What is photo electric effect?

In this topic we will explain the photoelectric effect and also will explain hertz’s observation and Hallwach’s and Lenard’s observation.

Photoelectric Effect: Hertz, Hallwachs and Lenard’s Observations

1. What is the Photoelectric Effect?

The photoelectric effect is the phenomenon in which electrons are emitted from the surface of a metal when electromagnetic radiation of a suitable frequency falls on it.

The emitted electrons are called photoelectrons, and the current produced by their motion is called photoelectric current.

Example: When ultraviolet light falls on a clean zinc plate, electrons may be emitted from its surface.

Important terms

• Photoelectrons: Electrons emitted from a metal surface due to incident light.
• Work function (ϕ): The minimum energy required to remove an electron from a metal surface.
• Threshold frequency (${\nu }_0$): The minimum frequency of light required to produce photoelectric emission.
• Stopping potential ($V_s$): The minimum reverse potential required to stop even the fastest photoelectrons.

Einstein’s photoelectric equation is

$$\boxed{{\displaystyle h\nu =\varphi +K_{\max }}}$$

where:

• $h$= Planck’s constant
• $\nu$ = frequency of incident light
• $\varphi$ = work function of the metal
• $K_{\max }$ = maximum kinetic energy of emitted electrons

Since $\varphi =h{\nu }_0$

$$\boxed{{\displaystyle K_{\max }=h(\nu -{\nu }_0)}}$$

This equation explains the main features of the photoelectric effect.

2. Hertz’s Observation (1887)

Heinrich Hertz

Heinrich Hertz was conducting experiments to demonstrate electromagnetic waves. During his experiments, he observed that ultraviolet radiation helped an electric spark pass between two metal electrodes.

Experimental observation

1. Hertz produced an electric spark between two metal electrodes.
2. He observed that ultraviolet light falling on the electrodes made the spark easier to produce.
3. When the ultraviolet radiation was blocked, the spark became more difficult to produce.

Conclusion

Hertz’s observation indicated that ultraviolet radiation could help release electrons from a metal surface, facilitating the discharge.

Importance: His observation was an early discovery of the photoelectric effect.

3. Hallwachs’s Observation (1888)

Wilhelm Hallwachs

Wilhelm Hallwachs studied the effect of ultraviolet light on electrically charged metal plates, especially a clean zinc plate.

Experiment

1. A clean zinc plate was connected to a negatively charged gold-leaf electroscope.
2. The electroscope’s gold leaf diverged because the plate and electroscope carried negative charge.
3. Ultraviolet light was allowed to fall on the zinc plate.
4. The gold leaf gradually collapsed, indicating that the electroscope was losing its negative charge.

Observation and conclusion

• Negatively charged zinc plate: It loses negative charge when exposed to suitable ultraviolet light.
• Positively charged zinc plate: It does not readily discharge through photoelectric emission because the emitted electrons are attracted back to the positively charged plate.
• Uncharged zinc plate: It can become positively charged after photoelectrons escape.

Conclusion: Ultraviolet light causes electrons to leave the zinc surface.

This provided clear evidence that light can cause the emission of electrons from a metal.

4. Lenard’s Observation (1902)

Philipp Lenard

Philipp Lenard investigated photoelectric emission using a metal surface and an apparatus in which emitted electrons could be collected and measured.

Experimental setup

In a typical arrangement:

• Light falls on a metal cathode.
• Photoelectrons are emitted from the cathode.
• A collector electrode (anode) collects the electrons.
• The resulting photoelectric current is measured using an external circuit.

Main observations of Lenard

1. Photoelectric current depends on light intensity.

For light of frequency above the threshold frequency, increasing the intensity generally increases the number of emitted electrons per second. Therefore, the photoelectric current increases.

2. Maximum kinetic energy depends on frequency.

Increasing the frequency of incident light increases the maximum kinetic energy of photoelectrons, provided the frequency is above the threshold.

3. Existence of stopping potential.

A reverse potential can be applied to oppose the motion of photoelectrons. At a particular reverse potential, even the fastest electrons cannot reach the collector, and the current becomes zero.

$$\boxed{{\displaystyle K_{\max }=e{V}_s}}$$Kmax​=eVs​​

Here, $e$e is the magnitude of the electron’s charge and $V_s$Vs​ is the stopping potential.

4. Threshold frequency exists.

Below a certain minimum frequency, no photoelectrons are emitted, even if the intensity of light is increased.

Conclusion: Lenard’s experiments established important properties of photoelectric emission and showed that the energy of photoelectrons is related to the frequency of incident light.

5. Comparison of Hertz, Hallwachs and Lenard

Scientist	Main observation	Significance
Hertz (1887)	UV light made an electric spark easier to produce.	Early discovery of photoelectric emission.
Hallwachs (1888)	UV light discharged a negatively charged zinc plate.	Demonstrated that electrons leave the metal surface.
Lenard (1902)	Studied emitted electrons, their current, kinetic energy and stopping potential.	Established important experimental laws of the photoelectric effect.

6. Key results to remember for exams

Intensity of light

Increasing intensity increases photoelectric current, provided the frequency is above the threshold.

Frequency of light

Increasing frequency increases the maximum kinetic energy of photoelectrons.

Threshold frequency

Below the threshold frequency, photoelectric emission does not occur, regardless of intensity in the ordinary one-photon photoelectric effect.

Stopping potential

The stopping potential depends on the maximum kinetic energy, and therefore on the frequency of light for a given metal.