THE INFLECTION OR DIFFRACTION OF LIGHT.
In this part of the subject it is absolutely necessary to return to the
theory of undulations with which the present subject was commenced. The
inflection of light offers a third method by which rays of light may be
decomposed and colour produced. The phenomena are extremely beautiful,
although the explanation of them is almost too intricate for a popular
work of this kind.
The cases where colour is produced by inflection are more numerous than
might at first be supposed; thus, if we look at a gaslight or the
setting sun through a wire gauze blind, protecting the eye with a little
tank of dilute ink, a most beautiful coloured cross is apparent. An
extremely thin film of a transparent matter, such as a little naphtha or
varnish dropped on the surface of warm water or soap bubbles, or a very
thin film of glass obtained by blowing out a bulb of red-hot glass till
it bursts, or an exquisitely thin plate of talc or mica, all present the
phenomena of colour, although they are individually transparent, and in
ordinary thicknesses quite colourless.
[Illustration: Fig. 314. The two lenses, with the plate or film of air
between them, and producing seven coloured rings when the lenses are
brought sufficiently close to each other by the screws.]
Sir Isaac Newton brought his powerful intellect to bear on these facts,
and as a preliminary step invented an instrument for measuring the exact
thickness of those transparent substances that afforded colour, and the
apparatus displaying Newton's rings is still a favourite optical
experiment. It consists of a plano-convex lens, A. (Fig. 314), a slice,
as it were, from a globe of glass twenty-eight feet in diameter, or the
radius of whose convex surface is fourteen feet. This plano-convex lens
is placed on another double convex lens, B., whose convex surfaces have
a radius of fifty feet each, consequently the lenses are very shallow,
and the space (C C) included between them being filled with air, can of
course be accurately measured. (Fig. 314.) It is usual to mount the
lenses in brass rings which are brought together with screws, when the
most beautiful coloured rings are apparent, and are produced by the
extreme thinness of the film or plate of air enclosed between the two
lenses; and [Page 329] the relative thicknesses of the plates of air at
which each coloured light is reflected are as follows:--
Red 133 10 millionths of an inch.
Orange 120 " "
Yellow 113½ " "
Green 105½ " "
Blue 98 " "
Indigo 92½ " "
Violet 83½ " "
By dividing an inch into ten millions of parts, and by taking 133 of
such parts, the thickness of the film of air required to reflect the red
ray is obtained, and in like manner the other colours require the minute
thicknesses of air recorded in the table above. When the thickness of
the film of air is about 12/178,000dths of an inch, the colours cease to
become visible, owing to the union of all the separate colours forming
white light, but if the Newton rings are produced in mono-chromatic
light, then a greater number of rings are apparent, but of one colour
only, and alternating with black rings, _i.e._, a dark and a yellow
succeeding each other; this fact is of great importance as an
illustration of the undulatory theory, and demonstrates the important
truth, that _two rays of light may interfere with each other in such a
manner as to produce darkness_.
Sir David Brewster remarks that, "From his experiments on the colours of
thin and of thick plates, Newton inferred that they were produced by a
singular property of the particles of light, in virtue of which they
possess, at different joints of their paths, _fits_ or dispositions to
be reflected from or transmitted by transparent bodies. Sir Isaac does
not pretend to explain the origin of these _fits_, or the cause which
produces them, but terms them _fits of transmission_ and _fits of
reflexion_."
Sir Isaac Newton objected to the theory of undulations because
experiments seemed to show that light could not travel through bent
tubes, which it ought to do if propagated by undulations like sound; and
it was reserved for the late Dr. Young to prove that light could and
would turn a corner, in his highly philosophical experiments
illustrating the inflection or bending in of the rays of light.
