Properties of Light

Light and its various phenomena present some of the most interesting studies in the whole realm of physics. They are interesting because the results of many experiments are revealed through the sense of vision as colour phenomena. Equally important and every bit as interesting is the historical development and the discovery of the various principles, concepts, and properties of light which give rise to this phenomena.

All of the various known properties of light are conveniently described in terms of the experiments by which they were discovered and the many and varied experiments by which they are now continually demonstrated. Numerous as they are, these experiments may by grouped together and classified under one of the three following heads: 1) geometrical optics, 2) physical optics, and 3) quantum optics. Each of these may be subdivided as follows: geometrical optics: rectilinear propagation, finite velocity, reflection, refraction; physical optics: diffraction, interference, polarization, double refraction; quantum optics: photoelectric effect, Compton effect, atomic excitation, pair production.

The rectilinear propagation of light is another way of saying that “light travels in straight lines”. The fact that objects may be made to cast fairly sharp shadows is an experimental demonstration of this principle. Another illustration is the image formation of an object which is produced by light passing through a small opening. An object can be ordinary incandescent light bulb. In order to see how an image is formed, consider the rays of light emanating from a single point a near the top of the bulb. Of the many rays of light radiating in all directions, the ray that travels in the direction of the hole passes through the point a' near the bottom of the image screen. Similarly, a ray leaving b near the bottom of the bulb and passing through the hole will arrive at b' near the top of the image screen. Thus, it may be seen that an inverted image is formed.

If the image screen is moved closer to the pinhole screen, the image will be proportionately smaller, whereas if it were moved farther away the image would be proportionately larger. The same thing happens when either the object or the pinhole is moved. Excellent photographs can be made with this arrangement by making a pinhole in one end of a small box a placing a photographic film or plate at the other. Such an arrangement is called a pinhole camera. For good, sharp photographs the hole must be very small, because its size determines the amount of blurring produced.