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‘Unit - >, DUAL NATURE OF MATTER, AND RADIATION, , , , , , , , , , , , , , de Broglie Hypothesis Following observations led de Broglie to put forward the, duality hypothesis for matter,, (a) In this universe, the whole energy is in form of matter and electromagnetic, radiation’s. ., ()) Nature love symmetry, as the radiation’s have get dual nature, matter should, also possess dual nature., IMPORTANT FORMULAE, $$ PU RMIOCAS, _ he, 1. Energy of photon (E) = hv = 7, 2. Momentum of photon (P)= pi = 4, c, zi he, 3. Work function (g,) = hu, =, Ay, 4. K.E. of emitted photo electron sm = (hv — $)) = bv —hv, = ie =, 5. eV, =h(v— v9) =he deel, » eV y 0 a’, 6& eV, = 5 Tt where V, = stopping potential., ‘ s a h_h, 7. De-Broglie wavelength associated with moving particle is given by A = > om, mv, 8 A=, , le where E = K.E. of particle., 2 mE, , 145

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—/—O——E—————oEe + &;X«4, , 146, , 9. For electron A =, , ACCURATE BOARD PAPERS PHysics TERY, 1, , gu 1227 5, , {2 meV orem Vv, , , , , , [ Questions Carrying One or Two Marks L, , (KNOWLEDGE, UNDERSTANDING AND APPLICATION paseD) ae}, , Q.1., Ans., , Q. 2., Ans., , Q. 3., Ans., , Q. 4., Ans., , Q. 5., Ans., , Q. 6., , Ans., , Q.7., , Ans., , Ans., , Define threshold wavelength ? (P.S.E.B. 2010, 2012), The maximum wavelength of incident light for a metal surface, above which incident, light cannot eject electrons is called threshold wavelength., , Define work function ? (P.S.E.B. 2010, 2015), The minimum energy, which must be supplied to the electron so that it can just come out, of a metal surface, is called work function of the metal., , Define photoelectric effect ? : :, , The phenomenon of ejection of electrons from a metal surface, when light of sufficiently, high frequency falls on it, is known as photoelectric effect., , Define stopping potential ? (P.S.E.B. 2012, 2014, 2015), Stopping potential is that minimum value of the negative potential (V)) which should be, applied to the anode in a photo cell, so that photoelectric, current becomes zero., , Define Threshold frequency ? (P.S.E.B. 2012, 2015), The minimum frequency (vg) which the incident light must possess so as to eject, photoelectrons from a metal surface, is called threshold frequency of the metal., , How will stopping potential change if RenenCZ of radiation incident on a metal, surface is increased. (PS.E.B. 2010), The stopping potential will increase, if frequency of radiation incident on a metal surface, is increased., , If intensity of incident radiation is doubled, what happens to the kinetic energy, photoelectron ? : (P.S.E.B. 2010), If intensity of incident radiation on a metal is doubled, the K.E. of emitted electrons, remains unchanged, as K.E. of emitted electrons is independent of intensity of incident, radiation., , . If intensity of radiation is doubled. How does the stopping potential will change?, , (P.S.E.B. 2010), , As @ Vy =h (v-V) 23, “ : . eG ._etont, so stopping potential V, remains unchanged, by increase in intensity. of incides", , radiation.

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ESS TT, , DUAL NATURE OF MATTER AND RADIATION, , Q.9., Ans., , Q. 10., Ans., , Q. 11., Ans., , Q. 12., , Ans., Q. 13., Ans., , Q14., Ans,, Q. 15,, Ans,, , Q. 16,, , Ans,, , Re, , 147, , What is effect of decrease of wavelength of incident light on velocity of, photoelectrons ? (P.S.E.B. 2010), , If wavelength of incident light is decreased, the velocity of emitted photoelectrons will, increases,, Which photon is more energetic Red or Violet ?, , Energy of a photon is given by, , >, , I, , =, , =hy=— feva.n=$}, , Since A, > A, so E, <Ey, Thus, violet photon is more energetic., , Which photon is more energetic Green or Blue ? (P.S.E.B. 2014), Energy of a photon is given by, , E=hv=, , Ag > Ay or Eg < Eg, Thus, blue photon is more energetic., Can X rays show photoelectric effect with visible light ? If yes. Why ?, , (P.S.E.B. 2012), , Yes, X rays can show photoelectric effect in sodium, zinc, copper etc. The energy of X, ray photon is quite large, in comparison to that of visible light or ultraviolet light., , Write down relation between energy & momentum of photon? _—(P.S.E.B. 2013), , E (Energy), , M ti f phot eT, iment af plete () c (Velocity of light in Vaccum), , On what factor does the threshold frequency depend ? (P.S.E.B. 2016), The threshold frequency depends on the metal or material of the cathode., What must be the main feature of metal used for photoelectric emission ?, , Metals should posses low work functions. Metals which are very sensitive to light are, very essential to photoelectric emission. For example : caesium and potassium etc,, , It is easier to remove an electron from a photosensitive surface sodium than from, copper, which has higher value of threshold frequency ? (P.S.E.B. 2012), , The work function of sodium is less than that of copper. Therefore, it is easier to remove, an electron from sodium than’ from copper. This implies that threshold frequency of, copper is more than that of sodium.

