When electrically charged particle moves under acceleration, it emits electromagnetic radiation.
So, the question is that when an electrically-charged particle, say a proton or an electron is at rest, then it will emit no EM radiation?
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$\begingroup$ Correct, either at rest or moving with constant speed $\endgroup$– user65081Commented Aug 6, 2021 at 5:09
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Accelerating charged particles, electrons or protons, emit electromagnetic waves. This means if they are moving with a constant velocity, or they are stationary, they will not emit electromagnetic radiation (photons).
An electron in an atom (in an excited state) can emit a photon, and drops to a lower energy level. But a free, isolated, non-accelerating electron will not spontaneously emit a photon, because conservation of momentum/energy will not hold.
Also, a proton (in a nucleus) can go through a different radiative process, called beta decay where a proton decays into neutron with the emission of a positron and a neutrino. And a free proton does not spontaneously decay (as far as all experiments have shown).
Also, at the macroscopic level, large material objects emit thermal electromagnetic waves, or thermal radiation. This is because again, the charged protons and electrons (in the atoms that make up a macroscopic object), are constantly vibrating. When these charged particles vibrate, since they are being rapidly accelerated and decelerated, they will emit these electromagnetic waves. Again, an acceleration of charged particles is required for the emission of electromagnetic waves.
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$\begingroup$ But objects at rest like a chair is visible. So does it means that chair emits EM radiation or it just reflects photons which fall on it so we see it or both? $\endgroup$– AnubisCommented Aug 6, 2021 at 6:15
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1$\begingroup$ It does indeed simply reflect photons that originated from a light source. Without the source, you would not see the chair. Also note that all objects emit thermal radiation (eg., infrared light emitted by hot bodies), that you cannot see without a thermal camera. This is because the atoms that make up a substance are continuously vibrating. Very good point. Cheers. $\endgroup$– joseph hCommented Aug 6, 2021 at 6:43
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$\begingroup$ I read somewhere that since sub-atomic particles are constantly moving and vibrating, they emit EM radiation but cancel each other out a lot due to destructive interference nonetheless they emit little EM radiation because the charge is not uniformly distributed. So this means neutral objects also emit EM radiation however small and the reflection of photos is there. So these two factors make up the EM radiation. Am I getting the hang of it? $\endgroup$– AnubisCommented Aug 6, 2021 at 8:38
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$\begingroup$ Sorry to bother you again. But can you clarify this: A neutral object in the dark still emits EM radiation as it is hot (above than absolute zero) and it vibrates so it is somewhere in EM spectrum but we cannot see it as it is not hot enough for its frequency to be in the range of visible light. $\endgroup$– AnubisCommented Aug 6, 2021 at 9:48
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$\begingroup$ That is correct. If the object was continuously heated, it would eventually reach a point where the emitted radiation would become visible. It is also important to note that most objects emit very small amounts of visible, but undetectable light. For example, humans do radiate in the visible range, but in such small amounts that it isn't detectable. If you look at a Planck black body power density versus $\lambda$ curve for any object with about human body temperature, the small $\lambda$ part of the curve is not zero in the visible range, but still it's $\gt 0$. You're correct. Cheers. $\endgroup$– joseph hCommented Aug 6, 2021 at 10:43