. A text book of physics, for the use of students of science and engineering . ities of illumination may be conlparedby several means. Law of inverse squares.—It is common experience that at greatdistances from a source of light the intensity of illumination is lessthat at points near it. The rate at which the intensity of illumination joudiminishes, as the distance from the source increases, follows fromthe fact that light travels in straight lines. Consider a point source of light S (Fig. 493). The rays proceedfrom S in all directions, and a small screen placed at A will casta shadow. If now
. A text book of physics, for the use of students of science and engineering . ities of illumination may be conlparedby several means. Law of inverse squares.—It is common experience that at greatdistances from a source of light the intensity of illumination is lessthat at points near it. The rate at which the intensity of illumination joudiminishes, as the distance from the source increases, follows fromthe fact that light travels in straight lines. Consider a point source of light S (Fig. 493). The rays proceedfrom S in all directions, and a small screen placed at A will casta shadow. If now a screen be placed at B, of exactly the same sizeand shape as the shadow at B, then on removing A, the light thatoriginally fell on A will fall on B. If the distance SB is twice SA, we ILLUMINATING POWER 545 now from geometry that the area of the screen B is four times thatA, and hence the intensity of illumination at B is one-quarter oflat at A, because the same amount of light falls on four times the:ea. Similarly, if SC is three times SA, the intensity of illumination. Fig. 493.—Illustration of the law of inverse squares. -j C is one-ninth of that at A, and so on. In general, since the area: any perpendicular section of the pyramid SABC is proportional tole square of the distance of the section from S, the amount of light?r unit area is inversely as the square of the distance. Hence thetensity of illumination due to a point source of light varies inversely ase square of the distance from the source. Although ordinary sources: light are not points, yet at considerable distances they may beeated as such. Exit. 114.—Law of inverse squares. Cut three cardboard squares, thedes of which are respectively, 5, 10, and 15 cm., whose areas are therefore the ratios 1, 4, and 9. Place one square near a candle flame and adjustte other two so that the shadow of the first falls exactly on them. Measureteir distances from the candle, when they will be found to be in the,tios 1:2:3. Il
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