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“we know about absorption & emission spectra. We get colours; so what is orgin of colurs? why the atoms emit colour spectra”
Light is emitted when an excited atom returns to its ground state. When excited, the electrons jump from lower level to higher level absorbing the energy equal to the difference in energy levels corresponding to the orbits. When these electrons come back to the original states the energy is emitted.
As we know E = hν=hc/λ
Accordingly light of different wavelengths (and hence different colours) are emitted depending on the energy difference between the levels of transition.
For more details, please refer to these links.
“In [wiki]photoelectric effect[/wiki] electrons emit out leaving behind positive charge then why [wiki]electron emission[/wiki] not stop after sometime.”
The photoelectric current can continue for long only if the lost electrons are replenished by some means. Unless, the positive potential will go on increasing until a stage is reached when no more electrons can be released with the the particular frequency of incident radiation.
Photoelectric effect takes place in the case of metals which have plenty of free electrons. The electrons lodged off by light could easily be regained as there is not much energy difference from a stray electron and a free electron on the surface of a metal.
Further, in a metal, the free electrons does not belong to a single atom, each atom is surrounded by a sea of electrons. SO it is not right to say that each electron released creates an ion each – but creates a net positive charge for each electron lost.
(Comments and further explanations from visitors solicited)