explaination about photo electric effect

1. B Y
S R I N I V A S V A D D E P A L L Y
Photoelectric effect

2. Introduction to Photoelectric effect
 The phenomenon of Photo electric effect was discovered in 1887 by Heinrich Hertz during his
observations on electromagnet wave experiments. In his experimental investigation on the production
of electromagnetic waves by means of a spark discharge. Hertz observed that high voltage sparks
across the detector loop were enhanced when the emitter plate was illuminated by UV light from an arc
lamp.
 Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of photoelectric
emission in detail during 1886-1902. Lenard observed that when UV radiations allow to fall on the emitter
plate of an evacuated glass tube enclosing two electrodes, current flows in the as shown in fig. The UV
radiations were stopped, the current flow also stopped.
 The observations indicates that when UV radiations incident on the
emitter plate C, electrons are ejected from it, which are attracted
towards the positive collector plate A by the electric field.
 The electrons flows through the evacuated glass tube,
resulting in the current flow. Thus light incident on
the surface of the emitter causes current in the external
Circuit. Hallwachs and Lenard are also studied the variation of
Photoelectric current with collector plate potential, frequency and
Intensity of the incident light.

3.  After the discover of the electron in 1897, it was became evident that the incident light causes electrons
to be emitted from the emitter pate. Due to negative charge, the emitted electrons are pushed towards
the collector plate by the electric field. Hallwachs and Lenard also observed that , When UV light
incident on the emitter plate , no electrons were emitted at all when the frequency of the incident light
was smaller than a certain minimum value, called the threshold frequency . This minimum frequency
depends on the nature of the material of the emitter plate.
 It was found that certain materials like Zn,Cd,Mg,etc responded only to UV light and having short
wavelengths to cause photoelectric emission from the surface. However, some alkali metals such as K,
Na, Li, Ru, Ce were sensitive even to visible light. All these photosensitive substances emit electrons
when they are illuminated by light. These electrons were termed as photoelectrons. The phenomenon
is called as Photoelectric effect.

4. Experimental study of Photoelectric Effect