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Transcribed Image Text:A 25-W incandescent light bulb radiates uniformly in all directions.
(a) Calculate the average intensity 2.1 m from the bulb.
lavg @ 2.1 m =
W/m²
(b) What is the average intensity 4.5 m from the bulb?
lavg @ 4.5 m =
W/m²
Transcribed Image Text:EM waves, like all other waves, are generated a source. The power of a source measures the rate that it radiates EM energy. The energy spreads
outward as the wave carries it away from the source so the amount crossing a given area decreases with distance.
The intensity at a given point measures the rate per unit area
that EM energy is being transported to that point (i.e. received).
Intensity = Source Power/Surface Area
The surface area takes into account the specific details of how
energy spreads outward as it radiates away from the source.
A light source that radiates uniformly in all directions like the
one shown in the figure is called isotropic or omni-directional.
The energy is spread uniformly across spherical surfaces that
are centered on the source so the intensity, , a distance, d,
from an isotropic source that with power, P, is given by:
Il
=
P
4zd²
Observation
Point
Surface Area = 4nd²
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