Why electron do not eject out even when there is photon of less threshold energy with increasing in time When photon having certain energy less than threshold energy  strikes on the electron of metallic plate. Electron do not eject out. But my question is when photon are falling continuously then electron must gain the energy to eject out.as electron have the energy transferred from earlier photon and next photon will transfer his energy too.so combination of these energy must be greater than the required energy for moving out from his path.
I know it's a silly question but i have doubt so asking it...
 A: It's not that simple. Electron can only absorb such photons which exactly hits electron quantum jump levels, for example check Lyman, Balmer series, etc. Otherwise non-compatible photons "are ignored" by electrons. Unless incident light is very strong, i.e. you shine with an intensive laser light on metal, even laser wavelength does not pass metal work function,- in this case non-linear effect can happen, such as multi-photon absorption. It is explained that intensive laser light weakens atom potential barrier, so electrons can escape atom by tunneling ionization process. But non-linear tunneling ionization frequency is covered by such law :
$$ {{\omega }_{t}}=\frac{eE}{\sqrt{2m_e{ {\mathcal E} }_{i}}} $$
where $E$ is amplitude of incident electric field, ${ {\mathcal E} }_{i}$ - ionization potential. So answer is that electrons can eject out of metal even photons does not pass ionization energy barrier, but... just in case strong electric field applied when non-linear effects can begin to happen.
A: The claim is, light with too long a wavelength cannot get electrons to leave atoms. They do not store energy from that light until they have enough to leave. The light has to have a short enough wavelength or it won't happen.
This was unexpected and for awhile no one understood how it could happen. There were two choices:

*

*There is something special about atoms.

*There is something special about light.

Maybe atoms have a sort of short-pass filter and only short-wavelength light can affect their electrons. When short-wavelength light hits an atom it can store the energy until it has enough to lose an electron, but longer-wavelength light won't do that.
Or maybe light does not actually travel in waves, but in energy packets. Atoms never store light energy, but must either absorb an entire energy packet or else not absorb it. Similarly whenever anything emits light, it either emits a whole quantum of light or none at all. If it does emit a photon, that photon travels until something absorbs the whole thing.
Einstein proved beyond any possible doubt that the latter explanation is right and the former explanation is wrong. I don't understand exactly how he did it, but he got a Nobel prize for it so he must have been right.
A: This is just a matter of statistics. The number of atoms is typically many orders of magnitude greater than the number of photons. Therefore the mean time between excitations of the same atom is very long -- much longer than the time required for the atom's excitation energy to be thermalized.
