What i know is that light is an electromagnetic wave. Is heat also an electromagnetic wave of certain wavelength other than light ? Why the light wave cause heat in an object when it falls on the object ? and why does an electric heater emits light ? It means both of these are very closely related but i am not understanding the relation between the two?
[Physics] the difference between heat and light
thermodynamicsvisible-light
Best Answer
(Source: Wikipedia)
So, to put it simply, we talk of "heat" whenever there is transfer of energy without work or transfer of matter. This is what is stated in the first law of thermodynamics:
$d U = \delta Q - \delta W$
which holds for a closed system (we rule out the transfer of matter).
The variation of the internal energy of a closed system which is not due to work is what we call "heat".
The word "light" usually refers to visible light, that portion of the electromagnetic spectrum whose wavelength is between $\sim400$ and $\sim700$ nm. But we can more generally talk about "electromagnetic radiation" and consider the whole spectrum.
As stated in the Wikipedia article I quoted, heat can be radiation between two bodies. This just means that the "$\delta Q$" in the first law of thermodynamics can be due to electromagnetic radiation.
Every body at a temperature $T$ different from $0$ will emit electromagnetic radiation (thermal radiation). This is because the atoms inside the material are moving, and this causes accelerations and decelerations of the charged particles the atoms are made of, and therefore electromagnetic radiation.
Therefore, an object at temperature $T$ will emit electromagnetic radiation in the surroundings (which are at temperature $T_e$) and absorb radiation from them. At the same time, the surroundings will emit and absorb electromagnetic radiation in the same way. The higher the temperature, the more the radiation.
This process will stop only when $T_e=T$, because then the absorbed radiation will be equal to the emitted radiation. So, the object and its surroundings will start at different temperature and end up having the same temperature. Since temperature and internal energy are related, this will mean that the internal energy of the bodies has changed. Seeing as how there has been no work or transfer of matter, we call heat this change of energy, which was due to electromagnetic radiation.
This one of the possible connections between heat and electromagnetic radiation.
Update: clarification
Of course there is also EM radiation which isn't thermal, i.e. it is not originated by the acceleration/deceleration of charges due to random motion. An example is given by transition between atomic energy levels, which is the mechanism behind the laser. This kind of radiation can of course be used to transfer energy without work/transfer of matter, so it is a kind of heat. The EM radiation is always the same: what changes is the mechanism originating it.
It is also clear that you can have heat without EM radiation. For example you put two objects in contact and transfer energy by conduction.
To sum up: heat is a change in energy without work/transfer of matter. This transfer can be realized via EM radiation or via other means (conduction, convection etc.). Also, EM radiation can originate in different ways: thermal motion, electronic transitions etc.