NEET Physics Notes Chapter 25 PDF

60 El ect r on, Phot on, Phot oel ect r i cEf f ectandXRay s1 Chapt er E3 25 El ect r on, Phot on, Phot oel ect r i cEf f ectandXr ay s whol et ubei sf i l l edwi t hbr i ghtl i ghtcal l edposi t i v ecol umnand col ourofl i ghtdependsupont henat ur eofgasi nt het ubeas showni nt hef ol l owi ngt ab...

60 El ect r on, Phot on, Phot oel ect r i cEf f ectandXRay s1 Chapt er E3 25 El ect r on, Phot on, Phot oel ect r i cEf f ectandXr ay s whol et ubei sf i l l edwi t hbr i ghtl i ghtcal l edposi t i v ecol umnand col ourofl i ghtdependsupont henat ur eofgasi nt het ubeas showni nt hef ol l owi ngt abl e. ID El ect r i cDi schar geThr oughGases Tabl e25. 1:Col ourf ordi f f er entgases Gas Ai r H2 N2 Cl 2 CO2 Neon Pur pl e r ed Bl ue Red Gr een Bl ui sh whi t e Dar k r ed U Atnor malat mospher i c pr essur e,t he gases ar e poor conduct orofel ect r i ci t y.I fweest abl i shapot ent i aldi f f er ence( of t heor derof30kV)bet weent woel ect r odespl acedi nai rata di st anceoff ew cm f r om eachot her ,el ect r i cconduct i onst ar t s i nt hef or m ofspar ks. D YG The di schar ge of el ect r i ci t yt hr ough gases can be sy st emat i cal l yst udi edwi t ht hehel pofdi schar get ubeshown bel ow Hi gh pot ent i al – di f f er ence+ Lengt hoft ube 30t o40cm Di amet er4cm Gas ( 5)Atapr essur eof1. 65mm ofHg: Skycol ourl i ghti spr oducedatt hecat hodei ti scal l edas negat i v egl ow.Posi t i v ecol umnshr i nkst owar dst heanodeand t hedar kspacebet weenposi t i v ecol umnandnegat i v egl ow i s cal l edFar aday sdar kspace( FDS). t i v ecol umn Negat i v egl owPosi – + Manomet er U Vacuum pump F. D. S. ST As t he pr essur ei n s i d e t h schar ge t ube i s gr adual l y F i g. 25. 1e di r educed,t hef ol l owi ngi st hesequenceofphenomenont hatar e obser v ed. ( 1)Atnor mal pr essur enodi schar get akespl ace. ( 2)Att hepr essur e10mm ofHg,azi gzagt hi nr edspar k r unsf r om oneel ect r odet oot herandcr acki ngsoundi shear d. – Col our St r eamer s + Fi g.25. 2 ( 3)Att hepr essur e4mm.ofHg, ani l l umi nat i oni sobser v ed att heel ect r odesandt her estoft het ubeappear sdar k.Thi s t y peofdi schar gei scal l eddar kdi schar ge. ( 4)Whent hepr essur ef al l sbel ow 4mm ofHgt hent he Fi g.25. 3 mm H g:Att ( 6)Atapr essur eof0. 8 hi spr essur e,negat i v e gl ow i sdet achedf r om t hecat hodeandmov est owar dst he anode.Thedar kspacecr eat edbet weencat hodeandnegat i v e gl owi scal l edasCr ook' sdar kspace.Lengt hofposi t i v ecol umn hodecal l edcat hodegl ow. f ur t herr educed.Agl owappearatcat Negat i v egl ow – Posi t i v ecol umn + Cat hodegl ow D. S. C. D. S. F. ( 7)Atapr essur eof0. 0 5 mm o f eposi t i v ecol umn Fi g.25. 4 Hg:Th spl i t si nt odar kandbr i ghtdi scofl i ghtcal l edst r i at i ons. ( 8)Att hepr essur eof0. 01or10–2mm ofHgsomei nv i si bl e 2Electron, Photon, Photoelectric Effect and X-Rays particles move from cathode which on striking with the glass Cathode B Anode tube on the opposite side of cathode cause the tube to glow. P These invisible rays emerging fromFilament cathode are called cathode A + X C rays. P –4 Y – (9) Finally when pressure mm drops of Hg, to nearly 10 there is no discharge in tube. P V L.T. Cathode Rays Magnetic field ZnScoated (1) Cathode rays, discovered by Sir William Crooke screen In this case; Electricforce eE= evB = Magn (2) They are streams of fast moving electrons. Fig. 25.5 (H.T.) 60 of electron v= velocity (3) They can be produced by using a discharge tube E3 –2 containing gas at a low pressure mm of Hg.of the order of 10 (5) As electron beam accelerated f (4) The cathode rays in the discharge tube are the electrons loss in potential energy appears as g produced due to ionisation of gas and that emitted by cathode V is the potential difference b If suppose due to collision of positive ions. anode then, loss ine potential energy V (5) Cathode rays travel in straight lines. And gain in kinetic energy at anod (6) Cathode rays are emitted normally from the cathode D YG U ID surface. Their direction is independent of the position of the anode. i. e.   (7) Cathode rays exert mechanical force on the objects they strike. Thomson found, (8) Cathode rays produce heat when they strikes a metal surface. If one includes the relativistic v (9) Cathode rays produce fluorescence. , then specific charge o (10) When cathode rays strike a solid object, specially a decreases with the increase in its ve X-rays metal of high atomic weight and high aremelting point (6) The deflection of an electron i emitted from the objects. (11) Cathode rays are deflected by an electric field and also l=where y= given by ; Length of each pl by a magnetic field. v =in deflection of electron speed the field of th (12) Cathode rays ionise the gases through which they are electron. passed. (13) Cathode rays can penetrate through thin foils of metal. + y (14) Cathode rays are found to have velocity ranging – E e l of velocity of light. U to – J.J. Thomson's Experiment Fig. 25.6 Millikans Oil Drop Experiment ST (1) Millikan the pionee (1) It's working is based on the fact that if aperformed beam of for the precise measurement of the ch electron is subjected to the crossed electric field and (2) By applying suitable electri magnetic field , it experiences a force due each field. In plates, theto charged oil droplets cou case the forces on the electrons in the electron beam due to even held stationary in the field o these fields are equal and opposite, the beamthat remains time. He found the charge on an o undeflected. –19 C. integral multiple of an elementary 10 c (2) When no field is applied, the electron beam produces (3) In this experiment charge on th P. illuminations at point (3) In the presence of any field (electric and magnetic) electron beam deflected up or down (illumination at or ) Atomizer (4) If both the fields are applied simultaneously and Oil drops X-ray tube adjusted such that electron beam passes undeflected and V+ P. produces illumination at point arc light + + + + ++ ++ + + + + + –––– – V– P – – –– –––– Microscope S Electron, Photon, Photoelectric Ef gas. This is doneq/ by m of measuring singly ionised ion of the gas. 60  = Coefficient of v where viscosity of air, 1 = Terminal velocity of drop when no electric field applied between theproduce (1)is The positive ions are These ionsis are accelerate v2 = Terminal velocity of drop hand plates, when side. electric field of the positive ions pass through th applied between the plates. Eand This fine ray of positive ions is sub between = the plates, V = Potential difference d Band magnetic field then allowed to strike a = density =of Separation between plates, Density oil, of air. ( but or ). Positive Rays ⊝  ⊝ ⊝  ⊝  D YG  U ID E3 (2) If the initial motion of the io electric and the magnetic fields are ap When potential difference is applied across electrodes –3 y-axis due to electric field is and in the due of a discharge of (10 electrons are force emitted from mmtube Hg), z magnetic field -direction. it is along the perforated cathode. As they move towards anode, they gain Y energy. These energetic electrons when collide with the atoms S of the gas in the discharge tube, they ionize the atoms. The y positive ions so formed at various places between cathode and +q v anode, travel towards the cathode. Since during their motion, Z z the positive ions when reach the cathode, some pass through the holes in the cathode and a faint luminous glow comes out N from each hole on the backside of the cathode. It is called D positive rays, which are coming out from the holes. The deflection electr Fig. due 25.10 to.....(i) The deflection due to magnetic f Positive rays (1) PositiveFig. rays are positive ions having same mass if the 25.8.....