I was wondering what happens to a ray of light after it is completely absorbed by a surface (making it appear black). I mean, obviously, the energy should be conserved. So does the energy change into heat energy? If yes, then what amount of light would heat a unit area (say, 1 metre squared) to raise it's temperature by 1 degree centigrade? Is there any way to calculate this?
So does the energy change into heat energy?
That's the most common case.
But for example if the light is absorbed by a solar panel, some of it could be converted to electrical energy.
If yes, then what amount of light would heat a unit area (say, 1 metre squared) to raise it's temperature by 1 degree centigrade?
That depends on the properties of the material the absorber is made of and how thick the material is. In practice it also depends on what processes are cooling the material (for example, is there air flowing across the surface).
Is there any way to calculate this?
You can, but there are typically many unknowns (mostly related to cooling mechanisms), so you usually can only get a rough estimate or a range of values.
what if we exclude all the external influences like air, etc. What if all of the light energy is converted to heat, how will then we calculate this?
Then you need to know the heat capacity of the absorbing object.
If the heat capacity is 1 J/K, then each joule of light energy it absorbs will raise its temperature by 1 K.
But more often we're interested in the temperature the object reaches when it's absorbing optical power continuously rather than when it just receives a fixed amount of energy. In this case we need to find the temperature that produces thermal equilibrium between the absorbed power and heat transferred to surrounding objects through conduction, convection, and radiation.