Class Physics English Volume

AND MATTER Unit dual nature of radiation and matter The minimum energy needed for an electron to escape from the metal surface is called work function of that metal . The work function of the metal is denoted by φ and is measured in electron volt (eV). The SI unit of energy is joule. But electron volt is a commonly used unit of energy in atomic and nuclear physics.

One electron volt is defined as the kinetic energy gained by an electron when accelerated by a potential difference of V. eV = KE gained by the electron = Work done by the electric field = q V = . × – C × V = . × – J Note Suppose the maximum kinetic energy of the free electron inside the metal is .

eV and the energy needed to overcome the surface barrier of a metal is eV, then the minimum energy needed for electron emission from the metallic surface is – . = . eV. Here .

eV is the work function of the metal. The work function is different for different metals and is a typical property of metals and the nature of their surface. Table . gives the approximate value of work function for various metals.

The material with smaller work function is more effective in electron emission because extra energy required to release the free electrons from the metal surface is smaller. charge. However, later experiments showed that under certain circumstances, they exhibit wave-like properties also. In this unit, the particle nature of waves (radiation) and the wave nature of particles (matter) – that is, wave-particle duality of radiation and matter is discussed with the relevant experimental observations supporting this dual nature.

. . Electron emission In metals, the electrons in the outer most shells are loosely bound to the nucleus. Even at room temperature, there are a large number of free electrons which are moving inside the metal in a random manner.

Though they move freely inside the metal, they cannot leave the surface of the metal. The reason is that when free electrons reach the surface of the metal, they are attracted by the positive nuclei of the metal. It is this attractive pull which will not allow free electrons to leave the metallic surface at room temperature. In order to leave the metallic surface, the free electrons must cross a potential barrier created by the positive nuclei of the metal.

The potential barrier which prevents free electrons from leaving the metallic surface is called surface barrier . Free electrons possess some kinetic energy and this energy is different for different electrons. The kinetic energy of the free electrons is not sufficient to overcome the surface barrier. Whenever an additional energy is given to the free electrons, they will have sufficient energy to cross the surface barrier and they escape from the metallic surface .

The liberation of electrons from any surface of a substance is called electron emission . Unit dual nature of radiation and matter Electrons Hot filament Figure . Thermionic emission from hot filament of cathode ray tube or x-ray tube ii) Field emission Electric field emission occurs when a very strong electric field is applied across the metal. This strong field pulls the free electrons and helps them to overcome the surface barrier of the metal (Figure .

). Examples: Field emission scanning electron microscopes, Field-emission display etc. Strong electric field Metal Electrons emitted – – – – – – – Figure . Field emission So the metal selected for electron emission should have low work function.

The electron emission is categorized into different types depending upon the form of energy being utilized. There are mainly four types of electron emission which are given below. i) Thermionic emission When a metal is heated to a high temperature, the free electrons on the surface of the metal get sufficient energy in the form of thermal energy so that they are emitted from the metallic surface (Figure . ).

This type of emission is known as thermionic emission . Metal Electrons liberated Termal energy (b) Electrons (a) Figure . Electrons in the (a) metal (b) heated metal The intensity of the thermionic emission (the number of electrons emitted) depends on the metal used and its temperature. Examples: cathode ray tubes, electron microscopes, x-ray tubes etc (Figure .

). Table . Work function of some materials Metal Symbol Work function (eV) Metal Symbol Work function (eV) Cesium Cs . Aluminium Al .

Potassium K . Mercury Hg . Sodium Na . Copper Cu .

Calcium Ca . Silver Ag . Molybdenum Mo . Nickel Ni .

Lead Pb . Platinum Pt . Unit dual nature of radiation and matter . PHOTO ELECTRIC EFFECT .

. Hertz, Hallwachs and Lenard’s observation Hertz observation Maxwell’s theory of electromagnetism predicted the existence of electromagnetic waves and concluded that light itself is just an electromagnetic wave. Then the experimentalists tried to generate and detect electromagnetic waves through various experiments. In , Heinrich Hertz was successful in generating and detecting electromagnetic wave with his high voltage induction coil causing a spark discharge between two metallic spheres (we have learnt this in Unit of XII standard physics).

When a spark is formed, the charges will oscillate back and forth rapidly and the electromagnetic waves are produced. The electromagnetic waves thus produced were detected by a detector that has a copper wire bent in the shape of a circle. Although the detection of waves is successful, there is a problem in observing the tiny spark produced in the detector. In order to improve the visibility of the spark, Hertz made many attempts and finally noticed an important thing that small detector spark became more vigorous when it was exposed to ultraviolet light.

The reason for this behaviour of the spark was not known at that time. Later it was found that it is due to the photoelectric emission. Whenever ultraviolet light is incident on the metallic sphere, the electrons on the outer surface are emitted which caused the spark to be more vigorous. iii) Photo electric emission When an electromagnetic radiation of suitable frequency is incident on the surface of the metal, the energy is transferred from the radiation to the free electrons.