Dr. Young placed before a hole in a shutter a piece of thick paper
perforated with a fine needle, and receiving through it the diverging
beams on a paper screen, found that when a slip of cardboard
one-thirtieth of an inch in breadth was held in such a beam of light,
that the shadow of the card was not merely a dark band, but divided into
light and dark parallel bands, and instead of the centre of the shadow
being the darkest part, it was actually white. Dr. Young ascertained
that if he intercepted the light passing _on one side_ of the slip of
card with any opaque body, and allowed the light to pass freely on the
other side of the slip of cardboard, that all the bands and the white
band in the centre disappeared, and hence he concluded that the bands or
fringes within the shadow were produced _by the interference [Page 330]
of the rays bent into the shadow by one side of the card, with the rays
bent into the shadow by the other side_. (Fig. 315).
[Illustration: Fig. 315.]
In order to show how two waves may interfere so as to exalt or destroy
each other, two sets of waves may be propagated on the surface of a
still tank or bath of water, from the two points A A (Fig. 315), the
black lines or circles representing the tops of the waves. It will be
seen that along the lines B B the waves interfere just half way between
each other, so that in all these directions there will be a smooth
surface, provided each set of waves is produced by precisely the same
degree of disturbing force, so as to be perfectly equal and alike in
every respect, and the first wave of one set exactly half a wave in
advance of the first wave of the other, while at the curve in the
direction of all the line C C, the waves coincide, and produce
elevations or undulations of double extent; in the intermediate spaces,
intermediate effects will, of course, be produced.
Professor Wheatstone has invented some very simple and beautiful
acoustic apparatus for the purpose of proving that the same laws of
interference exist also in sound, which, as already stated, consists in
the vibrations or undulation of the particles of air.
[Page 331]
The nature and effects of interference are also admirably illustrated by
the following models of Mr. Charles Woodward, President of the Islington
Scientific Institution, and to whom we have already alluded.
[Illustration: Fig. 316.--No. 1. A model of waves with moveable
rods.--No. 2. A model of fixed waves.--No. 3. Intensity of waves doubled
by the superposition and coincidence of two equal systems.--No. 4. Waves
neutralized by the superposition and interference of two equal systems,
the raised part of one wave accurately fitting into and making smooth
the hollow of the other, illustrating the fact that two waves of light
or sound may destroy each other.]
[Illustration: Fig. 317. Appearance of Newton's rings when produced in
yellow light, 1, 3, 5, 7, being the yellow rings, and 2, 4, 6, 8, the
dark rings. Light by the odd numbers; darkness by the even numbers. The
central spot, where the two surfaces are in contact, is dark.]
Returning again to the coloured rings, we find that Newton discovered
that at whatever thickness of the film of air the coloured ring first
appeared, there would be found at twice that thickness the dark ring, at
three times the coloured, at four times the dark, and so on, _the
coloured rings_ regularly occurring at the _odd numbers_, and the _dark
ones_ at the _even numbers_. This discovery is well illustrated by the
models (Fig. 316); and it maybe noticed at No. 3 that the highest and
the lowest parts of the waves [Page 332] interfere, but coincide and
produce a wave of double intensity; the little crosses of the upper
model are in a straight line with the numbers 1, 3, 5, 7, and are
supposed to represent the coloured rings, whilst in No. 4 the upper
series of waves is half an undulation in advance of the lower; and if
the eye is again directed from the little crosses downward, the figures
2, 4, 6, 8, even numbers, are apparent, and represent the dark rings,
when the waves of light destroy each other. The phenomena of thin
plates, such as colours from soap bubbles, and the films of varnish, are
well explained by the law of interference. The light reflected from the
second surface of the film of air (which must of course, however thin,
have two surfaces, viz., a upper and a lower one) interferes with the
light reflected from the first, and as they come from different points
of space, one set of waves is in advance of the other, No. 4, Fig. 316;
they reach the eye with different lengths of paths, and by their
_interference_ form alternately the luminous and dark fringes, bands, or
circles. Bridge's diffraction apparatus, manufactured only by Elliott
Brothers, offers itself specially as a most beautiful drawing-room
optical instrument. The purpose of this apparatus is to illustrate in
great variety, and in the most convenient and compact form, the
phenomena of the diffraction or interference of light. This is attained
by the assistance of photography. Transparent apertures in an opaque
collodion film are produced on glass, and a point of light is viewed
through the apertures. [Page 333] The forms of the apertures are
exceedingly various,--triangles, squares, circles, ellipses, parabolas,
hyperbolas, and combinations of them, besides many figures of fanciful
forms, are included in the set. When an image of the sun is viewed
through these apertures, figures of extraordinary beauty, both of form
and colour, are produced; and of each of these many variations may be
obtained by placing the eye-glass of the telescope at different
distances from the object glass. Many of the figures produced,
especially when the telescope is out of focus, might suggest very useful
hints to those concerned in designing patterns. Although the phenomena
are chiefly of interest to the student of science, in consequence of
their bearing on theories of light, yet their beauty and variety render
them amusing to all. A few words on the mode of using the apparatus may
be of service. (Fig. 318.)
