Absorption of the Spectrum--Analysis of Colour--Vibrations of
Rays--Absorption by Pigments--Phosphorescence--Interference.
We must now briefly consider what is the origin, or at all events the
cause, of the colour which we see in objects. It is not proposed to
enter into this by any means minutely, but only sufficiently to enable
us to understand the subject which is to be brought before you. What for
instance is the cause of the colour of this green solution of
chlorophyll, which is an extract of cabbage leaves? If we place it in
the front of the spectrum apparatus and throw the spectrum on the
screen, we find that while there is a certain amount of blue
transmitted, the green is strong, and there are red bands left, but a
good deal of the spectrum is totally absorbed. Forming a colour patch of
this absorption spectrum on the screen, we see that it is the same
colour as the chlorophyll solution, and of this we can judge more
accurately by using the reflected beam, and placing the rod in position
to cast shadows. (The light of the reflected beam is that of the light
entering the slit.) The colour then of the chlorophyll is due to the
absence of certain colours from the spectrum of white light. When white
light passes through it, the material absorbs, or filters out, some of
the coloured rays, and allows others to pass more or less unaffected,
and it is the re-combination of these last which makes up the colour of
the chlorophyll. We have a green dye which to the eye is very similar in
colour to chlorophyll, but putting a solution of it in front of the
spectrum, we see that it cuts off different rays to the latter. It would
be quite possible to mistake one green for the other, but directly we
analyze the white light which has filtered through each by means of the
spectrum, we at once see that they differ. Hence the spectrum enables
the eye to discriminate by analysis what it would otherwise be unable to
do. Any coloured solution or transparent body may be analyzed in the
same way, and, as we shall see subsequently, the intensity of every ray
after passing through it can be accurately compared with the original
incident light. There are some cases, indeed the majority of cases, in
which the colour transmitted through a small thickness of the material
is different to that transmitted through a greater thickness. For
instance, a weak solution of litmus in water is blue when a thin layer
is examined, and red when it is a thicker or more concentrated layer.
Bichromate of potash is more ruddy as the thickness increases. This can
be readily understood by a reference to the law of absorption. Suppose
we have a thin layer of a liquid which gives a purple colour when two
simple colours, red and blue, pass through it, and that this thin layer
cuts off one-quarter of the red and one-half of the blue incident on it,
another layer of equal thickness will cut off another quarter of the
three-quarters of red passing through the first layer, and half of the
one-half left of the blue; we shall thus have nine-sixteenths of the red
passing and only a quarter of the blue. With a third layer we shall have
twenty-seven sixty-fourths of red and only one-eighth of blue left,
showing that as the thickness of the liquid is increased the blue
rapidly disappears, leaving the red the dominant colour. Now what is
true of two simple colours is equally true of any number of them, where
the rates of absorption differ from one another, and what is true for a
solution is true for a transparent solid. In some opaque bodies, such as
rocks, the reflected colour often differs slightly from that of the same
when they are cut into thin and polished slices, through which the
light can pass. The reason is that when opaque, light penetrates to a
very small distance through the surface, and is reflected back, whilst
in these layers the colour has to struggle through more coloured matter,
and emerges of a different hue.
The question why substances transmit some rays and quench others, brings
us into the domain of molecular physics. Of all branches of physical
science this is perhaps the most fascinating and the most speculative,
yet it is one which is being built up on the solid foundations of
experiment and mathematics, till it has attained an importance which the
questions depending on it fully warrants. We have to picture to
ourselves, in the case in point, molecules, and the atoms composing
them, of a size which no microscope can bring to view, vibrating in
certain definite periods which are similar to the periods of oscillation
of the waves of light. At page 26 we have given the lengths of some of
the waves which give the sensation of coloured light. Now as light, of
whatever colour it may be, is practically transmitted with the same
velocity through air which has the same density throughout, it follows
that the number of vibrations per second of each ray can be obtained by
dividing the velocity of light in any medium by the wave-length. The
following table gives roughly the number of vibrations per second of the
ether giving rise to the colours fixed by the dark solar lines.
+-----------------------+-----------------+
| Name of Line. | Millions of |
| | Millions of |
| | Vibrations |
| | per Second. |
+-----------------------+-----------------+
| A in the Red | 395 |
| B " " | 437 |
| C " " | 458 |
| D " Orange | 510 |
| E " Green | 570 |
| F " Blue | 618 |
| G " Violet | 697 |
| H " Ultra-Violet | 757 |
+-----------------------+-----------------+
If we endeavour to gauge what this rate of oscillation means we shall
scarcely be able to realize it, even by a comparison with some
physically measurable rate of vibration. A tuning-fork, for instance,
giving the middle C, vibrates 528 times per second. Compare this with
the number of vibrations of the waves of light, and we still are as far
as ever from realizing it, yet the velocity of light, and the lengths of
the different waves have been accurately determined; the latter,
although the much smaller quantity, with even greater accuracy than the
first. These rates of vibration must therefore be--cannot help being--at
all events approximately true. This being so, we know that some of the
atoms of the molecules at least, and perhaps in some cases the
molecules themselves, are vibrating at the same rate as those waves of
light, which they refuse to allow to pass. If we have a child's swing
beginning to oscillate, we know that it is only by well-timed blows that
the extent of the swing is permanently increased, and the energy exerted
by the person who gives the well-timed blow is expended on producing the
increased amplitude. In the same way if the rate of vibration of a wave
of light is in accord with that of a molecule or atom, the amplitude or
swing of the atom or molecule is increased, and the energy of the wave
and therefore its amplitude is totally or partially destroyed; and as
the amplitude is a function of the intensity of the light, the ray fails
to be seen at all, or else is diminished in brightness.
