Notes : Hallwachs’ and Lenard’s Observations Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter - Param Himalaya

Notes : Hallwachs’ and Lenard’s Observations Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter - Param Himalaya

1. Hallwachs’ Observation

In 1888, Wilhelm Hallwachs observed that when ultraviolet light falls on a negatively charged zinc plate connected to a gold-leaf electroscope, the divergence of the gold leaves decreases.

This shows that the zinc plate loses negative charge. Hence, Hallwachs concluded that ultraviolet light causes the emission of negatively charged particles (electrons) from the metal surface. These emitted particles are called photoelectrons.

2. Lenard’s Observation

Philipp Lenard studied the effect of light on a metallic surface. He observed that: When ultraviolet light falls on the cathode (metal surface), electric current flows in the circuit. The current increases when UV light is incident on the cathode.Little or no current flows when UV light is removed.

Lenard also concluded that photoelectric emission occurs only when the incident light has a suitable minimum frequency, called the threshold frequency.

The threshold frequency is the minimum frequency of light required to eject electrons from a metal surface. Its value is different for different materials.

Result / Conclusion of These Observations : 

These observations led to the conclusion that: “When radiation of suitable frequency falls on a metallic surface, electrons are emitted from the surface.”This phenomenon is called the Photoelectric Effect.

The emitted electrons are called photoelectrons. The current produced due to these electrons is called photoelectric current.