HERTZ’S OBSERVATION
The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz, during his electromagnetic wave experiments. In his experimental investigation on the production of electromagnetic waves by means of spark discharge, Hertz observed that high voltage sparks across the detector loop were enhanced when the emitter plate was illuminated by ultraviolet light from an arc lamp.
Light shining on the metal surface somehow facilitated the escape of free, charged particles which we now know as electrons. When light falls on a metal surface, some electrons near the surface absorb enough energy
from the incident radiation to overcome the attraction of the positive ions in the material of the surface. After gaining sufficient energy from the incident light, the electrons escape from the surface of the metal into the surrounding space and metal gets more +ve charge.
HALLWACHS’ AND LENARD’S OBSERVATION
Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of photoelectric emission in detail during 1886-1902.
Lenard observed that when ultraviolet radiations were allowed to fall on the emitter plate of an evacuated glass tube enclosing two electrodes (metal plates), current flows in the circuit. As soon as the ultraviolet radiations were stopped, the current flow also stopped.
These observations indicate that when ultraviolet radiations fall on the emitter plate(cathode), electrons are ejected from it which are attracted towards the positive plate(anode). The electrons flow through the evacuated glass tube, resulting in the current flow. Thus, light falling on the surface of the emitter causes the current in the external circuit.
Hallwach in 1888, undertook the study further and connected a negatively charged zinc plate to an electroscope.
An electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects charge by the movement of a test object due to the Coulomb electrostatic force on it.
He observes that the zinc plate lost its charge when it was illuminated by UV light. Further, the uncharged zinc plate became -ve ly charged when it was irradiated by UV light. +ve charge on a +vely charged zinc plate was found to be further enhanced when it was illuminated by UV light. From these all observations, he concluded that -ve ly charged particles were emitted from the zinc plate under the action of UV light.
After the discovery of the electron in 1897, it became evident that the incident light causes electrons to be emitted from the emitter plate.
Hallwahs and Lenard also observed that when UV light fell on the emitter plat, 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 metals like zink, cadmium, magnesium etc responded only to UV light. However, some alkali metals such as lithium, sodium, potassium, caesium and rubidium are sensitive even to visible light.
The electrons emitted are known as photoelectrons and this phenomenon is called the photoelectric effect.
| Introduction | Electron Emission |
| Photoelectric Effect | Experimental Study of Photoelectric Effect |
| Photoelectric Effect and Wave Theory of Light | Einstein’s Photoelectric Equation: Energy Quantum of Radiation |
| Particle Nature of Light: The Photon | Wave Nature of Matter |
| Davisson and Germer Experiment | |