the temperature of a body is due to the vibrations of the atoms of a body.so when a hot and a cold body come in contact the vibrations of the atoms become equal due to collisions and thus we say heat is transferred.but how can the same transfer of heat occur due to radiations that can travel without any medium?
Bodies can emit infrared radiation and lose heat and also can absorb infrared heat and get hotter. Hence two bodies inside a hypothetical box will reach the same temperature by infrared “exchange”
what i meant to ask was how can heat be in the form of radiations?
‘Heat’ is the kinetic energy associated with the random movement of the molecules of the hot substance. But it still can be transmitted by EM radiation, when a molecule absorbs the radiation it gets the energy from it.
Exactly how the molecule ‘absorbs’ the radiation is a mystery to me. We had an ‘Atoms and Light’ course in the penultimate year of my degree but I didn’t get past the first page 
I imagine you need quantum mechanics for the specifics. Even so, you can still easily grasp the key idea, that EM-radiation carries energy, and conservation assures that a molecule that absorbs radiation gets the energy.
Electrons in atoms can absorb photons by moving to a higher state of energy. Molecules like H2O can also be directly excited by changing magnetic fields, like in a microwave oven.
Well I knew that, I guess I was just saying that I’ve never seen a picosecond-by-picosecond account of what the wavefunctions of the photon and electron look like over the course of the absorption. However, even if I did, I wouldn’t understand it (My understanding of wavefunctions never really got beyond single particles)
However I’ve just had a bizarre notion… in general, the energy of the incoming photon won’t be exactly equal to the energy gap between two states of the electron… so… where does the extra energy go? Thermal motion of the atom?
I’ve wondered the same thing. What is the probability that the photon’s energy is exactly equal to the difference in energy between two states of the electron? If you had a high-resolution spectrogram, how narrow would the absorption and emission lines be? From what I’ve read about research-grade atomic clocks, they go to a lot of trouble to cool the atoms, minimizing motion and collisions.
Excess energy can indeed go into thermal energy – I did most of my grad work on energy transfer between molecular ions in a crystal matrix. Sometimes energy would be transferrred to a molecular ion which contained a rarer isotope. The energy difference went into or camer out of the matrix as a whole. One neat tresult was that, if the isotope sat at a lower energy level, and you cooled the entire crystal down to close to absolute zero, it couldn’t extract energy from the crystal matrix, and it would be trapped at that isotope.
In the real world, I need to point out, your IR radiation comes in a whole broad band of wavelengths, and your objects aren’t single component materials. You sneakers have a lot of dfifferent components in them, and the polymers have huge numbers of different vibrational frequencies. A lot of photons will by chance have just the right energies to coincide with some absorbtion in the material, and the energy levels will be broadened by thermal motion anyway.
To try and simplify a lot for anyone confused by the above
Atoms can possess two types of energy - moving around (which is what you normally think of as heat). The second main type is the energy of the electrons - normally they are in the bottom state but can be easily excited by light, infrared etc to higher energy states. These excited states are very unstable and usually only survive for a billionths of a second before dropping back to normal. The energy they gained by jumping up has to be released somehow - either as light (in which case we say it is fluorescent) or as heat. Hence the electrons provide a pathway for infrared light to be converted to heat.