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THE NATURE OF LI G'H T
The intensity of light refers to its brightness, for example, a sunshiny day possesses a more intense or brighter light (degree of illumination) than a cloudy day.
The intensity of light diminishes in proportion to the square of the distance from its source. For instance, let us refer to Figure No. 4, which represents light rays emanating from z small source, such as an arc lamp or the flame of a candle. Let the square A represent screen one foot square placed at a distance
Fig. 4.
The intensity of light falling upon a given area varies inversely as the square of the distance from which it is removed from the light source. The black squares marked X are the whole, one-fourth, and one-ninth, respectively, of the larger squares A, B and C. A is one foot, B, two feet and C, three feet away from the light source S. The black squares being of the same size will receive less light as they are removed from the arc light.
of one foot from the light and the square B screen placed at a distance of two feet from the light. These two squares are in a line with the light, square A exactly shades square B. If we remove square A the same amount of light which fell upon square A will now fall upon square B. Square B is twice the diameter of square A, or four times its area. Since the same amount of light which fell upon square A covers a surface four times as great as twice the distance, it follows that the intensity of the light falling upon B is only one-fourth of the intensity of