[Illustration: Fig. 318. Elliott Brothers' diffraction apparatus.]
Choose a very bright day, for then only can the apparatus be used. Place
the mirror in the sun, and let the light be reflected on the back of the
blackened screen. The lens which is inserted into this screen will then
form an exceedingly bright image of the sun. Then at the distance of not
less than twelve feet, clamp the telescope to a table in such a position
as to view the image thus formed. Put the eccentric cap on the end of
the telescope, clean the glass objects carefully, and attach them to the
cap so that they may be turned each in order before the telescope. In
this manner, all those which consist of a series of figures may be
viewed. Then detach the eccentric cap, and replace it by the other. Into
it place any of the single objects. In viewing some of the figures,
brightness is advantageous--in others, delicacy; in the former case, let
the lens of long focus be inserted in the screen--in the latter case,
that of shorter focus. In every case, let the phenomena be observed not
only when the telescope is in focus, but also when the eye-glass is
pushed in to various distances.
Mr. Warren de la Rue has ingeniously taken advantage of the colours
produced by thin films of varnish, and actually _fixed_ the lovely
iridescent colour produced in that manner on highly polished paper,
which is termed "iridescent paper." A tank of warm water at 80° Fahr.,
about [Page 334] six inches deep, and two feet six inches square, is
provided, and a highly glazed sheet of white or black paper being first
wetted on a perforated metallic plate, is then sunk with the plate below
its surface, care being taken to avoid air bubbles. A peculiar varnish
is then allowed to trickle slowly down a sort of tongue of metal placed
in the middle of one of the sides of the tank, and directly the varnish
touches the surface of the water it begins to spread out in exquisitely
thin films, and by watching the operation close to a window and skimming
away all the imperfect films, a perfect one is at last obtained, and at
that moment the paper lying on the metal plate is raised from the bottom
of the tank, and the delicate film of varnish secured. When dry, the
iridescent colours are apparent, and the paper is employed for many
ornamental purposes. An extremely simple and pretty method of producing
Newton's rings has been invented by Reade, and is called "Reade's
iriscope." A plate of glass of any shape (perhaps circular is the best)
is painted on one side with some quickly drying black paint or varnish,
and after the other side has been cleaned, it is then rubbed over with a
piece of wet soap, and this is rubbed off with a clean soft duster. A
tube of about half an inch in diameter, and twelve inches long, is
provided, and is held about one inch above the centre of the soaped side
of the glass plate, and directly the breath is directed down the tube on
the glass, an immense number of minute particles of moisture are
deposited on the glass, and these by inflection decompose the light, and
all the colours of the rainbow are produced. (Fig. 319.)
[Illustration: Fig. 319. Reade's iriscope.]
The iridescent colours seen upon the surface of _mother-of-pearl_, which
Mr. Simonds' excellent commercial dictionary tells us is "the name for
the iridescent shell of the pearl oyster, and other molluscs," are
referrible to fine parallel lines formed by its texture, and are
reproducible, according to Brewster's experiments, by taking impressions
of them in soft wax. The gorgeous colours of certain shells and fish,
the feathers of birds, Barton's steel buttons, are not due to any
inherent _pigment_ or colouring matter that could be extracted from
them, but are owing either to the peculiar fibrous, or parallel-lined,
or laminated (plate-like) surfaces upon which the light falls, and being
reflected in paths of different lengths, interference occurs, and
coloured light is produced.
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