What phenomenon can occur when electromagnetic radiation strikes a black body?

When electromagnetic radiation strikes a black body, the correct phenomenon observed is that it is all absorbed by the black body. A black body is defined as an idealized physical object that perfectly absorbs all incident electromagnetic radiation, regardless of the wavelength or angle of incidence. This characteristic makes black bodies important in the study of thermal radiation and thermodynamics, as they serve as the perfect absorbers and emitters of radiation.

The significance of a black body lies in its ability to absorb energy completely without reflecting any part of it. This property is foundational in understanding concepts such as Planck's law, which describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature.

In contrast, other options are not applicable to a true black body. A black body does not reflect wavelengths nor does it change color based on the radiation it encounters; it absorbs and re-emits energy. Additionally, the notion that it could heat up without absorbing any radiation contradicts the definition of a black body as a perfect absorber. Therefore, the defining feature of a black body is its complete absorption of all incident electromagnetic radiation, making the first answer the most accurate representation of the phenomenon involved.