Light is the name which we give to the external agency which enables us to see. In order to see things we must have something which enters the eye and a brain to explain it to us. That which enters the eye is what we call light.
The eye consists of two principal parts and can best be understood by analogy with the camera. In front it has a lens which forms an image on the sensitive surface, which is called the retina, the retina playing the same part in the eye that the film does in the camera. The retina, however, differs from the film in that when light falls upon the film it produces a permanent change, which can be developed into a picture, and if the light falls upon the film for too long a time the film is spoiled, while the retina merely acts as a medium to transmit to the brain the sensation of the light that falls upon it, and when the light stops, the sensation stops and the retina is ready to make a new record. The retina behaves, in fact, like a film in which the sensitive material is continually renewed.
It is probable that this sensitive material in the eye is really of a chemical nature because it is apparently produced all the. time, and when the eye is kept in the dark the sensitive material accumulates for some time so that the eye becomes more sensitive, while when a strong light falls upon the eye, the sensitive substance is destroyed more rapidly than it is produced and the eye becomes less sensitive.
In this way, the eye has a very great range of sensitiveness. In bright sunlight it is as much as a million times less sensitive than it is after it has been kept for an hour in the dark, and it changes very rapidly, only a few minutes being necessary for an eye that has been in almost complete darkness to adapt itself to the glare of out-door lighting. In order to lessen the shock of changing light intensity, the lens of the eye is provided with an iris diaphragm just like that of a camera, but with the additional advantage that it operates automatically, opening and closing according to the intensity of the light. Measurements of the movements of the iris of the eye have been made by taking motion pictures of the eye when suddenly illuminated by a bright light, and these show what a wonderful instrument the eye is in its adaptation to changing conditions in the world around it.
Fig. 6. Diagram of Human Eye.
The retina is connected with the brain by a great many nerve fibers, each fiber coming from a different part of the retina, so that when light falls upon any part of the retina, the intensity of the light is communicated by the tiny nerve coming from that part of the retina to the brain and the brain forms an idea of the image on the retina by means of the multitude of impressions from different parts of the retina.
The image on the retina is inverted like all lens images, so that we really see things standing on their heads, but the brain interprets an inverted image on the retina as corresponding to an upright external world, and although the eye sees things upside down, the brain has no idea of it.
What we observe is the light which falls on the retina, but this light comes originally from some external source which, in the case of daylight, of course, is the sun. The light from the sun is reflected by the objects in the world around us according to their nature, and entering the eye it enables us to see the objects. When we look at a landscape we see that the sky is bright and the roads and fields are less bright, and the shadows under the trees are dark, because much of the light of the sun is reflected from the sky, less from the fields and roads and still less from the shadows under the trees. All these rays from the sun reflected from the natural objects in the landscape enter the eye and make a picture on the retina which is perceived by the brain by means of the tiny nerve fibers coming from the retina to the brain.
Fig. 7. Iris Opening and Closing.
But the eye not only perceives differences in the brightness of the light - it also observes differences in colour - and in order to understand how this can be we must search further into the nature of light itself.
The nature of light has long been a source of speculation, and at one time it was generally held that the light which entered the eye consisted of small particles shot off from the source of light, just as at one time it was held that sound consisted of small particles shot off from the source of a sound which struck the drum of the ear. This theory of light has the advantage that it immediately explains reflection; just as an india rubber ball bounces from a smooth wall, while it will be shot in almost any direction from a heap of stones, so the small particles of light would rebound from a polished surface at a regular angle, while a rough surface would merely scatter them.
This theory of the nature of light was satisfactory until it was found that it was possible by dividing a beam of light and slightly lengthening the path of one of the halves, and then reuniting the two halves together again, to produce alternate periods of darkness and light similar to the nodes of rest produced in an organ pipe, where the interference of the waves of sound is taking place. It could not be imagined that a reinforcement of one stream of particles by another stream of particles in the same direction could produce an absence of particles, while the analogy of sound suggested that just as sound was known to consist of waves in the air, so light also consisted of waves.
FiG. 8. Relative Wave Length of Red, Green and Blue.
Fig. 9. Simple Arrangement of Spectrum.
Light cannot consist of waves in the air, partly because we know that it travels through interstellar space, where we imagine that there is no air but through which we can still see the light of the stars, and also because the velocity of light - nearly 200,000 miles per second - is so great that it is impossible that it could consist of a wave in any material substance with which we are acquainted. It is, therefore, assumed that there exists, spread through all space and all matter, something in which the waves of light are formed, and this something is termed ether, so that it is generally held that light consists of waves in the ether.
Just as in sound we have wave notes of high frequency, that is, with many waves per second falling upon the ear. which form the high pitched notes, and also notes of low frequency where only a few waves a second fall upon the ear forming the bass notes, so with light we may have different frequencies of vibration. Since the velocity of light is the same for waves of different frequencies, it is clear that the waves of high frequency will be of different wave length from those of low frequency, the wave length being the distance from the crest of one wave to the crest of the next, and if we obtain waves of different lengths separated out, we shall find that the color depends upon the wave length. Fig. 8 shows the average length of wave corresponding to light of various colors, the diagram being drawn to scale.
White light consists of mixtures of waves of various lengths, but if instead of letting the, mixture of waves, which forms white light, fall directly on the eye we pass white light through an instrument known as a spectroscope, which changes the direction of the different waves by amounts which differ according to their lengths, we get the white light spread out into a band of colors which we call the spectrum, and we can scale this spectrum by means of numbers representing the lengths of the waves.
Fig. 9 gives a simple arrangement of the spectrum, the numbers representing the wave lengths in units which are millionths of millimeters. It will be seen that the visible spectrum extends from 700 to 400 units, wave lengths of 700 units corresponding to the extreme red and 400 to the darkest violet that can be seen, while the brightest region of the spectrum stretches from 500 to 600 units and includes the green and yellow colors. The spectrum is equally divided into three regions which may be broadly termed - red 700-600, green 600-500, and blue-violet 500-400.
If we get a piece of colored glass which lets through only the portion of the spectrum between 600 and 700, then we should have a piece of red glass; a glass which let through from 500 to 600 would be a green glass, and one which let through from 400 to 500 would be blue-violet in color, so that from the spectrum we already derive the idea that light can be conveniently divided into three colors, which we may call the primary colors - red, green and blue-violet. It is probable that this is connected with the structure of the retina, and one theory holds that there are three sets of receiving nerves in all parts of the retina, corresponding to the three primary colors - red, green and blue-violet.
Fig. 10. Portions of Spectrum Transmitted by Primaries.
If we let white light fall upon anything, such as a piece of white paper, which reflects all the wave lengths to the same extent, then the reflected light remains white and we should say that the object on which it falls is uncolored, but if the object absorbs some of the wave lengths of the spectrum more than others, then it will appear colored. Thus, a piece of red paper appears red because from the white light falling upon it it absorbs some of the green and blue-violet light, but reflects all the red light and, therefore, appears red. In the same way a green object absorbs both red and blue-violet more than it absorbs the green light and so looks green, and a yellow object absorbs the blue, reflecting the red and green of the spectrum and so appears yellow.
Light waves differ not only in their length but in their amplitude, that is, in the height of the wave, and the amplitude controls the intensity of the light just as the wave length controls the color. The eye, therefore, can detect differences in brightness which depend upon amplitude, and also differences of color which depend upon wave length.