Till now we have talked of the sunbeam as if it were made of only
one set of waves of different sizes, all travelling along
together from the sun. These various waves have been measured,
and we know that the waves which make up red light are larger and
more lazy than those which make violet light, so that there are
only thirty-nine thousand red waves in an inch, while there are
fifty-seven thousand violet waves in the same space.
How is it then, that if all these different waves making
different colours, hit on our eye, they do not always make us see
coloured light? Because, unless they are interfered with, they
all travel along together, and you know that all colours, mixed
together in proper proportion, make white.
I have here a round piece of cardboard, painted with the seven
colours in succession several times over. When it is still you
can distinguish them all apart, but when I whirl it quickly round
- see! - the cardboard looks quite white, because we see them all
so instantaneously that they are mingled together. In the same
way light looks white to you, because all the different coloured
waves strike on your eye at once. You can easily make on of
these card for yourselves only the white will always look dirty,
because you cannot get the colours pure.
Now, when the light passes through the three-sided glass or
prism, the waves are spread out, and the slow, heavy, red waves
lag behind and remain at the lower end R of the coloured line on
the wall (Fig. 7), while the rapid little violet waves are bent
more out of their road and run to V at the farther end of the
line; and the orange, yellow, green, blue, and indigo arrange
themselves between, according to the size of their waves.
And now you are very likely eager to ask why the quick waves
should make us see one colour, and the slow waves another. This
is a very difficult question, for we have a great deal still to
learn about the effect of light on the eye. But you can easily
imagine that colour is to our eye much the same as music is to
our ear. You know we can distinguish different notes when the
air-waves play slowly or quickly upon the drum of the ear (as we
shall see in Lecture VI) and somewhat in the same way the tiny
waves of the ether play on the retina or curtain at the back of
our eye, and make the nerves carry different messages to the
brain: and the colour we see depends upon the number of waves
which play upon the retina in a second.
Do you think we have now rightly answered the question - What is
a sunbeam? We have seen that it is really a succession of tiny
rapid waves, travelling from the sun to us across the invisible
substance we call "ether", and keeping up a constant cannonade
upon everything which comes in their way. We have also seen
that, tiny as these waves are, they can still vary in size, so
that one single sunbeam is made up of myriads of different-sized
waves, which travel all together and make us see white light;
unless for some reason they are scattered apart, so that we see
them separately as red, green, blue, or yellow. How they are
scattered, and many other secrets of the sun-waves, we cannot
stop to consider not, but must pass on to ask -
What work do the sunbeams do for us?
They do two things - they give us light and heat. It is by means
of them alone that we see anything. When the room was dark you
could not distinguish the table, the chairs, or even the walls of
the room. Why? Because they had no light-waves to send to your
eye. But as the sunbeams began to pour in at the window, the
waves played upon the things in the room, and when they hit them
they bounded off them back to your eye, as a wave of the sea
bounds back from a rock and strikes against a passing boat.
Then, when they fell upon your eye, they entered it and excited
the retina and the nerves, and the image of the chair or the
table was carried to your brain. Look around at all the things
in this room. Is it not strange to think that each one of them
is sending these invisible messengers straight to your eye as you
look at it; and that you see me, and distinguish me from the
table, entirely by the kind of waves we each send to you?
Some substances send back hardly any waves of light, but let them
all pass through them, and thus we cannot see them. A pane of
clear glass, for instance, lets nearly all the light-waves pass
through it, and therefore you often cannot see that the glass is
there, because no light-messengers come back to you from it.
Thus people have sometimes walked up against a glass door and
broken it, not seeing it was there. Those substances are
transparent which, for some reason unknown to us, allow the ether
waves to pass through them without shaking the atoms of which the
substance is made. In clear glass, for example, all the light-
waves pass through without affecting the substance of the glass;
while in a white wall the larger part of the rays are reflected
back to your eye, and those which pass into the wall, by giving
motion to its atoms lose their own vibrations.
Into polished shining metal the waves hardly enter at all, but
are thrown back from the surface; and so a steel knife or a
silver spoon are very bright, and are clearly seen. Quicksilver
is put at the back of looking-glasses because it reflects so many
waves. It not only sends back those which come from the sun, but
those, too, which come from your face. So, when you see yourself
in a looking-glass, the sun-waves have first played on your face
and bounded off from it to the looking-glass; then, when they
strike the looking-glass, they are thrown back again on to the
retina of your eye, and you see your own face by means of the
very waves you threw off from it an instant before.
But the reflected light-waves do more for us than this. They not
only make us see things, but they make us see them in different
colours. What, you will ask, is this too the work of the
sunbeams? Certainly; for if the colour we see depends on the
size of the waves which come back to us, then we must see things
coloured differently according to the waves they send back. For
instance, imagine a sunbeam playing on a leaf: part of its waves
bound straight back from it to our eye and make us see the
surface of the leaf, but the rest go right into the leaf itself,
and there some of them are used up and kept prisoners. The red,
orange, yellow, blue, and violet waves are all useful to the
leaf, and it does not let them go again. But it cannot absorb
the green waves, and so it throws them back, and they travel to
your eye and make you see a green colour. So when you say a leaf
is green, you mean that the leaf does not want the green waves of
the sunbeam, but sends them back to you. In the same way the
scarlet geranium rejects the red waves; this table sends back
brown waves; a white tablecloth sends back nearly the whole of
the waves, and a black coat scarcely any. This is why, when
there is very little light in the room, you can see a white
tablecloth while you would not be able to distinguish a black
object, because the few faint rays that are there, are all sent
back to you from a white surface.
Is it not curious to think that there is really no such thing as
colour in the leaf, the table, the coat, or the geranium flower,
but we see them of different colours because, for some reason,
they send back only certain coloured waves to our eye?
Wherever you look, then, and whatever you see, all the beautiful
tints, colours, lights, and shades around you are the work of the
tiny sun-waves.
Again, light does a great deal of work when it falls upon plants.
Those rays of light which are caught by the leaf are by no means
idle; we shall see in Lecture VII that the leaf uses them to
digest its food and make the sap on which the plant feeds.