LIGHT AND ITS SOURCES—WHAT IS LIGHT?—VELOCITY OF LIGHT—REFLECTION AND
REFRACTION—RELATIVE VALUE OF LIGHTS.
The subject of Light and the science of Optics are so interesting to
all of us that some short history of light is necessary before we can
enter upon the scientific portion of the subject. The nature of the
agent (as we may term light) upon which our sight depends has employed
man’s mind from a very early period. The ancients were of opinion that
the light proceeded _from_ the eye to the object looked at. But they
discovered some of the properties of light. Ptolemy of Alexandria, who
was born A.D. 70, made some attempts to discover the law of
Refraction; and we are informed that Archimedes set the Roman fleet
on fire with burning-glasses at Syracuse. The Arabian treatise of
Alhagen, in 1100 A.D., contains a description of the eye and
its several parts; and the writer notices refraction and the effects of
magnifying glasses (or spectacles). Galen, the physician, practically
discovered the principle of the stereoscope, for he laid down the law
that our view of a solid body is made up of two pictures seen by each
eye separately.
Still the science of optics made little progress till the law
determining the path of a ray of light was made known, and the laws
of refraction discovered. Refraction means that a ray is deflected
from its straight course by its passage from one transparent medium
to another of different density. The old philosophers found out the
theory of sound, and they applied themselves to light. Newton said
light consisted of minute particles emanating from luminous bodies.
Huyghens and Euler opposed Newton’s theory of the emission of light;
and it was not till the celebrated Thomas Young, Professor at the Royal
Institution, grappled with the question that the undulating or wave
theory of light was found out. He based his investigations upon the
theory of sound waves; and we know that heat, light, and sound are most
wonderfully allied in their manner of motion by vibration. But he was
ridiculed, and his work temporarily suppressed by Mr. Brougham.
Light, then, is a vibratory motion (like sound and heat), a motion
of the atoms of our ether. But how is the motion transmitted? Sound
has its medium, air; and in a vacuum sounds will be very indistinctly
heard, if heard at all. But what is the medium of communication of
light? It is decided that light is transmitted through a medium called
_ether_, a very elastic substance surrounding us. The vibrations,
Professor Tyndall and other philosophers tell us, of the luminous
atoms are communicated to this ether, or propagated through it in
waves; these waves enter the pupil of the eye, and strike upon the
retina. The motion is thus communicated by the optic nerve to the
brain, and then arises the great primary faculty, Consciousness. We see
light, the waves of which, or ether vibrations, are transversal; air
waves or sound vibrations are longitudinal.
We have spoken of radiant heat. Light acts in the same way through the
ether; and when we consider Sound we shall learn that a certain number
of vibrations of a string give a certain sound, and the quicker the
vibration the shriller the tone. So in light. The more quickly the
waves of luminosity travel to our eye, and the faster they strike it,
the greater the difference in the _colour_, or what we call colour.
Light as we see it is composed of different colours, as visible in
the rainbow. There are seven primary colours in the sunlight, which
is white. These can be divided or “dispersed,” and the shortest rays
of the spectrum are found to be violet, the longest red. It has been
calculated that 39,000 red waves make an inch in length. Light travels
at a rate of nearly 190,000 miles a second, so if we multiply the
number of inches in that distance by the number of red waves, we shall
have millions of millions of waves entering the eye in a single second
of time. The other waves enter more rapidly still, and “the number of
shocks corresponding to the impression of violet is seven hundred and
eighty-nine millions of millions” per second! Or taking the velocity
of light at 186,000 miles in a second, it would be six hundred and
seventy-eight millions of millions (Tyndall). There may be other
colours which we cannot see because the impressions come too rapidly
upon the retina; but the violet impression has been thus accurately
determined. See page 168.
We have seen that heat is a kind of motion of particles in a body—a
vibratory motion which, instead of being apparent to the ear, is
apparent to the eye in rays of light. Thus heat, sound, and light are
all intimately connected in this way. We have also learnt that rays of
light radiate and travel with tremendous speed to our eyes, but without
any shock. There is no feeling connected with the entrance of light to
the eye any more than there is any sensation of sound when entering the
ear, except when the light is vividly and very suddenly revealed, or
when a very piercing sound is heard. Then the nerves are excited, and
a painful sensation is the result; but under ordinary circumstances we
are not physically conscious of the entrance of light or sound.
Heat and light are considered to be one and the same thing in different
degrees of intensity. The sources of light are various. The sun and
fixed stars, heat, electricity, many animals, and some plants, as well
as decaying animal matter, give out light. There are luminous and
non-luminous bodies. The moon is non-luminous, as she derives her light
from the sun, as does the earth, etc.
