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

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

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.

Hallwachs’ Observation

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’s Observation

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.

Frequently Asked Questions (FAQs)

1. What did Hallwachs observe in the photoelectric effect?

Hallwachs observed that when ultraviolet light falls on a negatively charged zinc plate, the plate loses its negative charge. This happens because electrons are emitted from the metal surface due to the photoelectric effect.

2. What was Lenard's contribution to the study of the photoelectric effect?

Lenard performed detailed experiments on photoelectric emission and studied the effect of light intensity, frequency, and applied potential on the emitted electrons. His observations provided important evidence for the quantum nature of light.

3. What is photoelectric emission?

Photoelectric emission is the phenomenon in which electrons are emitted from the surface of a metal when light of suitable frequency falls on it.

4. How does light intensity affect photoelectric current?

An increase in light intensity increases the number of emitted photoelectrons and hence increases the photoelectric current, provided the frequency is above the threshold frequency.

5. What is threshold frequency?

Threshold frequency is the minimum frequency of incident radiation required to eject electrons from a metal surface. Below this frequency, photoelectric emission does not occur.

6. Does increasing the intensity of light increase the kinetic energy of photoelectrons?

No. The maximum kinetic energy of photoelectrons depends on the frequency of the incident light, not on its intensity.

7. Why was classical wave theory unable to explain Lenard's observations?

Classical wave theory could not explain the existence of threshold frequency, the instantaneous emission of electrons, and the dependence of kinetic energy on frequency rather than intensity.

8. Which theory successfully explained Hallwachs' and Lenard's observations?

Einstein's photoelectric theory, based on Planck's quantum hypothesis, successfully explained all the observations related to the photoelectric effect.

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