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
1.0instead 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.
16