Light is distributed in rays. These rays are straight in all
directions. The velocity of light is almost inconceivable. It travels
at a rate of 186,500 miles a second. The latest computation with
electric light has given a rate of 187,200 miles a second; but the blue
rays in the light experimented on probably account for the difference,
for blue rays travel quicker by one per cent. than red rays. Römer
first found out the velocity of light, which comes to us from the
sun—ninety millions of miles—in eight minutes. Fizeau calculated the
velocity by means of a wheel, which was set moving with tremendous
speed by making the light pass between the teeth of the wheel and back
again.
When rays of light meet substances they are deflected, and the
phenomena under these circumstances are somewhat similar to the
phenomena of heat and sound. There are three particular conditions of
rays of light: (1) they are absorbed; (2) they are reflected; (3) they
are refracted.
Firstly. Let us see what we mean by light being absorbed; and this is
not difficult to understand, for any “black” substance shows us at once
that all the sunlight is taken in by the black object, and does not
come out again. It does not take in the light and radiate it, as it
might heat. The rose is red, because the rays of light pass through it,
and certain of them are reflected from within. So colour may be stated
to be the rays thrown out by the objects themselves—those they reject
or reflect being the “colour” of the object.
[Illustration: Fig. 85.—Angle of reflection, etc.]
Secondly. Bodies which reflect light very perfectly are known as
mirrors, and they are termed plane, concave, or convex mirrors,
according to form. A plane mirror reflects so that the reflected ray _d
i_ forms the same angle with the perpendicular as the incident ray _r
i_; in other words, the angle of incidence is always equal to the angle
of reflection, and these rays are perpendicular to the plane from which
they are reflected. The rays diverge, so that they appear to come from
a point as far behind the mirror as the luminous point is in front, and
the images reflected have the same appearance, but reversed. There is
another law, which is that “the angular velocity of a beam reflected
from a mirror is twice that of the mirror.” The Kaleidoscope, with
which we are all familiar, is based upon the fact of the multiplication
of images by two mirrors inclining towards each other.
[Illustration: Fig. 86.—Concave mirror.]
A concave mirror is seen in the accompanying diagram, and may be called
the segment of a hollow sphere—V W. The point C is the geometrical
centre, and O C the radius; F is the focus; the line passing through it
is the _optical_ axis; O being the _optical centre_. All perpendicular
rays pass through C. All rays falling in a direction parallel with the
optical axis are reflected and collected at F. Magnified images will
be produced, and if the object be placed between the mirror and the
focus, the image will appear at the back; while if the object be placed
between the geometrical centre and the focus, the image will appear to
be in front of the mirror.
We can understand these phenomena by the accompanying diagrams. Suppose
a ray A _n_ passes from one object, A B, at right angles, it will be
reflected as _n_ A C, the ray A C being reflected to F. These cannot
meet in front of the mirror, but they will if produced meet at _a_,
and the point A will be reflected there; similarly B will be reflected
at _b_, and thus a magnified image will appear behind or at the back
of the mirror’s surface. In the next diagram the second supposed case
will produce the image in the air at _a b_, and if a sheet of paper be
held so that the rays are intercepted, the image will be visible on the
sheet. In this case the perpendicular ray, A _n_, is reflected in the
same direction, and the ray, _a c_, parallel with the axis is reflected
to the focus. These rays meet at _a_ and corresponding rays at _b_,
when the image will be reproduced; viz., in front of the mirror.
[Illustration: Fig. 87.—Reflection of mirrors (I).]
[Illustration: Fig. 88.—Reflection of mirrors (II).]
The concave mirror is used in the manufacture of telescopes, which,
with other optical instruments, will be described in their proper
places. We will now look at the _Refraction_ of light.
Bodies which permit rays of light to pass through them are termed
transparent. Some possess this property more than others, and so
long as the light passes through the same medium the direction will
remain the same. But if a ray fall upon a body of a different degree
of density it cannot proceed in the same direction, and it will be
broken or _refracted_, the angle it makes being termed the angle of
refraction.
For instance, a straight stick when plunged into water appears to be
broken at the point of immersion. This appearance is caused by the rays
of light taking a different direction to our eyes. If in the diagram
(fig. 89) our eye were at _o_, and the vessel were empty, we should
not see _m_; but when water is poured into the vessel the object will
appear higher up at _n_, and all objects under water appear higher than
they really are.
[Illustration: Fig. 89.—Refraction in water.]
[Illustration: Fig. 90.—A water-bottle employed as a convergent lens.]
