ample, we saw 90% of the mirror as a perfect reflector and 10% as
imperfections, 90% of the mirror would reflect; the remaining 10% would
emit. Therefore, the emissivity equals 0.1.
Consider for a moment the exact opposite of
a perfect mirror, which is a perfect emitter.
The eye looks at a perfect emitter and sees
no reflection at all, only the emitting surface.
Since 100% of the surface emits, and 0%
reflects, the emissivity equals 1.0. This type
of object is called a blackbody.
And finally, consider a good emitter. The eye
sees a small amount of reflection interspersed
with the large amount emitting. If, for ex-
ample, 10% of the surface did not emit, and
instead reflected, then we would have 10%
reflecting and the remaining 90% emitting.
Therefore, the emissivity equals 0.9. Accordingly, we can state the
following rule of emissivity:
The emissivity of the surface is simply
the percentage of the surface that emits. The remaining percent-
age of the surface reflects.
Good Emitter
Emissivity = 0.9
Reflectivity = 0.1
1.0
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