Kumar said:
Sorry, I think it is not clear as spectral lines of Carbon & Oxygen do not matches with the number of electrons in their atoms.
Okay.
Each electron can have more than one possible transition.
If an electron normally sits in orbit A, it might also have higher possible orbits B and C. If we heat up our test material, we might find that some electrons jump from A to B, and then fall back again, releasing photons with the characteristic energy of B - A.
If we heat it more, we might get some electrons all the way to orbit C. Now they can either drop back from there to B, releasing a photon of energy C - B, and then to A as before, or they could drop straight back to A, releasing a higher energy photon C - A.
So one electron can give us any of three different energies (and hence wavelengths).
Can this heat keep body's atoms/moleclues-partly heated/excited all the time?
It can, of course, keep the body's atoms and molecules heated.
It can't keep them "partly excited", because this is physically impossible.
Read what I was just saying about electron orbits. An electron can be in orbit A, or B, or C. Orbit A is its "ground state"; orbits B and C represent excited states.
An electron can never be found between orbits A and B, or between B and C. It cannot ever happen.
This is one of the key predictions of Quantum Mechanics. It has been tested extremely thoroughly, and has been shown to be correct beyond any doubt.
How it become yellow>>white on increasing heat? White may mean all visible spectrum--how then it happens?
Because at any time it is radiating in a whole range of frequencies. Remember, this is blackbody radiation, which is completely different to the emission of photons by excited electrons.
When an object is relatively cool, its radiation is entirely in the infrared (but in a broad range of infrared frequencies). When you heat it to red hot, it is still radiating in the infrared, but now it is also radiating at the red end of the visible spectrum.
But the time it is white hot, it is radiating right across the visible spectrum, so it appears white.
Whether spectrum of sunrays is mostly yellow--or how yellow colour is particularily related to Sunlight? I want to compare it with similar yellow light emitted by burning of candle/oil lamp.
The spectrum of sunlight is not mostly yellow. You can see this for yourself with a prism - the spectrum is fairly evenly spread from red through to violet. The yellow and red parts are a bit brighter than the blue and violet, but it's still a continuous spectrum.
Remember how white light contains all the colours? Well, yellow light can contain all the colours too; it's just that it has a bit less light on the blue/violet end and a bit more on the yellow/red end.
You can also have purely yellow light - what is referred to as
monochromatic light. Lasers put out monochromatic light. Individual spectral lines are monochromatic. But the Sun is not, and neither are common light sources like light bulbs or candles.