Lenard’s observations

Introduction

Lenard’s observations are concerned with the phenomenon of photoelectric emission done during 1886-1902. Philip Lenard along  with Wilhelm Hallwachs observed that when electromagnetic waves were allowed to fall on metal plates in an evacuated tube, current flows in the circuit which was the result of photoelectric emissions from the metal plate. The current stopped as soon as the ultraviolet radiations were stopped. Hence, this current is referred to as photocurrent. In Lenard’s observation it was mainly noted that how this photocurrent varied with the potential of collector plate, intensity and frequency of the ultraviolet radiations. The main observations made was that the photoelectric emissions occurred only above a particular frequency which was thus named as threshold frequency. Another important observation made was that the number of photoelectrons emitted per second was directly proportional to the intensity of light.

Procedure of the experiment

In 1862-1947, Lenard carried out an experiment in which ultraviolet radiations were allowed to fall on one of the two metal plates enclosed in an evacuated glass tube. One plate was connected to the negative terminal and named the emitter plate. The other was connected to a positive plate and named the collector plate. The two plates had a circuit between them consisting of a galvanometer rheostat and an ammeter connected in series. The ammeter detected the presence of current in the circuit.The electromagnetic waves were incident on the emitter plates. 

Observations of photoelectric effect

It was noted that when ultraviolet rays fell on the emitter plates, current started flowing in the circuit connecting the two plates. This was because energy was imparted to the metal plates by the electromagnetic waves. This provided sufficient kinetic energy to the free electrons in the metal plate to escape from the metal plate. These electrons were then attracted by the collector plate connected to the positive terminal. After striking the positive collector plate, the electrons start flowing in the circuit on being attracted by the negative potential on the other end of the circuit.

Further to confirm this observation, electromagnetic waves were made incident on a zinc plate. After sometime the zinc plate was seen to have become positive. On illuminating it with electromagnetic radiation for a longer period of time, the zinc plate became more positive. This reassured that the zinc plate emitted electrons when exposed to electromagnetic radiation.

Again a negatively charged plate was illuminated with electromagnetic waves. In this case, the plate lost its charge. It meant that the plate emitted electrons and hence lost its negativity on emitting electrons.

Some of the alkali metals such as potassium, lithium, sodium, caesium and rubidium were also sensitive to electromagnetic waves and even to visible light. Such photosensitive substances emit electrons when illuminated by electromagnetic waves. After discovery of electrons, these emitted electrons were named as photoelectrons. The process of emission of photoelectrons is called photoelectric emission.

However, some alkali metals including sodium, lithium, potassium, rubidium and caesium were sensitive even to visible light. Such photosensitive substances have the tendency to emit electrons upon being illuminated by light. Upon the discovery of electrons, they were termed as photoelectrons. The phenomenon is called the photoelectric effect.

Effect of frequency on photoelectric effect                      

Now, few variations were made in some parameters. The frequency of the incident radiation was varied. Consequently, it was noted that below a specific frequency of the radiation the galvanometer showed no deflection. It meant that there was no current and no photoelectric emission. So a very interesting observation was made that metal plates emitted electrons only above a threshold frequency. On further increasing the frequency above the threshold frequency there was no change in the magnitude of current. Since current is the measure of the number of photoelectrons emitted per second, it was noted that no change was made in the number of photoelectrons emitted per second on varying the frequency of the electromagnetic radiations above the threshold frequency. The threshold frequency was dependent on the nature of the material used. 

The energy of electromagnetic waves corresponding to threshold frequency is called work function. Some alkali metals were sensitive to radiation of very low frequency whereas some emitted electrons only in presence of radiations of very high frequency like ultraviolet radiations.

Effect of intensity on photoelectric effect

Now the intensity of the incident electromagnetic radiation was varied. The intensity of electromagnetic waves means the energy passing per second per unit area which is perpendicular to the direction of propagation of electromagnetic waves. It was noted that on increasing the intensity, the deflection of the galvanometer was greater. This means that increased intensity increases the rate of electron emission.

Effect of collector plate potential on photoelectric effect

Now, the collector plate potential was varied. The collector plate was connected to positive potential. The positive potential attracts the emitted electrons and helps in moving the electrons away from the metal plate. So, once the metal has started emitting electrons, an increase in collector plate potential increases the rate of electron emission and hence  increases the photocurrent. The graph below depicts it clearly.

To stop the photocurrent, negative potential is given to the collector plate. The value of negative potential at which the negativity repels the electrons and causes the photocurrent to become zero is called stopping potential. The stopping potential does not depend on the intensity but only on the threshold frequency. The stopping potential depends on the kinetic energy with which the emitter electrons approach the collector plate. Greater is the kinetic energy with which the emitted electrons approach, greater is the stopping potential required to stop the electrons from reaching the collector plate. Since the kinetic energy depends on the frequency of the incident radiations, we can say that the stopping potential is dependent on the frequency of the electromagnetic waves.

Conclusion

1. For a photosensitive material, the value of photocurrent is independent of the frequency of incident electromagnetic radiations. But, photoelectric emission starts only above a threshold frequency.
2. The saturation current , the steady current achieved at given collector potential, is directly proportional to the intensity of incident radiation.
3. The stopping potential is independent of intensity of electromagnetic radiation.
4. Photoelectric emission is an instantaneous process.