Black Body Radiation: Why Does the Black Spot Appear Brighter?

[Amith2006]

Sir,
A polished metal plate with a rough black spot on it is heated to about 1400 Kelvin and quickly taken into a dark room. It is said that the black spot will appear brighter than the plate. Is it because the black spot is a perfect black body and hence is a better absorber of heat than the plate?

 

[Andrew Mason]

Sir,
A polished metal plate with a rough black spot on it is heated to about 1400 Kelvin and quickly taken into a dark room. It is said that the black spot will appear brighter than the plate. Is it because the black spot is a perfect black body and hence is a better absorber of heat than the plate?

The plate has a lower emissivity than the black spot (ie it is not a perfect absorber of radiation - it reflects radiation whereas the black spot absorbs almost all incident radiation). So in the black-body radiation equation:

$$P = \sigma Ae(T^4)$$

the power emitted by the black spot is higher for the black spot (higher e value).

AM