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The Intensity of Illumination

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-440

2025-12-16

644

is measured by the quantity of light that falls on a unit of surface. This differs with the intensity of the source of the light, and with the distance from the source.

The intensity of illumination upon any surface is inversely proportional to the square of its distance from the source of light. This law is true for very small sources of light only. It can be verified as follows:

Demonstration. - Place a lamp close to a screen through which there is a small hole. At a distance of 1 ft. place a cardboard disk 1 in. in diameter. Place a screen 2 ft. from the light, and the shadow cast by the disk will be 2 in. in diameter, and its area four times that of the disk. Since the disk cuts off the light from a part of the screen four times as great as itself, the intensity of the light falling upon the screen must be only one fourth as great as that falling upon the disk. If the screen is placed three feet from the light the intensity will be only one ninth as great.