Any point light source spreads out like an expanding sphere. In this exhibit a point light source shines through a perforated metal screen projecting an array of dots – in evenly spaced rows – onto a movable white screen. The farther the screen is moved from the light, the wider the dot spacing becomes. A small square is drawn on the screen. Depending upon the distance of the screen from the light, more or fewer dots fit in the square. If the distance is doubled, for example, the number of dots is reduced to 1/4 of the original number. If the distance is tripled, the number of dots is reduced to 1/9. In general, the light drops by the inverse of the square of the increase of the distance. Anything that seems to radiate uniformly from a point acts in this manner: gravity, electrical and magnetic forces, and sound are all examples.
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