(ii) ST U experimental gas does not have isotopes. However if the gas From equation (i) and (ii), has isotopes then positive rays are group of positive ions having different masses. (2) They travel in straight lines and cast shadows of objects where ; This is the equation o placed in their path. But the speed of the positive rays is much smaller than that of cathode rays. means all the charged particles movi butand of same strike scree q / m value will yzplane (3) They are deflected by electric magnetic fields butthe on a parabolic track as shown in the ab the deflections are small as compared to that for cathode rays. q/ m moving (3) All the positive ions of with same. dif (4) They show a spectrum of velocities. Different positive velocity lie on the same parabola. Hi ions move with different velocities. Being heavy, their velocity is the value of yand z.The ions of different spec much less than that of cathode rays. on different parabola. (5) q/ m ratio of these rays depends on the nature of the Y q/ m q/ m q/ m q/ m gas in the tube (while in case q/ m is of the cathode rays Light V4 large small small large constant and doesn't depend on the nature the tube). mass V2 V3 of gas in V1 q/ m for hydrogen is maximum. Heavy Z (6) They carry energy and momentum. The kinetic energy of mass V V V V 1> 2> 3> 4 positive rays is more than that of cathode rays. (7) The value of charge on positive rays is an integral multiple of electronic charge. (A) (B) Fig. 25.11 (8) They cause ionisation (which is much than (4) The more number of that parabola tells t produced by cathode rays). present in the given ionic beam. Thomson's Mass Spectrograph Bainbridge Mass Spectrograph It is used to measure atomic masses of various isotopes in S P Low Cathode – pressure gas – Screen or Photo plate 4Electron, Photon, Photoelectric Effect and X-Rays In Bainbridge mass spectrograph, field particles of same Hence de-Broglie wavelength velocity are selected by using a velocity selector and then they are subjected to a uniform magnetic field perpendicular to the velocity of the particles. The particles corresponding to Å, Å , different isotopes follow different circular paths as shown in the figure. (1) Velocity selector The positive : ions having a certain Å, Å velocity vgets isolated from all other velocity particles. In this chamber the electric and magnetic fields are so balanced that associa (3) de-Broglie wavelength the particle moves undeflected. For this the necessary condition Neut r o: nde-Broglie particles For wavelength is and E,Band vshould be mutually perpendicular to 60 is each other. E3 Energy of thermal neutrons at ordi B (2) Analysing chamber In this chamber : magnetic is field ; where T = Absolute applied perpendicular to the direction of motion of the particle. As a result the particles move along a circular path of radius temperature, k= Boltzman's constant Joul e/ ke = l vi n, also So,     wavelength of a photon Eis given of energy by While the wavelength of an electro Kis given by D YG    v m   B   +q     2r 1 m 1   2r 2   E B m2   r       Photographic plate. Therefore, for the s U   Velocity spectrum ID In this way the particles of different masses gets deflected on circles of different radii and reach on different points on the photo (4) Ratio of wavelength of photon The plate. the ratio Characteristics of Matter Wav Separation between two traces Fig. 25.12 U (1) Matter wave represents the pr particle in space.. (2) Matter waves are not electroma (3) de-Brogile or matter wave is in Matter Waves (de-Broglie Waves) on the material particle. It means, According to de-Broglie a moving material particle wave is associated with every moving sometimes acts as a wave and sometimes as a particle. or uncharged). The wave associated with moving particle is called matter (4) Practical observation of matt wave or de-Broglie wave and it propagates the form of wave when thein de-Broglie wavelength is of packets with group velocity. particles. (1) de-Broglie wavelength According to : de-Broglie theory, (5) Electron microscope works on the wavelength of de-Broglie wave is given by waves. ST (6) The phase velocity of the matt than the speed of the light. Where = h= Plank's constant, m = Mass of thevparticle, (7) Matter waves can propagate in v E= Speed of the particle, Energy of the particle. not mechanical waves. The smallest wavelength whose measurement is possible nth (8) The number of de-Broglie