Objects that absorb radiant energy must therefore also somehow release energy again. This also applies to black painted objects, which can be regarded as ideal black bodies in very good approximation. At some point a thermal equilibrium will be established. In reality, however, one notices that the temperature of irradiated objects does not rise permanently. Figure: Heating of an object by absorption of radiation As long as the radiation is only partially absorbed, the temperature should rise steadily due to the permanently absorbed radiant energy. It does not matter whether it is an ideal or a real black body. Experience shows, however, that objects placed in the sun and thus irradiated do not heat up permanently. The absorbed radiant energy becomes noticeable in a temperature increase of the object (increase of the internal energy)!Īs long as a black body is irradiated and absorbs energy, its temperature would have to rise steadily. If this energy is absorbed by an object, then the absorbed energy cannot simply disappear. Electromagnetic radiation is always connected with radiant energy, depending on the wavelength. From an energetic point of view, however, this conclusion is quickly disproved. One could now jump to the conclusion that a blackbody does not emit any electromagnetic radiation. Figure: Color vision of a black objectĪ blackbody is an idealized object that absorbs all incident electromagnetic radiation! Thermal radiation For this reason, the charcoal shown in the figure below also shows slight shading on the left hand side. That’s why even black objects often show slight shading. In reality, every object will always reflect a part of the incident radiation. A perfect black body, however, only exists in the ideal conception. An object that absorbs all incident radiation is therefore also called a blackbody (or black body). A black object therefore does not emit any visible radiation. The object appears black to us under normal conditions.
If, on the other hand, an object absorbs all incident wavelengths of visible light, then obviously no visible radiation is reflected. Thus, the reflected light contains exactly the wavelength range that gives us the green color impression.
These reflected wavelengths determine the perceived color of the object.Ī green leaf absorbs almost all wavelengths except the green wavelength range between 500 nm and 550 nm. Depending on which wavelengths are absorbed, the reflected light consists of certain wavelengths. When white light hits an opaque object, a certain part of that light is always absorbed and the rest is reflected.
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