Hertz and Lenard's experiments, discovery and observation of photoelectric effect, practice problem, FAQs

The existence of electromagnetic waves moving at light speed was one of the important predictions of Maxwell theory of electromagnetism which concluded that light itself is just such a wave. This motivated many experimentalists to generate and detect electromagnetic waves with Heinrich Hertz being the first to make a successful attempt. Although he was successful in generating the waves, he discovered an important phenomena later called photoelectric effect during his experiments.

Table of contents

Hertz observation
Hallwachs’ and Lenard’s Observations
Practice problems
FAQs

Hertz observation

The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz (1857—1894) the phenomena of photoelectric effect was first observed by Heinrich Hertz in 1887.

While studying experimentally the production of electromagnetic waves by means of spark discharge. He found that when the emitter plate was illuminated by ultraviolet light, high-voltage sparks across the detector loop were enhanced. This observation led him to conclude that light facilitated the emission of some electrons.From this it was concluded that when suitable radiation 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.

Hertz has contributed to studying electromagnetic waves. He produces the EM waves by using spark plugs. He found that when UV light is placed on the emitter plate a high voltage spark is generated . This led him to conclude that there is potential difference generated due to UV light on the emitter plate.light helps the electron to lose the surface of the metal and this creates the potential difference.so if suitable frequency of light lead to emission of electron from the surface of metals.

Hallwachs’ and Lenard’s Observations

Wilhelm Hallwachs and Philipp Lenard studied in detail the phenomenon of photoelectric effect during

1886—1902.

Lenard (1862-1947) observed flow of current when ultraviolet radiation is exposed on the emitter plate of an evacuated glass tube enclosing two electrodes (metal plate). The current flow stops as soon as the ultraviolet radiation is stopped. These observations indicate that electrons are ejected from emitter plate C, when ultraviolet radiations fall on it which are attracted towards the positive, collector plate A by the electric field.

Thus current in the external circuit is due to light falling on the surface of the emitter.

Hallwachs and Lenard studied variation of photocurrent with collector plate potential and with frequency and intensity of incident light. Hallwachs, in 1888 for further study, connected a negatively charged zinc plate to an electroscope. He found that when a zinc plate was illuminated by ultraviolet light it lost its charge. When an uncharged zinc plate was illuminated by ultraviolet light, it became positively charged. Further when a positively charged zinc plate was illuminated by ultraviolet light it was found to be further enhanced. He concluded from these observations that under the action of ultraviolet light negatively charged particles were emitted from the zinc plate.

It became evident after the discovery of the electron in 1897 that the incident light causes electrons to be

emitted from the emitter plate. The emitted electrons due to its negative charge are pushed towards the

collector plate by the electric field. Halwwachs and Lenard also observed that when the frequency of the incident light was smaller than a certain minimum value, no electrons were emitted at all from the emitter plate. This minimum frequency is called the threshold frequency and it depends on the nature of the material of the emitter plate.

It was found that sorne alkali metals such as lithium, sodium, potassium, caesium and rubidium were sensitive even to visible light whereas certain metals like zinc, cadmium, magnesium etc. responded only to ultraviolet light, having short wavelength for electron emission from the surface. Electrons are emitted, when photosensitive substances are illuminated by light. These electrons were termed as photoelectrons after the discovery of electrons. This phenomenon is known as the photoelectric effect. The electric current constituted by photo-electrons is known as photoelectric current.

The minimum amount of energy required to remove an electron from the metal surface is called work function (𝜙). Work function of some common metals are provided in the table below.

Metal	Work function(eV)
Cs	2.14
K	2.30
Na	2.75
Ca	3.20
Mo	4.17
Pb	4.25
Al	4.28
Hg	4.49
Cu	4.65
Ag	4.70
Ni	5.15
Pt	5.65

Practice problems

Q1. What is the photoelectric effect?
Answer: The ejection of electrons from a metal plate when illuminated by light or any other electromagnetic radiation of suitable wavelength (or) frequency is called photoelectric effect.

Q2. How will you define threshold frequency?
Answer: For a given surface, the emission of photoelectrons takes place only if the frequency of incident light is greater than a certain minimum frequency. The minimum frequency is called threshold frequency.

Q3. What were Lenard’s Observations regarding the kinetic energy of the photoelectron's dependence on the intensity of incident light?
Answer: It was observed by Lenard that intensity of incident light has no effect on the maximum kinetic energy of the photoelectrons. A bright light and a dim light of same frequency ejects electrons with the same energy. Although more electrons got ejected from the bright light as compared to the dim light.

Q4. What is the importance of the photoelectric effect?
Answer: It demonstrates that light has particle-like qualities. It is established that light is photons (packets) of energy where each photon interacts with the electrons and each photon must have sufficient energy to remove each electron. This is why, in the original photoelectric experiment, increasing the intensity of the light didn't excite the electrons in the metal surface. The effect has significant importance in the field of research in material science and forms the basis for a variety of useful devices.

FAQs

Q1. List some devices based on photoelectric effect.
Answer: Photoelectric cell or photodiodes, photoconductive cells, photovoltaic devices etc

Q2. Why do we not observe the photoelectric effect with non-metals?
Answer: For non metals work function is very high.

Q3. How did the experimentalist know that it was electrons that are getting emitted?
Answer: Since the charged particles were attracted by the positive plate, therefore they might be carrying negative charge. Later further studies showed that these were electrons.

Q4. What led Hertz to the discovery of the photoelectric effect?
Answer: The enhancement of the spark length when the emitter plate is illuminated by ultraviolet radiation led Hertz to discover the effect.