wave is that of -rays. orbital electron n. is The wavelength of matter waves associated with the (9) Only those circular orbits a microscopic particles like electron, proton, neutron, stable whose circumference is int particle et c. is of the order m. of Broglie wavelength associated with (2) de-Broglie wavelength associated with the charged Davision and Germer Experimen particles The energy : of a charged particle accelerated through (1) It is used to study the scatteri potentialV difference is or to verify the wave nature of elect F Electron gun Electron, Photon, Photoelectric 5 Ef emitted by electron gun is made to fall (5) on The nickel Bragg's crystal formula cutcan be re along cubical axis at a particular behaves angle. like a Nicrystal containing interatomic Dand angle  distance three dimensional diffraction grating and it diffracts the electron  beam obtained from electron gun. Using =  Dsin  60 Heisenberg Uncertainty Princ (1) According to Heisenberg's un impossible to measure simultaneousl momentum of the particle. E3 (2) Let xand pbe the uncertainty in th (2) The diffracted beam of electrons is received the measurement of theby position and mom detector which can be positioned at any angle by rotating it –34 then ; where and  10 Jh= 6.63 sis about the point of incidence. The energy of the incident beam of electrons can also be varied by changing the applied voltage to the Planck's constant. the electron gun. ID U D YG Incident beam (3) According to classical physics, the intensity of scattered A more rigorous treatment gives beam of electrons at all scattering angle will be same but Davisson and Germer, found that the intensity of scattered (3) x If = 0 then p=andif x= p= 0 then beam of electrons was not the same but different at different we are ableangle to measure the ex i. e. ,if angles of scattering. It is maximum °at for diffracting 50 particle (say an electron) then t vol tpotential difference. 54 measurement of the linear momentum o Similarly, if we are able to measure of the particle ,p= 0, then we can not measu i. e. position of the particle at that time o 50 V 54 Fig. 25.14 Viewer Incident Reflected photon photon (4) If the de-Broglie waves exist for electrons then these should be diffracted as the Bragg's formula X-rays. Using Original momentum , we can determine the wavelength of these waves. of electron Final momentum of electron An electron cannot Fig. 25.16be observed w = glancing angle for incident beam = Bragg's angle. between diffracting U where d = distance momentum (4) Uncertainty principle success ST 0° =65 ° =5 D  planes, (i) Non-existence of electrons in (ii) Finite size of spectral lines d (5) The Heisenberg uncertainty pr to energy and time, angular mome Atomic planes Hence Fig. 25.15 diffracting The distance between crystaldisplacement. for planes in Ni o and this experiment isthe Bragg's. This angle = 65 d=0.91 Åand (6) If the radius of thethe nucleus probab is rthen n= Å gives for 1, finding the electroninside the nucl x= 2 rand uncertai Now the de-Broglie wavelength can also be determined by in momentum is using the formula. Thus the deBroglie hypothesis is verified. Photon 6Electron, Photon, Photoelectric Effect and X-Rays According to Eienstein's quantum theory light propagates 0= Threshold wavelengthin the bundles (packets or quanta) of energy, each bundle being Work function in electron volt W0( eV) called a photon and possessing energy. (1) Energy of Energy photon of : photon is given by Table 25.2 : Work function of se c = Speed ofh light, where = Plank's Element Work function Element Work function eV) eV) ( ( Jsec,  = Frequency Hz, =in constant 10 = 6.6 Platinum6.4 Aluminum4.3 Wavelength of light. –34  Gold 5.1 Silver 4.3 Nickel 5.1 Sodium 2.7 Carbon 5.0 Copper 4.7 60 In electron volt Lithium 2.5 (2) Mass of photon Actually : rest mass of the photon is Silicon 4.8 Potassium 2.2 zero. But it's effective mass is given as Cesium 1.9. This mass is also 0) T (2) Threshold frequency :he minimum ( freque known as kinetic mass of the photon incident radiations required