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UAL NATURE OF RADIATION AND MATTER—Dual Natur, , LATEST SYLLABUS, , et) I i, | nature of radiation, P, = hotoelectric effect, Hertz, , __ are of light. Matter waves - wave nature of parti, : art, | mooie omitted; only conclusion should be lied, , & of Radiation and Matter, , rca ; Einstein's photoelectric equation - particle, fl. Davisson - Germer experiment (experimental details, , and Lenard's ob, de Broglie relatio, , 40.1. INTRODUCTION, , ], Trace the history which links matter and waves, , , , , , , , , , , , , , The glow on the wall was assumed to be due to the rays emitted by cathode. ., Hence, these te named cathode rays. \t was found that these cathode rays -_ — i -« Andes, have the following properties :, , (i) They carry negative charges. U, , (ii) They travel in straight lines. Figure 1 +, , (iii) These rays are deflected by electric and magnetic fields., On the basis of these properties, William Crookes concluded that cathode rays consist of streams of negatively charged particles., , Thomson determined the ratio of (e/m) of these particles. J.J. Thomson experiments showed that cathode rays move with very, high speed (of the order of 1/10th of speed of light = 3 x 10’ ms~!). The value of (e/m) for cathode rays was found to be, 1-76 x 10!! C kg"!. The value of (e/m) was independent of the nature of the material of the cathode and the nature of the gas filled, im the discharge tube., , Later on, at the end of the 19th century, it was found that negatively charged particles are emitted when certain metals are, “exposed to ultraviolet rays. Negatively charged particles are also emitted by an incandescent filament (heating the filament)., Thomson determined the value of (e/m) of negatively charged particles emitted by metals when exposed to ultra-violet rays and of, _ the negatively charged particles emitted by the incandescent filament. In both the cases, Thomson found that the value of (e/m) of, - Regati vely charged particles is same as that of the value of (e/m) of cathode rays negatively charged particles. Therefore, he, concluded that negatively charged particles emitted either by applying a strong electric field (in case of gas dicharge tube) or by, ‘ultraviolet rays or by heating the metal are identical. Thomson named these negatively charged particles as electrons., , ; - rmed his famous oil-drop experiment in 1913 to determine the value of charge on an, R.A. Millikan, the American physicist perfo dto be 1-602 x 10-!9 C. Moreover, Millikan concluded that charge on any body is, , = The value cae ——_ abe .. e(i.e., 1-602 x 10° 19 ©). Thus, the property of quantization of electric charge, , w, : ' =" - — st Be mccicectat be calculated as the values of (e/m) and e for electrons are now known., , - Oe at - pees s of radiation. According to this concept, radiations e.g., light waves, radio waves,, , a Be Ate 1to carry energy in packets or bundles known as photons s quae., , sa). oh i : ‘no that any moving particle or an object is associated with a wave known as, , Jt See a gave an bes c. later on verified experimentally by Davisson and Germer., ~€t Wave or de-Broglie wave., , | Thus radiation and matter have dual nature as pet de-Broglie hypothesis., ELECTRON EMISSION, , , , , , , , , , , , , , , , , , , , , , , , ferent types of electron emissions., , surface by supplying external energy is, , cnet Orb! 0 an atom are loosely es, metal even at room temperature. |

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DINESH NEW MILLENNIUM PHYSICS-x gy 0, , - ~ ae > os of th i * i ae, , for the conductivity of a metal. However, these free electrons cannot leave the surface of the metal eir own. As S000, , t ) charge is developed on the surface of the metal. This positive cae, ge, , an electron tends to leave the metal surface, ; : desetin, pulls back ( or attracts) the free electron ten the metal surface (Figure 2). This :, f an external energy*® supplied to it to overcom,, , barrier of free electrons. The free electron can, , The external energy heeded to emit electrons from a metal surface can be supplied in, . bc 3 ' ae Emitie:, , , , a positive, ding to leave (, jeave the metal surface only 1, , , , 27 T Ton,, , , , , , mission as discussed below :, , , , , , , , differen o pes of electron ¢ NA, ($y Thermionic emission :, si 8 ; Figure 3, The process of emission of electrons from a metallic-stryee hen eu, piSure 3]. The free electrons in the Light Photo electrons, , , , ‘ace is heated is known as thermionic emission, efay and can overcome