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

 

[kyle8921]

I can confirm that the explanation for the black spot appearing brighter is due to the fact that it is a perfect black body. A black body is an idealized object that absorbs all radiation that falls on it, making it a perfect absorber of heat. This means that when the plate is heated, the black spot will absorb more heat energy compared to the surrounding polished metal, causing it to appear brighter.

Furthermore, the black spot also emits radiation at a higher rate due to its higher temperature, making it appear even brighter in the dark room. This phenomenon is known as black body radiation and is a fundamental concept in thermodynamics and quantum mechanics.

In contrast, the polished metal plate reflects a significant amount of radiation, resulting in a lower temperature and less emission of heat energy. This is why the black spot appears brighter in comparison.

Overall, the black spot's appearance is a result of its perfect absorber properties, making it an essential concept in understanding heat transfer and radiation. I hope this explanation helps clarify the phenomenon of black body radiation and its effect on the appearance of the black spot.