to ej (3) Momentum of the photon surface is defined as threshold freq E3  Momentum If incident frequency < 0 No photoelectron em E= energy of each photon where ID For most metals the threshold freq (4) Number of emitted The photons number of : photons (corresponding to wavelengths but nm),betw emitted per second from a source of monochromatic radiation for potassium and cesium oxidesit is of wavelength and power Pis given as ; between 400nm and ) 700 U (3) Threshold wavelength :he maximum ( wavel 0) T (5) IntensityI )of : Energy lightcrossing ( per unit arearadiations required to of incident normally per second is called intensity or energy flux metallic surface is defined as thres If incidentwavelength >0 No photoelectron e D YG i. e. (4) Einstein's photoelectric According equat to Ei photoelectric effect is the result o rfrom a point source Pintensity At a distance of power is between photon and electron in whic given by  absorbed n) I (6) Number of photons falling :P fis per the second ( E=h0 power of radiation and of a photon then Eis the energy E=h K. E.= max. K. E.= 0 The photo-electric effect is the emission of electrons e– (called photo-electrons when light strikes a surface. To escape – e from the surface, the electron must absorb enough energy from Metal the incident radiation to overcome the attraction of positive ions Fig. 25.17 Einstein's photoelectric equatio E=W0+Kmax in the material of the surface. ST U Photo-Electric Effect The photoelectric effect was first observed by Heinrich where maximum kinetic Hertz and it was investigated in detail by Whilelm Hallwachs and emitted electrons. Philipp Lenard. Experimental Setup for Photoe The photoelectric effect is based on the principle of Q) and cathod (1) Two conducting electrodes, th conservation of energy. ( P ) are enclosed in an evacuated glass W0) : (1) Work function (or threshold The energy) ( minimum energy of incident radiation, required to eject the electrons from metallic surface is defined as work function of that surface. 0= Threshold frequency; P – e – e– e – e Q V mA Fig. 25.18 Electron, Photon, Photoelectric 7 Ef (6) Compton Effect E3 60 (2) The battery or other source of potential difference (1) The scattering of a photon by creates an electric field in the direction from anode to cathode. Compton effect. (3) Light of certain wavelength or frequency falling on the (2) The energy and momentum is cons surface of cathode causes a current in the external circuit called photoelectric current. (3) Scattered photon will have (4) As potential difference increases, photo electric wavelength) as compare current to incident p also increases till saturation is reached. (4) The energy lost by the photon i i. e.plate Q isis (5) When polarity of battery atreversed ( energy. w. r. t.plate P) electrons startkinetic negative potential moving back towards the cathode. (5) The change in wavelength due t (6) At a particular negative potential Qno electron called of plate Compton shift. Comp will reachQ the andplate the current will become zero, this V0.by negative potential stopping is called potential denoted Maximum kinetic energy of photo electrons in terms of stopping o = 0 If , = 0 potential will therefore be o = 90 , of Light Effect of Intensity and Frequency ID (1) Effect of intensity If the intensity : of light is increased (called Compton wav (while it's frequency is kept the same) the current levels off at a higher value, showing that more electrons are being emitted per Compton scattering V0doesn'tpotential i. e. unit time. But the stopping change – i 2I D YG = constant U Intensity  no. of incident  no. photon of emitted photoelectron photo per time current I Target electron Recoil at rest electron h  –  i h Incident photon X-Rays f Scattered photon Fig. 25.21 (1) X-rays were discovered by scie V they are also called Rontgen rays. (2) Effect of frequency If frequency : of incident light Fig. 25.19 (2) Rontgen discovered that whe increases, (keeping intensity is constant) stopping potential –3 discharge tube of kept potential dif mmis H gand 