from the Particles of Light which enter their Composition?" [55] Newton also constructed a primitive
form of a frictional electrostatic generator, using a glass globe.[56]
In his book Opticks, Newton was the first to show a diagram using a prism as a beam expander, and
also the use of multiple-prism arrays.[57] Some 278 years after Newton's discussion, multiple-prism
beam expanders became central to the development of narrow-linewidth tunable lasers. Also, the
use of these prismatic beam expanders led to the multiple-prism dispersion theory.[57]
Subsequent to Newton, much has been amended. Young and Fresnel combined Newton's particle
theory with Huygens' wave theory to show that colour is the visible manifestation of light's
wavelength. Science also slowly came to realise the difference between perception of colour and
mathematisable optics. The German poet and scientist, Goethe, could not shake the Newtonian
foundation but "one hole Goethe did find in Newton's armour, ... Newton had committed himself to
the doctrine that refraction without colour was impossible. He, therefore, thought that the object-
glasses of telescopes must forever remain imperfect, achromatism and refraction being
incompatible. This inference was proved by Dollond to be wrong."[58]