An object radiates and absorbs electromagnetic waves when it is accelerated or decelerated due to the fundamental principles of electromagnetism, specifically Maxwell's equations.
Maxwell's equations describe the behavior of electromagnetic fields and their interaction with charges and currents. One of these equations, known as Maxwell's acceleration equation or the law of acceleration, states that a changing electric field (accelerated charges) produces a magnetic field, and a changing magnetic field produces an electric field. This relationship between changing electric and magnetic fields gives rise to the propagation of electromagnetic waves.
When an object undergoes acceleration or deceleration, the charges within the object are also accelerated or decelerated. This acceleration causes the charges to undergo changes in velocity, resulting in the generation of a time-varying electric field and, subsequently, a time-varying magnetic field. These changing electric and magnetic fields propagate away from the object as electromagnetic waves.
The accelerated charges act as sources of electromagnetic radiation, and the energy associated with this radiation is emitted as electromagnetic waves. The radiated waves carry energy away from the object in the form of electromagnetic radiation.
Similarly, when an object is exposed to an external electromagnetic wave, the electric and magnetic fields of the wave can induce currents in the object. If these induced currents experience acceleration or deceleration, they, in turn, radiate electromagnetic waves. This phenomenon is known as absorption of electromagnetic waves.
In summary, when an object is accelerated or decelerated, the charges within the object undergo changes in velocity, resulting in the generation of changing electric and magnetic fields. These changing fields propagate as electromagnetic waves, causing the object to radiate electromagnetic radiation. Conversely, when an object absorbs an external electromagnetic wave, the induced currents within the object may also accelerate or decelerate, leading to the emission of electromagnetic waves.