10 increases but their is no change inkept photoelectric 25 unknown radiations ( kV ,then somecurrent O –V0 2>1 i I= constant U 2 ST (ii) An arrangement to accelerate V ImportantFig. Formulae for 25.20 (1) and (2)  (3) (4) (5) (3) There are three essential r production of X-rays. (i) A source of electron 1 –V02–V01O by anode. (iii) A target of suitable materia high melting Effect point on which these hig Photoelectric Coolidge X-Ray Tube (1) It consists of a highly evacua cathode and target (also known as fi The cathode consist of a tungsten f coated with oxides of barium or stron of electrons even at low temperat surrounded by a molybdenum cylinder w. r. t.the target. (2) The target (It is a material of melting point and high thermal condu or molybdenum is embedded in a copper 8Electron, Photon, Photoelectric Effect and X-Rays o (5) X-rays are measured in Rontgen (3) The face of the target to the is incident set at 45 power). electron stream. V Lead (6) X-rays carry no charge so the chamber Anode magnetic field and electric field. C Water (7) T (8) They used in the study of crysta F (9) They ionise gases Filament W Target Window X-rays (10) X-rays do not pass through hea E3 60 (11) They affect photographic plat Fig. 25.22 (4) The filament is heated by passing the current through it. (12) Long exposure to X-rays is inj k Vto 80 kV) is applied A high potential 10 difference ( between (13) Lead is the best absorber of Xthe target and cathode to accelerate the electrons which are BaSO (14) For X-ray photography of huma 4 is emitted by filament. The stream of highly energetic electrons the best absorber. are focussed on the target. X-ray photons producedAbsorption from the target. The of X-Rays of X-rays emitted is directly proportional to the X-rays are absorbed when they inci emitted per second from the filament and this can be Intensity X -rays emergent by increasing the i nt ensi t yo filament fXcurrent.of So U number of intensity electrons increased ID (5) Most of the energy of the electrons is converted into (15) They produce photoelectric ef heat (above 98%) and only a fraction of the energy of the (16) X-rays are not emitted by hydr electrons (about 2%) is used to produce X-rays. (17) These cannot be used in Radar (6) During the operation of the tube, a huge quantity of heat reflected by the target. is produced in this target, this heat is conducted through the (18) They show all the important pr copper anode to the cooling fins from where it is dissipated by reflection, refraction, interferen radiation and convection. et c. (7) Control of intensity : Intensity of X-rays implies the r aysFi l amentcur r ent D YG So intensity of absorbed X-rays X-rays : (8) Control of quality or penetration power of Quality of X-rays implies the penetrating power of X-rays, which can be controlled by varying the potential difference where x= thickness of between absorbing = absorpti medi the cathode and the target. coefficient For large potential difference, energy of bombarding electrons will be large and hence larger is the penetration power I 0 Emergent X-rays of X-rays. Table 25.3 : Types of X-rays Hard X-rays I Incident X-rays Soft X-rays x U More penetration power Less penetration power Fig. 25.23 More frequency ofLess the order frequency of of the order of = Wavelength of X-ray) 16 10 Hz ST 19 10 Hz Lesser wavelength Å More range wavelength (0.1 Å – range (4 –4Å) 100 Å) Properties of X-Rays Frequency of X-ray) Atomic number of target) Classification of X-Rays In X-ray tube, when high speed elec (1) X-rays are electromagnetic waves with wavelength they penetrate the target. They los range Å 0.1 –100 Å. comes to rest inside the metal. The el stopped makes several collisions wi (2) The wavelength of X-rays is very small in comparison to At carry each collision of the followi the wavelength of light. Hence they much moreone energy get formed. (This is the only difference between X-rays and light) (3) X-rays are invisible. (1) Continuous X-rays (2) Characteristic (4) They travel in a straight line with speed of