Photoelectric Effect - Hallwachs’ and Lenard’s Observations

The emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency falls on it. This phenomenon is called the photoelectric effect.

Electrons emitted from the metal during photoelectric emission are called photoelectrons.

The minimum frequency of incident radiation required to just cause photoelectric emission from a given metal is called the threshold frequency.

At the end of the nineteenth century, scientists were studying the interaction between light and matter. Experiments by Hallwachs and Lenard showed that light can cause the emission of negatively charged particles from metals, which later became a major clue in the development of modern quantum physics.

Apparatus

Lenard used an evacuated glass tube containing two metal plates. One plate acted as the emitter, and the other acted as the collector. A potential difference was applied between the plates, and the circuit was used to detect electric current.

Procedure

1. Ultraviolet light was allowed to fall on the emitter plate.
2. Electrons were emitted from the illuminated plate.
3. These electrons moved toward the collector plate, producing current in the circuit.
4. When the ultraviolet light was removed, the current stopped.

Observation

• Current flowed only when suitable radiation fell on the emitter plate.
• The current disappeared when the incident light was removed.
• This showed that light was responsible for releasing charged particles from the metal surface.

Inference

Light can liberate electrons from certain metal surfaces. These emitted electrons are photoelectrons.

Apparatus

Hallwachs used a clean zinc plate attached to a gold-leaf electroscope. The zinc plate was given a negative charge and then exposed to ultraviolet light.

Procedure

1. A zinc plate was negatively charged.
2. The charged plate was connected to an electroscope.
3. Ultraviolet light was allowed to fall on the zinc plate.
4. The electroscope gradually lost its negative charge.

Observation

• The negatively charged zinc plate got discharged when ultraviolet light fell on it.
• This implied that negatively charged particles were leaving the metal surface.

Inference

The particles emitted from the surface were electrons. Therefore, light was causing electron emission from the zinc plate.

Observation 1: Existence of Threshold Frequency

For each metal, there is a minimum frequency below which photoelectric emission does not occur. Even a strong low-frequency light cannot produce emission if the threshold value is not reached.

Observation 2: Dependence on Material

Different metals respond to different minimum frequencies. Zinc, cadmium, and magnesium generally require ultraviolet radiation, whereas alkali metals such as lithium, sodium, potassium, rubidium, and caesium can emit even under visible light.

Observation 3: Emission is Linked to Incident Radiation

Photoelectric current appears only when suitable light falls on the emitter and stops when the light is removed. This confirmed a direct connection between incident light and electron emission.