The concept of a "quantum" in quantum physics refers to the fundamental unit or packet of energy associated with a particular type of particle or field. In the case of light, the quantum is referred to as a "photon." Photons are the elementary particles that make up electromagnetic radiation, including visible light, radio waves, X-rays, and other forms of electromagnetic waves.
The reason why we specifically refer to photons as quanta of light is due to the nature of the electromagnetic interaction. Electromagnetic waves, such as light, radio waves, and X-rays, are described by the electromagnetic field. Quantum theory explains that the energy of the electromagnetic field can be quantized into discrete packets or quanta, and each quantum corresponds to a single photon.
The energy of a photon is directly proportional to its frequency. Light, radio waves, and X-rays all have different frequencies, which means they carry different amounts of energy per photon. The distinction between light, radio waves, and X-rays lies in their respective frequencies and energies, but they all consist of photons.
Therefore, while we specifically use the term "photon" to describe quanta of light, the same concept applies to other forms of electromagnetic radiation. The term "quantum" is a general term referring to the smallest unit of energy associated with a particular particle or field, and it applies to all particles and fields in quantum physics.