the surface, the metal surface. The electrons emitted in this wa, , emitted due to thermal energy., , barrier. As a result, the free, , Metal aDse oh, y are known, , electrons are emitted from, as the mine tecausc UiCcy, (ii) Photo-electric emission :, The process of emission of electrons from a metallic surface, when light of suitable, is incident on the metal surface is known as photo-electric emissigu<“1 he free, , electrons in the metal absorb the energy of the inciden nd-eross over the surface, , barrier. Hence, the free electrons are emitted from the metal surface [Figure 4]. The, , electrons emitted so are known as photo-electrons because they are emitted due to light, , , , , , , , , , , , , , , , (iii) Secondary emission :, = process of emission of free electrons from a metallic surface, when high, energetic electron beam is incident on the metal surface is called secondary emission:, The electrons emitted so are known as secondary electrons [Figure Se, , (iv) Field emission :, seed process of emission of free electrons from a metallic surface, when a strong, electric field (= 10° V mw) is applied across the metal surface is known as fiele, , , , , , , , , , , , , , %, , , , , HOTS (Higher Order Thinking Skills Based Information), , ne, rs, , | At eri pressure, discharge Tisai ae, 4 atmospheric ; gas does not, positive and negative ions are so close to each other that they recombine, H ., , is no charge which can cause discharge through the gas. een, , difference across ee i ition potentials, so they require different potential, lige aed caiak ch discharge tube to start the discharge., , of them are : neon signs, fluorescent light, —~ tae practical applications. Some, , lamps ete. Ree YOPPT AF ARE eroUTY veRpur, , *

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i 4, 7, , tas, , i, , , , Cathode rays are fast moving electrons having charge e = _}. -19, emitted normally in a straight line from the surface of ihe cath: ia, helvesin tormiciibe eae, , fluorescence and phosphorescence. Th y are deflected by magnetic and electric field. Cathode, rays cause ore: . They have high ‘ . ; 5 ., iain the foils and without significant loss of mes ata: power and can pass through air and thin foils without, , ee h as tungsten. Cathode r, ; rays produce X-rays when they strike a target of high, atomic weight SUC @ys travel with very high s, of light (in vacuum). ry gn speed. The speed of cathode rays is about 10% of the speed, , 3, Specific charge of an electron, , The ratio of charge on an electron (e) and the mass of the electron (m) is known as specific charge of an electron., , J.J. Thomson used a special arrangement to determine the ratio e/m of electron as shown in figure 8. Helmholtz coil, , produces nearly uniform mangetic field. The anode and cathode are fed with electric current from induction coil. These two, fields act like crossed fields., , Fluorescent, , screen, , Electric field, , , , , , Cathode Anode Suge Helmholtz coil, , , , , , , --1- QO 8-9, , ===, \, , @@@@--B., , =, ~, , , , Figure 8, , 4, Charge/Mass/Radius of electron, , Millikan determined the value of: charge on an electron using oil drop method. This method was based upon the, measurement of, , (i) the terminal velocity of an oil drop under the influence of gravity alone and (ii) the terminal velocity under the joint, action of gravity and an electric field opposing the gravity., The experimental arrangement used by Millikan to determine the charge on an electron is shown in figure 9., , , , , , , , , , , , , , Circular, , Plates 7 i Atomizer, , X-rays, , Constant, , : temperature, TTP he bath, , , , , , , , From Thomson's method, —£- = 1°7598 x 10!" C kg”, , From Millikan’s method, gal 6 x 10-9 C, , , , , , , ( Millikan’, method is also used to determine

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10.2.1. Work Function (0)., , , , of a substance. Name and define the unit in which work function is measured, On Whay, , 3, Define the term "work function" é, factors, work function of a substance or metal depends :, The minimum energy required by the free electron to just leave the meta, , metal., Work function is denoted by ¢ and can be measured in electron volt (eV)., , bey 16% 10 OC x1V =1:6x10 "J, Definition of eV: One eV is the energy acquired by ane, 1V., , The work function of a metal depends upon : (Z) the properties of, (iii) impurities in metal etc., , Work function of some metals is given in Table 1., Table 1 ; Values of work functions of some metals, , | surface is known as the work function Of the, , , , , lectron