Question

Three objects coloured black, grey and white can withstand hostile conditions at ${2800}^{o}C$. These objects are thrown into furnace where each of them attains a temperature of ${2000}^{o}C$. Which object will glow brightest?

• A. The white object
• B. The black object
• C. All glow with equal brightness
• D. Grey object

Answer 1

The correct option is B The black object

Kirchoff's law of thermal radiation states that for a body of any arbitrary material emitting and absorbing thermal electromagnetic radiation at every wavelength in thermodynamic equilibrium, the ratio of its emissive power to its dimensionless coefficient of absorption is equal to a universal function only of radiative wavelength and temperature. That universal function describes the perfect black-body emissive power.
According to Kirchhoff’s radiation law a body that, at a given temperature, exhibits a stronger absorptivity must also exhibit a more intensive emission.
Black object absorbs maximum and can also emit maximum.