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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)
Varun G asked
“Do black holes produce thermal radiation, as expected on theoretical grounds and do they absorb light?”
Ans: Hope you are talking about the Hawking radiation.
Any body at a temperature above absolute zero emits radiations. If the temperature of black body is not absolute zero (It was Stephen Hawking who predicted that black holes should have a finite, non-zero temperature, and hence the name “Hawking Radiation”) it will emit radiation.
In a black hole emitting radiation, there is a loss of mass. If the mass decreased due to Hawking Radiation is more than the mass gained by the black holes via alternate means, the net mass of the black holes will go on decreasing. (This is called “black hole evaporation” Further, it has been noted that the black holes with lower mass emit more radiations than the heavier ones.
This answer may seem contradicting the definition of black hole itself.
“A black hole is a body whose gravitational force of attraction is so huge that even electromagnetic radiation cannot escape from it” as the definition goes.
But the Hawking Radiation is caused by Quantum effects. The processes behind the “escape” of radiation from a black hole is thought to be
- Vacuum Fluctuations and
- Quantum tunneling
The above terms and concept will be too high to be discussed at school level. However for the curious ones, I am giving some links to explore.