Metals consist of: - positively
charged ions called kernals and negatively charged electrons. Kernals are fixed
on their positions and does not move whereas electrons are free and they move
in between positively charged ions but still they do not allow to leave the
metal surface. Now the question arises that is it possible to knock out these
free electrons from metal surface? The answer will be YES.
If
we want to eject electrons from the metal surface, it is necessary to give some
energy to these free electrons. There are some methods which can cause the
ejection of electrons-
First is THERMIONIC EMISSION-
It is the phenomenon of emission of electrons from the metal surface with the help of thermal energy. When we heat the metal surface the free electrons start getting energy and they excited and consequently start ejecting out of the metal surface. More thermal energy will cause more ejection of electrons. Electrons ejected are called thermal electrons.
Second is FIELD EMISSION- It is the phenomenon of ejection of electrons from the surface of metal by applying strong electric field. Battery (denoted by B) connected with metal surface (denoted by A). Positive ions of battery will start attracting electrons from metal surface. First electrons will along battery side of metal and as electric field goes on increasing, it will cause the ejection of electrons out of metal.
Now let’s think- Is it possible to eject electrons from metal surface with the help of light?
Answer
will be- YES
This
process is named as PHOTOELECTRIC EMISSION
Photoelectric emission is a phenomenon of emission of electrons from the surface of metal when light of suitable frequency falls on it.
When light falls on metals, electrons get energy and get excited after that will ultimately comes out of the metal surface. This phenomenon was explained by Hertz, when he was conducting an experiment related to Maxwell electromagnetic radiation.
HERTZ OBSERVATION - Hertz prepared an apparatus in which he used two plates, one is cathode C and other is anode A. Plate C is photosensitive plate and is given negative potential and plate A is given positive potential. Whole apparatus is evacuated in glass tube. Through a window, light of suitable frequency is made to fall on plate C. Electrons get energy and will start coming out of the plate C and will start attracting towards positive anode A and thus will constitute current. Electrons thus ejected are called photoelectrons and current formed is called photo current.