light. X-rays Electron, Photon, Photoelectric 9 Ef Continuous X-Rays As an electron passes close to the positive nucleus of atom of the target, the electron is deflected from it's path aselectrons shown in To fill this vacancy fro figure. This results in deceleration of the electron. Thewe loss inthat the created vacancies, know energy of the electron during deceleration is emitted in the E1to lower E2form from a higher energy orbit energy , it orbit radia of X-rays. energy ( E1 – E 2). Thus this energy differen 60 form of X-rays of X-ray very small but def The X-ray photons so emitted form the continuous depends upon the target material. Th spectrum. of sharp lines and is called characte X-ray photon e– (1) K,L,M,…… ser i es:If the electron strik + Y D YG Intensit y U ID E3 eject an electron from of the the atom, a K-shell createdK-shell. in the Immediately an elec L-shell K-shell, outer shell, sayjumps to theemittin Fig. 25.24 photon of energy equal to the energy (1) Minimum wavelength When the : electron looses two shells. if an electro jumps t M-shell whole of it's energy in a single collision withSimilarly, the atom, an X-ray photon of maximum energy emitted hmaxis i. e. K-shell, X-ray photon of higher ene L,M,Nof el photons emitted due to the jump shells to the K-shells K,K,Klines gives of K-series the of spectrum. v= velocity of electron before collision with target where V = potential difference through which electron is n=5 atom, O 8 c= speed of  m/ s =3 accelerated, light 10 N n=4 M M Maximum frequency of radiations (X-rays) M n=3 Mseries L L L Minimum wavelength = cut off wavelength of X-ray L n=2 Lseries K K K K n=1 series (2) Intensity wavelength The continuous graph : X-rayKIf over the electron striking the targe spectra consist of all the wavelengths a given range. Fig. 25.27 These the target atom, an..electr shell L-shell of M,shows N… wavelength are of different intensities. Following figure the intensity variation of different forX-rays various L-shell jumpswavelengths to the so that photons accelerating voltages applied to X-ray tube. emitted. 30 kV 20 kV 10 kV These photonsL-series form theof the spec similar way theMformation series, of. may be N series et c explained. ST U (2) Intensity-wavelength At certain graph sharpl : wavelengths, the intensity of K X-ray , min K …. as shown in figure. These X-r Wave length For each voltage, the intensity curve starts at a particular characteristic X-rays. At other wav Fig. 25.25 minimum wavelength ( rapidly to a maximum and min). Rises gradually and these X-rays are calle then drops gradually. K Intensity The wavelength at which the intensity is maximum K depends on the accelerating voltage, being shorter for higher L voltage and vice-versa. LL Characteristic X-Rays K-series L-series Few of the fast moving electrons having high velocity penetrate the surface atoms of the target material and knock min Wavelength out the tightly bound electrons even from the inner most shells Fig. 25.28 of the atom. Now when the electron is knocked out, a vacancy is Mosley's Law created at that – place. e X-ray Mosley studied the characteristi spectrum X-ray photon number of a heavy elements and conclu – e e– different elements are very similar M L K + Fig. 25.26 10 Electron, Photon, Photoelectric Effect and X-Rays number, the spectral lines merely shift towards higher n= 2, 3, where …. 4, frequencies. He also gave the following relation Kfor While line k (iv) k Uses of X-Rays (i) In study of crystal structure : determined using X-ray diffraction. Z 60 (ii) In medical science (iii) In radiography (iv) = Frequency of Z where emitted = Atomic line, number of In radio therapy (v) In engineering target, constant, constant or a= Proportionality b= Screening (vi) In laboratories Shielding constant. (vii) In detective department ( Z–b) = Effective atomic number (viii) In art the change occurring aand bdoesn't depend on the nature of target. Different examined by X-rays. values of as follows bare b= 1 E3 Fi g.25. 