A blackbody is an idealized object that absorbs all incident electromagnetic radiation (radiant energy) across a wide range of frequencies and does not reflect or transmit any of it. This means that a blackbody absorbs and converts all the incoming radiation into heat energy. It is called a blackbody because it appears perfectly black when it is in thermal equilibrium with its surroundings.
In contrast, other objects, known as real objects, do not absorb all incident radiation in the same way as a blackbody. Real objects have various levels of reflectivity and transmissivity for different frequencies of electromagnetic radiation. Reflectivity refers to the ability of an object to reflect incident radiation, while transmissivity refers to the ability of an object to transmit radiation through it.
The relationship between an object's reflectivity, transmissivity, and absorptivity is governed by the principle of conservation of energy. According to this principle, the sum of the reflectivity, transmissivity, and absorptivity of an object for a given frequency of radiation is always equal to 1.
In practical terms, the ability of an object to absorb radiation depends on its material properties, surface characteristics, and the wavelength/frequency of the incident radiation. For example, objects that appear white to our eyes, such as snow or certain types of paint, have high reflectivity for visible light and tend to reflect most of the incident radiation. On the other hand, objects that appear black, like charcoal or a black matte surface, have high absorptivity for visible light and tend to absorb most of the incident radiation.
It's important to note that while a blackbody absorbs all incident radiation, real objects may absorb only a portion of the incident radiation depending on their material and surface properties. The concept of a blackbody is useful in physics and thermodynamics as a theoretical reference to understand radiation and energy transfer, but in reality, no object perfectly behaves as a blackbody across all frequencies.