11. DUAL NATURE OF RADIATION AND MATTER

Transcription

1 11. DUAL NATURE OF RADIATION AND MATTER Very shrt answer and shrt answer questins 1. Define wrk functin f a metal? The minimum energy required fr an electrn t escape frm the metal surface is called the wrk functin f the metal 2. Define 1eV 1eV is the energy gained by an electrn when it is accelerated thrugh a ptential difference f ne vlt. 3. Define therminic emissin? Emissin f electrn frm a metal surface when it is heated t sufficiently high temperature is called therminic emissin. 4. Define field emissin? Emissin f electrn frm metal surface when it is subjected t high electric field (f the rder f 10 8 V) is called field emissin. 5. Define phtelectric emissin? Emissin f electrns frm metal surface, when it is illuminated with light f suitable frequency is called phtelectric effect. 6. Wh discvered phtelectric effect? Henrich Hertz discvered phtelectric effect. 7. Define threshld frequency f a metal? Threshld frequency f a metal is the minimum cut-ff frequency f incident light belw which n phtelectric emissin takes place irrespective f intensity f incident light. 8. Hw pht electric current depends n intensity f incident light? Abve threshld frequency, phtelectric current is directly prprtinal t intensity f incident light. 9. What d yu mean by saturatin current? As the ptential f cllectr is increased fr a radiatin f certain high frequency and intensity, phtelectric current increases and reaches t a imum cnstant value. This cnstant current is called saturatin current. 10. Define stpping ptential f a given phtsensitive metal? Stpping ptential f a phtsensitive metal is defined as the minimum negative ptential applied t the cllectr at which the phtelectric current just drps zer. 11. Give the mathematical relatin between stpping ptential and imum kinetic energy f phtelectrn. ev imum kinetic energy e Charg e f electrn V Magnitude f stpping ptential 12. Give the graphical representatin f the variatin f phtelectric current with cllectr plate ptential.

2 13. Represent the variatin f stpping ptential with frequency f incident light graphically. 14. Give the graphical representatin f effect f frequency f incident radiatin n stpping ptential. 15. Define quanta? Radiatin energy is made up f discrete unit f energy called quanta. 16. What is a de-brglie wave? A wave assciated with mving particle is called de-brglie wave. 17. What is the experimental utcme f Davissn and Germer experiment? Davissn and Germer prvided experimental prf fr the wave nature f matter particle and verified the de-brglie s expressin fr wavelength f matter wave. 18. What happens t the kinetic energy f phtelectrns if the intensity f incident radiatin is increased? inetic energy remains same as kinetic energy is independent f intensity f incident radiatin 19. Why sufficiently pwerful AM radi signal cannt prduce phtelectric effect? The energy f radi phtn is less than the wrk functin f any metal s even sufficiently pwerful AM radi signal cannt prduce phtelectric effect. 20. Give the labeled schematic representatin f experimental arrangement fr the study f phtelectric effect.

3 21. Name the factrs n which imum kinetic energy f phtelectrns depends. Maximum kinetic energy f phtelectrns depends n the nature f the emitter and the frequency f incident radiatin. 22. Give the Einstein s phtelectric equatin and explain the terms. Einstein s phtelectric equatin is given by where h Wrk functin Maximum kinetic energy h plank ' s cns tan t Frequency f incident radiatin 23. What is the threshld frequency f a phtn fr phtelectric emissin frm a metal f wrk functin 1eV 19 1x1.6 x10 14 Threshld frequency 2.41x10 Hz 34 h 6.625x Why the phtelectrns emitted frm a metal surface fr a certain radiatin have different energies even if wrk functin f metal is a cnstant? Wrk functin is the minimum energy required fr the electrn in the highest level f the cnductin band t get ut f the metal. Nt all electrns in the metal belng t this level. They ccupy a cntinuus band f levels. Cnsequently, fr the same incident radiatin, electrns kncked ff frm different levels cme ut with different energies. 25. What is the significance f the slpe f graph f stpping ptential f an emitter verses frequency f incident radiatin? The slpe f graph f stpping ptential f an emitter verses frequency f incident radiatin is bserved t be a cnstant. The value f slpe is measured t be h/e which is independent f nature f emitter. Millikan calculated the value f h with the help f experimental value f slpe and knwn value f e. The calculated value bserved t be matching with Plank s cnstant exactly. 26. Draw labeled schematics diagram t shw the experimental arrangement f Davissn and Germer experiment.

4 27. Mentin the relatin fr de-brglie wavelength. Accrding t de-brglie thery, wavelength f matter wave assciated with particle f mmentum p (p=mv) is given by h...(2) p 28. Give the relatinship between the accelerating ptential and the de-brglie wavelength assciated with a charged particle. h de-brglie wavelength assciated with a charged particle is given 2mqV Where q=charge f the particle V=ptential thrugh particle is accelerated m=mass f the particle Lng answer questins 1. Explain Hallwachs and Lenard s experimental bservatins. Wilhelm Hallwachs and Philipp Lenard cnducted a detailed study f the phenmenn f phtelectric emissin. Lenard bserved that when ultravilet radiatins were allwed t fall n the emitter plate f an evacuated glass tube enclsing tw electrdes (metal plates), current flws in the circuit. As sn as the ultravilet radiatin is stpped, the current flw als stps. Thus, light falling n the surface f the emitter causes current in the external circuit. Hallwachs bserved that the uncharged zinc plate became psitively charged when it is irradiated by ultravilet light. Als psitive charge n a psitively charged zinc plate gets enhanced when it is illuminated by ultravilet light. Frm the experimental bservatins he cncluded that zinc plate emits negatively charged particles under the actin f ultravilet light.