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—, , 148 ACCURATE BOARD PAPERS PHYSICS TERM-I], , Q.17. Blue light can eject electrons from a photosensitive surface while orange light, cannot. Will violet and red light can eject electrons from metal surface >, Give reasons ? ’ (P.S.E.B. 2012), , The wavelength of red light is greater than that of orange light while the wavelength of, violet light is less than that of blue light. Hence red light cannot cause photoelectric, emission, while photoelectric emission will take place for violet light., , Ans., , Q.18. Green light can eject electrons from a certain photosensitive surface, yellow light, does not. What will happen in case of red and violet light ? Give reasons ?, , (P.S.E.B. 2014), , Ans. As the green light eject electrons from surface, wavelength of green light is less than, threshold frequency for the surface. Further as yellow light cannot eject electrons, the, wavelength of yellow light is greater than threshold frequency. The wavelength of red, light is greater than that of yellow light while wavelength of violet light is less than, green light. Hence, red light cannot eject electrons while violet can eject electrons., , Q. 19. Name a phenomenon which illustrates particle nature of light ? (P.S.E.B. 2018), , Ans. Photoelectric effect, , Q. 20. What will be value of stopping potential if frequency of incident radiation is just, equal to threshold frequency ? (P.S.E.B. 2018), , Ans. From Einstein’s photoelectric equation, hv =hvgt+eVo, When v= U9, hug = hv t+eVo, Vp =0, Q. 21. What is the main aim of Davison and Germer experiment ?, , Ans. The wave nature of slow moving electrons has been established experimentally by, Davison and Germer in 1927., , Q. 22. Write Einstein’s Photoelectric equation. (P.S.E.B. 2015), , , , Ans. sma = h(v-v9), , , , , , , , Q. 23. Why are alkali metal surface are most suited as Photosensitive surface ?, , Ans. Alkali metals have small work function. So even with a radiation of small energy», electrons can be ejected out of their surface. That is why alkali metals are more suitable, for Photosensitive surface.

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DUAL NATURE OF MATTER AND RADIATION 149, , Q.24. A Photon and electron have got the same de-broglie wavelength which has greater, total energy ?, , Ans. An electron will have higher total energy because energy of electron is E= mc”, where, cis velocity of light. For Photon E = mvc. Therefore electron has higher total energy., Q.25. Why do we do not simply talk of (e) and (m) separately ? ,, Or, , Why do we prefer to talk more about specific charge, rather than charge and mass, separately of a charged particle ?, , Ans. Charge e and mass m do not occur separately in the equations of motion of charged, particle. These relations take into account e/m, so specific charge is more important., , , , 1 2, e.g. eV =—mv, 2, e 1 v2, or — =-—, m 2V, , , , , , , , Q.26. Why is wave nature of matter not apparent in our daily observation ?, Give examples in support of your answer., , Ans. . The wave nature of matter’ can be apparent if the wavelength of the wave associated, with the moving matter is of the order of the size of the matter., , aot, , mv, Consider a stone of mass 6.6 kg moving with a velocity of 1 ms"! , then, , _ 6.6x10~*4, , a 6x1, , A =1x 10m, Since this wavelength is too small that cannot be detected. Hence wave nature of matter, is not apparent in our daily life., Q.27. Why does the maximum velocity of Photoelectron not depend upon the intensity of, radiation ?, , Aus. Maximum velocity depends upon maximum kinetic energy of the photoelectron which, further depends upon hy energy of incident Photon. It ‘is the frequency and not intensity, which matters for maximum velocity. :, , Q-28, Light from an incandescent bulb is falling on a ceramic sheet but no Photoelectrons, are emitted. Explain why ?