when it is accelerated through a potential difference of, , the metal, (ii) nature of the surface of the metal and, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Metal Work function (in eV), Caesium (Cs) Aluminium (Al), Potassium (K) Mercury (Hg) 4-50, Lithium (Li) Copper (Cu) 4:65, Sodium (Na) Silver (Ag) 4-7, Calcium (Ca) Nickel (Ni) 545, Platinum (Pt) 5-65 Si |, , , , ).3. PHOTOELECTRIC EFFECT, , 4. Define photo-electric effect. Discuss Hertz experiment, Write conclusion drawn from his observations,, , The phenomenon of emission of electrons from preferably metal surfaces’ exposed to light en, tabl uency Is, known as photo-electric effect. P § ergy of suitable frequency, , 10.3.1. Hertz Observations, , In 1887 , Heinrich Hertz performed an experiment to investigate the production of electromagnetic radiation by means of, spark discharge. He used a detector or receiver made of a wire bent into a circle with metallic spheres S,’ and Sy’ on ea“, end. A small gap was kept between these spheres (Figure 10). Electromagnetic radiations were cd with the help of, induction coil, plates P; and P, and metallic spheres S; and S, attached to plates P; and P,. Peskomaanetic radiation falling, , , , , , , , , , , , , , , , , on the detector induced a potential difference across the gap between spheres Si and S5: This was evident as the sparks, , jumped across the gap. Hertz observed that sparks across the gap S\S, jumped m, , ore I edt, ultraviolet light from an arc lamp. 5 readily when the detector was exp%, , , , = — SS SS, , Receiver, or, Detector

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AL NATURE DF RADIATION ANG) MATTER-Dual Nature af Radiation and! Matter, , 40.3.2. Hallwachs’ and Lenard’s Observations, , , , §, Discuss Hallwach's and Lenard's observations. What is the result of these observations ?, Wilhelm Hallwachs in 1888 observed that ultraviolet light thrown on a negatively, , charged zinc plate, connected to a gold-leaf electroscope, decreases the divergence, , of gold leaves (Figure 11).This observation indicates the decrease in the negative, , charge on zinc plate. Thus, Hallwachs concluded that a negatively charged zinc Ropasively, , plate when exposed to ultra-violet light emits negatively charged particles. The charged, emitted particles are infact photoelectrons emitted by the action of light. Zinc plate, , Philipp Lenard used the apparatus shown in figure 11 to study the effect of, light on a metallic surface. He observed that when ultra-violet light was incident, on a metallic electrode *C’ (called cathode), electric current appeared in the, circuit. This electric current in the circuit is known as photoelectric current., However, there was no current in the circuit, when electrode *C’ was not exposed, to ultra-violet light. He also concluded that only light of suitable minimum frequency, (called threshold frequency) results into photoelectric current. This minimum frequency, varies from material to material., , Ultraviolet, , Light, , Emitted negatively, charged particles, (Photoelectron), , et : ’ : 3 Gold leaf, The minimum frequency of incident light required to emit electrons from @ _ electroscope, , metal surface is known as threshold frequency. Thus, photoelectrons are emitted, from an emitter (say a metal) surface only if the frequency of the incident light is, greater than the threshold frequency. Figure 11, Conclusion ; From all these observations, it was concluded that when radiations of, suitable frequency fall on a metallic surface, electrons are emitted, This phenomenon is, known as photo electric effect., The electrons emitted by the metal surface are known as photo electrons. The electric current constituted by photo, , electrons is known as photo electric current., , % HOTS (Higher Order Thinking Skills Based Info, Nearly all metals emit photoelectron ., , , , , rmation), ultra-violet light. But alkali metals like lithium, sodium,, , , , (b) (c), Figure 12, , 4. EXPERIMENTAL STUDY OF PHOTOELECTRIC EFFE, , 7 The experimental set up to:study photoelectric effect is shown in Figure 13,, , cn POnsists of a highly evacuated tube having two electrodes ‘A’ and ‘C’. The electrode ‘C’ (called cathode) is a photo, e os €mitter which emits photoelectrons when exposed to ultra-violet radiation, The electrode ‘A’ (called anode) is a charge, , of rte Tons) collecting plate. The tube has a side window, made of quartz covered with a filter, through which the incident light, °d wavelength enters the tube and falls ‘on the photo-sensitive plate *C’., , , , , , a, , , , , , , fl