The same amount of light confined to a skinnier cone

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Unformatted text preview: n the illuminance on the sheet 2 m from P is four times less. This is because doubling the distance increases the area by a factor of four over which the light is spread. Similarly, 3 m from P the illuminance is nine times less, and 4 m from P it’s 16 from times less. Note the flux (amount of light) is not changing, but the illuminance is because the same amount is spread over different areas. In general, E is proportional to Φ and inversely proportional to the square of the distance. This is reminiscent of Newton’s inverse square law for gravitation. 1/16 1 P 1/4 1/9 1 m 2 m 3 m 4 m Solid Angles We can measure ordinary, “flat” angles by the ratio of arc length of a circle to the radius of the circle. Imagine two radii shooting out from the center, subtending part of the circumference. By definition this ratio is the measure of the angle between the radii in radians. There are 2 π radians in a circle since C = 2 π r. Now imagine a sphere instead of a circle and a cone shooting out from the cente...
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This note was uploaded on 11/17/2011 for the course PHYS 121 taught by Professor Burgeson during the Fall '11 term at BYU.

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