Question

In absorbtion spectra,we pass em radiation thorugh unexited atoms .

(a)they absorb some energy, of which corresponding black bands are seen on the photoelectric plate. If they absorb energy from the light, that means the atoms went to an excited state((do all of them absob energy from light?)). so when they would return back to the ground state, they should emit radiation of the same wavelength, if they do why do we see black bands then?(i m talking abt absorbtion spectra)

Answer 1

Hi,
1) First, all atoms that are introduced to em waves absorb energy. But all atoms don't excite, or some excite temporarily.( this is answer to 2nd part)

All atoms have tendency to absorb specific EM waves of particular wavelength. When available, they easily absorb waves.

2) As i told before, all atoms donot reach exact excitation point.
This is because , all atoms are not getting enough energy, above the treshold energy needed for excitation.

Well, in absorption spectra, we use atoms in gaseous forms. Gases are highly disturbed, and paticles move to and fro in random fashion. These atoms gain some energy from Em,moves to a partially excited stage, and their motion becomes even faster. Before gaining the treshold energy, atoms collide with one another or with wall and lose energy.

So, the em absorbed is lost as heat or other forms , before atoms can properly excite and re emit the light absorbed.

If you want atoms to emit energy, you have to provide them enough energy in some other form, so that their collisions are less, or treshold energy is easily attained.

Only if treshold energy is attained, an atom can properly emit radiation, and give a emission spectra.

So, in absorption spectra, dark lines remain dark, because the light absorbed is lost as heat or other forms and is not re emitted as light.