Hence, the free electrons get sufficient energy to cross the surface barrier and the photo electric emission takes place (Figure . ). The number of electrons emitted depends on the intensity of the incident radiation. Examples: Photo diodes, photo electric cells etc.

Free electrons Radiation Emitted electrons Metal Figure . Photo electric emission iv) Secondary emission When a beam of fast moving electrons strikes the surface of the metal, the kinetic energy of the striking electrons is transferred to the free electrons on the metal surface. Thus the free electrons get sufficient kinetic energy so that the secondary emission of electron occurs (Figure . ).

Examples: Image intensifiers, photo multiplier tubes etc. Free electrons Metal High Speed electrons Secondary electrons p d e Figure . Secondary emission of electrons Unit dual nature of radiation and matter Lenard’s observation In , Lenard studied this electron emission phenomenon in detail. His simple experimental setup is shown in Figure .

. The apparatus consists of two metallic plates A and C placed in an evacuated quartz bulb. The galvanometer G and battery B are connected in the circuit. _ Ultraviolet Radiation G C Electrons Quartz bulb Figure .

Experimental setup of Lenard When ultraviolet light is incident on the negative plate C , an electric current flows in the circuit that is indicated by the deflection in the galvanometer. On other hand, if the positive plate is irradiated by the ultraviolet light, no current is observed in the circuit. From these observations, it is concluded that when ultraviolet light falls on the negative plate, electrons are ejected from it which are attracted by the positive plate A . On reaching the positive plate through the evacuated bulb, the circuit is completed and the current flows in it.

Thus, the ultraviolet light falling on the negative plate causes the electron emission from the surface of the plate. Photoelectric effect The ejection of electrons from a metal plate when illuminated by light or any other electromagnetic radiation It is interesting to note that the experiment of Hertz confirmed that light is an electromagnetic wave. But the same experiment also produced the first evidence for particle nature of light. Hallwachs’ observation In , Wilhelm Hallwachs, a German physicist, confirmed that the strange behaviour of the spark is due to the action of ultraviolet light with his simple experiment.

A clean circular plate of zinc is mounted on an insulating stand and is attached to a gold leaf electroscope by a wire. When the uncharged zinc plate is irradiated by ultraviolet light from an arc lamp, it becomes positively charged and the leaves will open as shown in Figure . (a). Further, if the negatively charged zinc plate is exposed to ultraviolet light, the leaves will come closer as the charges leaked away quickly (Figure .

(b)). If the plate is positively charged, it becomes more positive upon UV rays irradiation and the leaves open further (Figure . (c)). From these observations, it was concluded that negatively charged electrons were emitted from the zinc plate under the action of ultraviolet light.

+ + + + + Zn –– –– – – – – – – – Zn – ––––– – ––––– ++ ++ + + + + + Zn (a) (b) (c) UV rays + ++++ Figure . Irradiation of ultraviolet light on (a) uncharged zinc plate (b) negatively charged plate (c) positively charged plate Unit dual nature of radiation and matter of suitable wavelength (or frequency) is called photoelectric effect . Although these electrons are not different from all other electrons, it is customary to call them as photoelectrons and the corresponding current as photoelectric current or photo current . Metals like cadmium, zinc, magnesium etc show photoelectric emission with ultraviolet light while some alkali metals lithium, sodium, caesium respond well even to larger wavelength radiation like visible light.

The materials which eject photoelectrons upon irradiation of electromagnetic wave of suitable wavelength are called photosensitive materials . . . Effect of intensity of incident light on photoelectric current Experimental setup The apparatus shown in Figure .

is employed to study the phenomenon of photoelectric effect in detail. S is a source of electromagnetic waves of known and variable frequency ν and intensity I . C is the cathode (negative electrode) made up of photosensitive material and is used to emit electrons. The anode (positive electrode) A collects the electrons emitted from C .

These electrodes are kept in an evacuated glass envelope with a quartz window that permits the passage of ultraviolet and visible light. The necessary potential difference between C and A is provided by high tension battery B which is connected across a potential divider arrangement PQ through a key K . C is connected to the centre terminal while A to the sliding contact J of the potential divider. The plate A can be maintained at a desired positive or negative potential with respect to C .

To measure both positive and negative potential of A with respect to C , the voltmeter is designed to have its zero marking at the centre and is connected between A and C . The current is measured by a micro ammeter m A connected in series. If there is no light falling on the cathode C , no photoelectrons are emitted and the microammeter reads zero. When ultraviolet or visible light is allowed to fall on C , the photoelectrons are liberated and are attracted towards anode.

As a result, the Photo sensitive plate Quartz window Evacuated glass tube V µA