In what way the atoms vibrate where more than one ray is absorbed is
still a matter of speculation, but no doubt as experimental methods are
more fully developed, and mathematicians investigate the results of such
experiments, we shall be able to form a picture of the vibrations
themselves. At page 137 a speculation as to the reason why solids or
liquids can absorb more waves of light than one which are adjacent to
each other is put forward, but it does not deal with the absorptions
which occupy various parts of the spectrum. Again, too, we have the fact
that the energy absorbed by these atoms and molecules from the waves of
light, must show itself as work done on them--it may be as heat or as
chemical action. We shall see by and by that in some cases, no doubt, at
least a part is expended in the latter form of work.
Perhaps this mode of looking at the question of colour in objects may
make the subject more interesting to the reader than it at first appears
to be deserving. The whole subject is one which enlarges the faculty of
making mental pictures, and this is one of the most useful forms of
scientific education.
But how can we distinguish between pigments which to the eye are
apparently the same? If we dye paper with the green dye referred to, we
can place it in the spectrum, and we shall see that the dye reflects
differently to the white paper. In fact we shall find that it refuses to
reflect in those parts of the spectrum which the transparent solution
refused to transmit. So long as the light passes through the dye-stuff,
it is indifferent, as regards the colour produced, whether the colouring
matter be at a distance from the paper or whether the latter be dyed
with it, as we can see at once. If we place the solution of the dye in
the reflected beam of the apparatus and form a patch on the screen, and
alongside throw the patch of white light from the integrated or
recombined spectrum upon the dyed paper, it will be found that the two
colours are alike; that is, the green-coloured light on the white paper,
or the white light on the green paper are the same. Similarly we may
experiment on other dyes, such as magenta, log-wood, &c., and we shall
see that like results are obtained. It should be said, however, that
when the paper is dyed with the colouring matter a _small quantity_ of
white light will be reflected from the surface of the paper itself. We
may now say that the general colour is given to a body by its refusal to
transmit or reflect, more or less completely, certain rays of the
spectrum. Should the solvent form a compound with the dye, perhaps this
would not be absolutely true, but in the large majority of cases the
statement is correct. When we have bodies which are also fluorescent,
this statement would also have to be modified, but we need not consider
these for the present.
Another source of colour in objects, though very rarely met with, and
which for our object we need not stay to explain in detail, is the
interference of light. Such is seen in soap-bubbles. Briefly it may be
said that the colours are due to rays of light reflected from the inner
surface of the film, which quench other rays of light of the same
wave-length reflected from the outer surface. If two series of waves of
the same wave-length are going in the same direction and from the same
source, each of which has the same intensity as the other, that is,
having the same amplitude, and it happens that the one series is exactly
half a wave-length behind the other, then the crest of one wave in the
first series will fill up the trough of the other in the second series,
and no motion would result, and this lack of motion means darkness,
since it is the wave motion which gives the sensation of light. If then
we have white light falling on two reflecting surfaces, such as the
front and back of a soap-film, part of the light will be reflected from
each, and if the film be of such a thickness that the latter reflects
light exactly 1/2 wave-length, 3/2 or 5/2 wave-length, &c., of some
colour behind the former, the colour due to that particular wave-length
will be absent from the reflected white light, and instead of white
light we shall have coloured light, due to the combination of all the
colours less this colour, which is quenched.
A very pretty experiment to make is to throw the image of a soap film on
the screen, and to watch the change in the colours of the film. Their
brilliancy increases as the film becomes thinner, and the bands, which
first appear close to each other, separate, and then we see a large
expanse of changing colour. A soap solution should be made according to
almost any of the published formulæ, and a piece of flat card be dipped
in it, and be drawn across a ring of wire some inch in diameter,
or--what the writer prefers best--the stop of a photographic lens. A
film will form and fill the aperture. The ring or stop may be placed
vertically in a clamp, and a beam of light caused to fall at an angle of
about 45 degrees on to the film. If a lens be placed in the path of the
reflected beam to form an image of the aperture, the colours which the
film shows can be exhibited to an audience, if the diameter of the image
be made four or five feet. Instead of this large image, a small image
may be thrown on the slit of the spectroscope, by using a lens of a
greater focal length, and if the beam be so directed that it falls on
the axis of the collimator, a very fairly bright spectrum may be also
thrown on the screen. The appearance of the spectrum is somewhat like
that shown in the above diagram (Fig. 9).
Fig. 9.--Interference Bands.
If we take a horizontal line across the spectrum, we shall see what
particular colours are missing from the reflected light which falls on
the part of the slit corresponding to that line. The colours of some
objects, such as of the opal, and the lovely colouring of some feathers
are due to interference of light. The partial scattering of different
rays by small particles will also cause light to be coloured, as we
shall see in the experiments we shall make to imitate the colour of
sunlight at various altitudes of the sun. We may, however, take it as a
rule that the colour of objects is produced by the greater or less
absorption of some rays, and the reflection in the case of opaque
bodies, or the transmission, in the case of transparent bodies, of the
remainder.