Physics 203/204

1. Physics 203/204 7: Introduction to Quantum Physics • Blackbody Radiation and Planck’s Hypothesis • The Photoelectric Effect • Compton Effect • Atomic Spectra • The Bohr Quantum Model of the Atom

2. Classical Physics • Material objects obey Newtons Laws of Motion • Electricity and Magnetism obey Maxwells Equations • Position and momentum are defined at all times • Initial Position and momentum plus knowledge of all forces acting on system predict with certainty the position and momentum at all later times. • Could not explain • Black Body Radiation • Photo Electric Effect • Discrete Spectral Lines

3. Blackbody Radiation and Planck’s Hypothesis Any object with a temperature T>0 K radiates away thermal energy through the emission of electromagnetic radiation Classical explanation heat causes accelerated charges (Maxwell like distribution of accelerations) that emit radiation of various frequencies

4. Incandescent Spectra produced from Thermal Radiation intensity frequency

5. Wiens Displacement Law l T = 2 . 898 ´ 10 mK - 2 max Rayleigh - Jeans Law 2 p ckT I ( l , T ) = l 4 Intensity of radiation of wavelength l at temp T However this only agrees with experiment at long l Lim I ( l , T ) = ¥ Ultraviolet Catastrophe l ¯ 0 ( Þ ¥ total energy density )

7. Planck’s Assumptions Oscillating molecules that emit the radiation only have discrete energies En = nhn n = quantum number En = energy of quantum state n of molecule Molecules emit or absorb energy in discrete units of light called QUANTA

8. E2 hn hn=E E1 E = E2-E1

9. The Photoelectric Effect Light A Electron • A is maintained at a positive potential by battery. • IG = 0 until monochromatic light of certain l is incident G V

10. high intensity light I low intensity light -V0 V plate A has negative potential Stopping Potential • When A is negative only electrons having K.E. > eV0 will reach A, independent of light intensity • Maximum K.E. of ejected electrons Kmax=eV0

11. Observed Properties 1 . No electrons ejected if n £ n (cut off frequency ) c 2 . If n ³ n the number of photo electrons µ light intensity c 3 . K is independent of light intensity max 4. K ­ as n ­ max 5 . Electrons are emitted instantaneously even at low light intensities Wave theory of light does not predict such properties

12. Einstein explained this by the hypothesis that light is quantized in energy packets = QUANTA with energy E = h n he called such quanta PHOTONS . The intensity of the light is proportional to the number of such quanta i . e . I µ nh n In order for electrons to be emitted they must pass through surface . \ use f amount of energy to overcome surface barrier º Ionization Potential º Work Function K = h n - f = h n - h n max c

13. Einsteins Theory Predicts 1 . K = h n - f ; so K depends on n max max 2 . h n ³ f ; for emission of electrons 3 . h n - f only depends on n not on intensity 4. K ­ as n ­ max 5 . single electrons are excited by light , not many gradually Þ instantaneous emission Kmax = hn-f slope = h Kmax nc

14. Compton Effect scattered photon q f scattered electron More Evidence that light is composed of particles

15. Observed scattering intensity I ( ) I = I l , q ; incident l ¹ scattered l - this contradicts classical theory 0 D l = l - l 0 Compton ( 1923 ) suggested treating photon as particle hc E = h n = l The Special Theory of Relativity gives E = pc [ ] p is the magnitude of the momentum of the photon hc h \ pc = Þ p = l l D E = D p = 0 tot tot h ( ) Þ D l = 1 - cos q m c e Þ l ­ ; n ¯ ; E ¯ during collision photon h Compton Wavelength of electron = m c e

16. Youngs Double Slit Experiment Light is composed of waves Photo Electric Effect Light is composed of particles Compton Effect Light is composed of particles Paradox? • Wave Particle Duality

17. Atomic Spectra

18. Absorption Spectra gas gas Emission Spectra

22. 1 . Electron moves in circular orbit about nucleus 2 . Electron can only exist in specific orbits ( ) Electron States are Quantized h L = m v r = I w = n = n ; n = 1 , 2 , h K e 2 p v é ù I = mr ; w = 2 ë r û 3 . Electrons in such orbits DO NOT radiate energy although they are accelerating. Such orbits are thus called STATIONARY STATES 4. Atoms radiate only when electron jumps from higher energy (large radius ) to lower energy (smaller radius ) orbits . The frequency of light they radiate is given by E - E n = h l h

23. kq q e 2 ( ) U r = = - k 1 2 r r k = coulombs constant - 1 e 2 ( ) r E r = K + U = m v - k 2 2 r e + If electrons speed is constant m v e e 2 2 2 F = m a = = k Þ m v = k e 2 c e c r r e r 2 1 1 e 2 \ m v = k 2 2 2 r e 1 e 2 ( ) Þ E r = - k 2 r

24. Quantization of Angular Momentum ß n n h h r = Û v = m v m r e e n ke h 2 2 2 \ m v = = 2 m r r e 2 e n h 2 2 Þ r = ; n = 1 , 2 , K m ke 2 e \ r = r i . e . r depends on n n h 2 Bohr radius is defined as r = m ke 0 2 e so that r = n r 2 n 0

25. using these values for r in the expression n for the energy we obtain m k e 1 æ 2 4 ö E = - ; n . = 1 , 2 , e K è ø 2 n n h 2 2 1 æ ö = - 13 . 6 eV è ø n 2 thus the frequencies of emitted photons are æ E - E m k e ö 1 1 2 4 ç ÷ n = = - 2 1 e h 2 n n 21 è ø h 2 2 2 1 2 æ 1 n m k e 1 1 ö 2 4 ç ÷ = = - e l c 2 c n n è ø h 2 2 2 1 2 Theoretical expression for Rydberg constant m k e 2 4 R = e 2 c H h 2 which is in good agreement with experimental value