One may also place a piece of money at the bottom of a basin, and then
stoop down gradually, until, the edge of the basin intervening, the
coin is lost to view. If an operator then fills the basin with water,
the piece of money appears as though the bottom had been raised. The
glass lenses used by professors may be very well replaced by a round
water-bottle full of water. A candle is lighted in the darkness, and
on holding the bottle between the light and a wall which acts as a
screen, we see the reflected light turned upside down by means of
the convergent lens we have improvised (fig. 90). A balloon of glass
constitutes an excellent microscope. It must be filled with perfectly
clear, limpid water, and closed by means of a cork. A piece of wire
is then rolled round its neck, and one end is raised, and turned up
towards the focus; viz., to support the object we wish to examine,
which is magnified several diameters. If a fly, for instance, is at the
end of the wire, we find it is highly magnified when seen through the
glass balloon (fig. 91). By examining the insect through the water in
the balloon, we can distinguish every feature of its organism, thanks
to this improvised magnifier. This little apparatus may also serve to
increase the intensity of a luminous focus of feeble power, such as a
lighted candle. It is often employed in this manner by watchmakers. If
a bottle full of water is placed on a table, and exposed to the rays
of the sun, the head of a lucifer match being placed in the brightest
centre of light caused by the refracted rays, the match will not fail
to ignite. I have succeeded in this experiment even under an October
sun, and still more readily in warm weather.
[Illustration: Fig. 91.—A simple microscope formed with a glass balloon
full of water.]
In the Conservatoire des Arts in Paris a visitor will always notice
a number of people looking at the mirrors in the “optical” cabinets.
These mirrors deform and distort objects in a very curious manner, and
people find much amusement in gazing into them till they are “moved on”
by the attendants. Such experiments create great interest, and a very
excellent substitute for these may be found in a coffee-pot or even in
a large spoon, and all the grotesque appearance will be seen in the
polished surface. The least costly apparatus will sometimes produce
the most marvellous effects. Look at a soap-bubble blown from the end
of a straw. When the sphere has a very small diameter the pellicule
is colourless and transparent; but as the air enters by degrees,
pressing upon all parts of the concave surface equally, the bubble gets
bigger as the thickness decreases, and then the colours appear,—feeble
at first, but stronger and stronger as the thickness diminishes.
The study of soap-bubbles and of the effects of the light is very
interesting. Newton made the soap-bubble the object of his studies
and meditations, and it will ever hold its place amongst the curious
phenomena of the Science of Optics. But before going into all the
phases of Lights and Optics we will proceed to explain the structure of
the eye, as it is through that organ that we are enabled to appreciate
light and its marvellous effects.
[Illustration: Fig. 92.—Grotesque effects of curved surfaces.]
It is often considered an embarrassing matter to fix precisely the
value of two lights. Nothing, however, can be easier in reality, as
we will show. In comparing different lights, it is necessary to bear
in mind the amount of waste, the colour of the light, the luminous
value of the source, and the steadiness of the flame. The luminous
value of a lamp-burner is generally equalled by that of a wax candle,
and we will take as an example one of those at six to the pound. Very
precise appliances are used for this experiment when great exactness
is required; but it is easy to calculate in a simple manner the
differences in ordinary lights. Supposing we desire to test the value
of light given by a lamp and a wax candle, they must both be placed
on the table at an equal height, B and A, (fig. 93), in front of some
opaque body, A, and then a large sheet of paper must be fixed as
vertically as possible to form a screen. When B and A are lighted,
two shadows, E and F, are produced, to which it is easy to give
exactly the same intensity, by advancing or withdrawing one of the
two sources of light. The intensities of the two lights will then be
inversely proportional to the squares of the measured distances, AB
and AC. By a similar careful calculation it has been possible to draw
up a table of the relative values of various ordinary lights. We have
not included here the electric light, which has recently attracted so
much attention, because this system of lighting can hardly be said to
have yet penetrated the domain of domestic life; but when we consider
electricity, as we hope to do in a future part, we intend to study
this question fully, for there is no doubt that electricity is becoming
more and more adapted to our daily life.
[Illustration: Fig. 93.—An elementary Photometer.]
The measurement of intensity of light is called _Photometry_, and the
instruments used are _Photometers_. Bunsen’s instrument consists of a
screen of writing-paper, saturated in places with spermaceti to make it
transparent. A sperm candle is placed on one side, and the light to be
compared on the other. The lights are provided with graduated bars, and
these lights are then removed farther and farther from the screen till
the spots of grease are invisible. The relative intensities are as the
squares of the distance from the screen.
[Illustration: Fig. 94.—The soap-bubble.]
We append a table showing the comparative cost of light given by Dr.
Frankland at the Royal Institution some few years ago. The standard of
comparison was 20 sperm candles burning for 10 hours at the rate of 120
grains an hour:—
_s._ _d._
Wax 7 2½
Sperm Oil 1 10
Paraffin 3 10
Spermaceti 6 8
Coal Gas 0 4½
Paraffin Oil 0 6
Tallow 2 8
Cannel Gas 0 3
Rock Oil 0 7⅔
There are many other interesting experiments connected with
Light,—Spectrum Analysis, etc., etc.,—all of which we will defer for a
time until we have examined the Eye and some effects produced upon it
by Light, illustrated by numerous diagrams in the pages next following.