5 2. Explain the effect f phtelectric current with cllectr plate ptential Phtelectric current increases with increase in accelerating (psitive) ptential. At sme stage, fr a certain psitive ptential f plate A, the phtelectric current becmes imum r saturates. If ptential f plate A is further increased, the phtcurrent remains same. This imum value f the phtelectric current is called saturatin current. When the ptential f the cllectr plate is made mre and mre negative (retarding) with respect t the plate emitter, the electrns are repelled and nly the mst energetic electrns reach the cllectr. The phtcurrent decreases rapidly until it drps t zer at a certain sharply defined, critical value f the negative ptential V 0. Fr a particular frequency f incident radiatin, the minimum negative (retarding) ptential V 0 given t the cllectr plate fr which the phtelectrns are cmpletely stpped frm reaching cllectr r phtcurrent becmes zer is called the cut-ff r stpping ptential. 3. Mentin the experimental bservatins f phtelectric effect. (i) Fr a given phtsensitive material and frequency f incident radiatin(abve the threshld frequency), the phtelectric current is directly prprtinal t the intensity f incident light. (ii) Fr a given phtsensitive material and frequency f incident radiatin, saturatin current is fund t be prprtinal t the intensity f incident radiatin whereas the stpping ptential is independent f its intensity. (iii) Fr a given phtsensitive material, there exists a certain minimum cut-ff frequency f the incident radiatin, called the threshld frequency, belw which n emissin f phtelectrns takes place, n matter hw intense the incident light is. Abve the threshld v frequency, the stpping ptential and the imum kinetic energy f the emitted phtelectrns increases linearly with the frequency f the incident radiatin, but is independent f its intensity. (iv) The phtelectric emissin is an instantaneus prcess, irrespective f intensity f the incident radiatin. 4. Explain the experimental bservatins with the help f Einstein s phtelectric equatin. a) Accrding t Einstein s thery, the basic elementary prcess invlved in phtelectric effect is the absrptin f a light quantum by an electrn. This prcess is instantaneus. Thus, irrespective f the intensity, phtelectric emissin is instantaneus. b) Accrding t Einstein s equatin, h depends linearly n as is a cnstant fr a given metal. Als is independent f intensity f radiatin. Abve cncepts are in gd agreement with the experimental bservatin. This is due t the fact that accrding t Einstein s thery, phtelectric effect arises frm the absrptin f a single quantum f radiatin by a single electrn.. c) is always nn-negative, Phtelectric emissin is pssible nly if h where h Thus, there exists a threshld frequency fr the metal surface, belw which n phtelectric emissin pssible, n matter hw intense the incident radiatin may be r hw lng it falls n the surface. d) Intensity f radiatin is prprtinal t the number f energy quanta per unit area per unit time. The greater the number f energy quanta available, the greater is the number f electrns absrbing the

6 energy. Hence the number f electrns cming ut f the metal is als higher. This explains why, fr >, phtelectric current is prprtinal t intensity. 5. Give the characteristics f phtn. (i) In interactin f radiatin with matter, radiatin behaves as if it is made up f particles called phtns. h (ii) Each phtn has energy E h where is the frequency, mmentum p where c is the c speed f light. (iii) All phtns f light f a particular frequency, r wavelength, have the same energy and mmentum, whatever the intensity f radiatin may be. By increasing the intensity f light f given wavelength, there is nly an increase in the number f phtns per secnd crssing a given area, with each phtn having the same energy. Thus, phtn energy is independent f intensity f radiatin. (iv) Phtns are electrically neutral and are nt deflected by electric and magnetic fields. (v) In a phtn-particle cllisin (such as phtn-electrn cllisin), the ttal energy and ttal mmentum are cnserved. Hwever, the number f phtns may nt be cnserved in a cllisin. The phtn may be absrbed r a new phtn may be created.. 6. Explain Davissn and Germer experiment. The experiment is perfrmed by varying the accelerating vltage frm 44 V t 68 V. A strng peak bserved in the intensity (I ) f the scattered electrn fr an accelerating vltage f 54V at a scattering angle 50 The appearance f the peak in a particular directin is due t the cnstructive interference f electrns scattered frm different layers f the regularly spaced atms f the crystals. Frm the electrn diffractin thery, the wavelength f matter waves prducing ima at 50 is calculated t be = nm Accrding t de-brglie thery nm V

7 Fr V = 54 V nm 54 Thus, there is an excellent agreement between the theretical value and the experimentally btained value f de Brglie wavelength. Davissn- Germer experiment thus strikingly cnfirms the wave nature f electrns, particles in general and the de Brglie relatin. *************************************************************************************