29 for Kseries L-series for b= 19.2 for M-series ID b= 7.4 (1) Mosley's law supported Bohr's theory of positive rays help Z) of  Discovery (2) It experimentally determined the atomic number ( isotopes. elements. D YG U  The de-Broglie wavelength of elec (3) This law established the importance of ordering of of an atom is equal to circumference o elements in periodic table by atomic number and not by atomic  A particle having zero rest mass a weight. and momentum must travels with a spee A= 43, data (4) Gaps in Moseley's 61, 72, for 75 suggested light. existence of new elements which were later discovered.  de-Broglie wave length associate (5) The atomicCnumbers of established u,Agand Ptwere is given as (Energy of ga to be 29, 47 and 78 respectively. ST U (6) When a vacancyK-shell, occurs in there the is still one electron remaining K-shell. inAn the electron L-shell in will the Tis at temperature ) feel an effective charge to +of (the nucleus Z–1) edue Zefrom  A photon is not a material partic K-shell electron, L-shell and – efrom the remaining because energy. K-shell orbit is well outside orbit. the  When a particle exhibits wave nat (7) Wave length of characteristic spectrum with a wave packet, rather then a wave and energy of X-ray  radiations. By coating the metal surface with a strontium oxide it's work function is  We must remember that intensity radiation is inversely proportiona between source of light and photos Pi. e., n2= n1= 1 K( (8) If transition takes 2 to place from  - line) so ) (i)  The photoelectric current can be some inert gas like Argon into the bu (ii) emitted by cathode ionise the gas by current is increased.  Compton effect shows that photon h (iii) In general theK wavelength -lines are given ofProduction allby the  of X-ray is the rever photoelectric effect. Electron, Photon, Photoelectric 11 Ef Uncertainty in the measurement  The thickness of medium at which intensity of emergent i. e. X-rays becomes half is called half value thickness photon within the nucleus is x1/2 ( ) and it is given as where nucleus =d= an d = diameter of thex uncertainty in the measurement of po.  Continuos X-rays are produced due to the phenomenon called "Bremsstrahlung". It means slowing down or braking radiation.  The wavelength of characteristic X-ray doesn't depend 60 Z) on accelerating voltage. It depends on the atomic number ( of the target material.  In characteristic X-ray spectrum and also E3  Nearly all metals emits photoelectrons when exposed to UV light. But alkali metals like lithium, sodium, potassium, rubidium and cesium emit photoelectrons even when exposed to visible light.  Oxide coated filament in vacuum tubes is used to emit ID electrons at relatively lower temperature.  Conduction of electricity in gases at low pressure takes because colliding electrons acquire higher kinetic energy due to increase in mean free path. U  Kinetic energy of cathode rays depends on both voltage and work function of cathode. D YG  Photoelectric effect is due to the particle nature of light.  Hydrogen atom does not emit X-rays because it's energy levels are too close to each other.  The essential difference -rays between is X-rays and of   that, -rays emits from nucleus while X-rays from outer part of atom.  There is no time delay between emission of electron and. e.the incidence i of photon electrons are emitted out as soon as the light falls on metal surface. U  If light were wave (not photons) it will take about an year to eject a photoelectron out of the metal surface.  Doze of X-ray are measured in terms of produced ions or ST free energy via ionisaiton.  Safe doze for human body per week is one Rontgen (One 4 Rontgon is the amount of X-rays 10 which emits 2.5 Jfree energy through g ionization m air at NTP of 1  The photoelectrons emitted from the metallic surface have different kinetic energies even when the incident photons have same energy. This happens because all the electrons do not exist in the surface layer. Those coming from below the surface loose more energy in getting themselves free.  Einstein was awarded Nobel prize for explaining the photoelectric effect.