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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.
 Fall '11
 Burgeson
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