Lecture II: Sunbeams and How They Work Continued …
Thus we see there are two ways of touching anything at a distance; 1st, by throwing some thing at it and hitting it; 2nd, by sending a movement of wave across to it, as in the case of the quivering boards and the air.
Now the great natural philosopher Newton thought that the sun touched us in the first of these ways, and that sunbeams were made of very minute atoms of matter thrown out by the sun, and making a perpetual cannonade on our eyes. It is easy to understand that this would make us see light and feel heat, just as a blow in the eye makes us see starts, or on the body makes it feel hot: and for a long time this explanation was supposed to be the true one. But we know now that there are many facts which cannot be explained on this theory, though we cannot go into them here. What we will do, is to try and understand what now seems to be the true explanation of the sunbeam.
About the same time that Newton wrote, a Dutchman, named Huyghens, suggested that light comes from the sun in tiny waves, travelling across space much in the same way as ripples travel across a pond. The only difficulty was to explain in what substance these waves could be travelling: not through water, for we know that there is no water in space – nor through air, for the air stops at a comparatively short distance from our earth. There must then be something filling all space between us and the sun, finer than either water or air.
And now I must ask you to use all you imagination, for I want you to picture to yourselves something quite as invisible as the Emperor’s new clothes in Andersen’s fairy-tale, only with this difference, that our invisible something is very active; and though we can neither see it nor touch it we know it by its effects. You must imagine a fine substance filling all space between us and the sun and the starts. A substance so very delicate and subtle, that not only is it invisible, but it can pass through solid bodies such as glass, ice, or even wood or brick walls. This substance we call “ether.” I cannot give you here the reasons why we must assume that it is throughout all space; you must take this on the word of such men as Sir John Herschel or Professor Clerk-Maxwell, until you can study the question for yourselves.
Now if you can imagine this ether filling every corner of space, so that it is everywhere and passes through everything, ask yourselves, what must happen when a great commotion is going on in one of the large bodies which float in it? When the atoms of the gases round the sun are clashing violently together to make all its light and heat, do you not think they must shake this ether all around them? And then, since the ether stretches on all sides from the sun to our earth and all other planets, must not this quivering travel to us, just as the quivering of the boards would from me to you? Take a basin of water to represent the ether, and take a piece of potassium like that which we used in our last lecture, and hold it with a pair of nippers in the middle of the water. You will see that as the potassium hisses and the flame burns round it, they will make waves which will travel all over the water to the edge of the basin,, and you can imagine how in the same way waves travel over the ether from the sun to us.
Straight away from the sun on all sides, never stopping, never resting, but chasing after each other with marvellous quickness, these tiny waves travel out into space by night and by day. When our spot of the earth where England lies is turned away from them and they cannot touch us, then it is night for us, but directly England is turned so as to face the sun, then they strike on the land, and the water, and warm it; or upon our eyes, making the nerves quiver so that we see light. Look up at the sun and picture to yourself that instead of one great blow from a fist causing you to see starts for a moment, millions of tiny blows from these sun-waves are striking every instant on you eye; then you will easily understand that his would cause you to see a constant blaze of light.
But when the sun is away, if the night is clear we have light from the starts. Do these then too make waves all across the enormous distance between them and us? Certainly they do, for they too are suns like our own, only they are so far off that the waves they send are more feeble, and so we only notice them when the sun’s stronger waves are away.
But perhaps you will ask, if no one has ever seen these waves not the ether in which they are made, what right have we to say they are there? Strange as it may seem, though we cannot see them we have measured them and know how large they are, and how many can go into an inch of space. For as these tiny waves are running on straight forward through the room, if we put something in their way, they will have to run round it; and if you let in a very narrow ray of light through a shutter and put an upright wire in the sunbeam, you actually make the waves run round the wire just as water runs round a post in a river; and they meet behind the wire, just as the water meets in a V shape behind the post. Now when they meet, they run up against each other, and here it is we catch them. Fir if they meet comfortably, both rising up in a good wave, they run on together and make a bright line of light; but if they meet higgledy-piggledy, one up and the other down, all in confusion, they stop each other, and then there is no light but a line of darkness.
And so behind your piece of wire you can catch the waves on a piece of paper, and you will find they make dark and light lines one side by side with the other, and by means of these bands it is possible to find out how large the waves must be. This question is too difficult for us to work it out here, but you can see that large waves will make broader light and dark bands than small ones will, and that in this way the size of the waves may be measured.
And now how large do you think they turn out to be? so very, very tiny that about fifty thousand waves are contained in a single inch of space! I have drawn on the board the length of an inch, and now I will measure the same space in the air between my finger and thumb. Within this space at this moment there are fifty thousand tiny waves moving up and down. I promised you we would find in science things as wonderful as in fairy tales. Are not these tiny invisible messengers coming incessantly from the sun as wonderful as any fairies? and still more so when, as we shall see presently, they are doing nearly all the work of our world.
We must next try to realize how fast these waves travel. You will remember that an express train would take 171 years to reach us from the sun; and even a cannon-ball would take from ten to thirteen years to come that distance. Well, these tiny waves take only seven minutes and a half to come the whole 91 millions of miles. The waves which are hitting your eye at this moment are caused by a movement which began at the sun only 7 1/2 minutes ago. And remember, this movement is going on incessantly, and these waves are always following one after the other so rapidly that they keep up a perpetual cannonade upon the pupil of your eye. So fast do they come that about 608 billion waves enter your eye in one single second.* I do not ask you to remember these figures; I only ask you to try and picture to yourselves these infinitely tiny and active invisible messengers from the sun, and to acknowledge that light is a fairy thing. (*Light travels at the rate of 190,000 miles, or 12,165,120,000 inches in a second. Taking the average number of wave-lengths in an inch at 50,000, then 12,165,120,000 X 50,000 = 608,256,000,000,000.)
But we do not yet know all about our sunbeams. See, I have here a piece of glass with three sides, called a prism. If I put it in the sunlight which is streaming through the window, what happens? Look! on the table there is a line of beautiful colours. I can make it long or short, as I turn the prism, but the colours always remain arranged in the same way. Here at my left hand is the red, beyond it orange, then yellow, green, blue, indigo or deep blue, and violet, shading one into the other all along the line. We have all seen these colours dancing on the wall when the sun has been shining brightly on the cut-glass pendants of the chandelier, and you may see them still more distinctly if you let a ray of light into a darkened room, and pass it through the prism as in the diagram (Fig. 7). What are these colours? Do they come from the glass? No; for you will remember to have seen them in the rainbow, and in the soap- bubble, and even in a drop of dew or the scum on the top of a pond. This beautiful coloured line is only our sunbeam again, which has been split up into many colours by passing through the glass, as it is in the rain-drops of the rainbow and the bubbles of the scum of the pond.
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.
Go to Lecture 2 – Part 3 here.