Chapter 33 Early Quantum Theory and models of atom
Chapter 33 Early Quantum Theory and Models of Atom
Revolution of classical physics World was well explained except a few puzzles? two dark clouds in the sky of physics M-Mexperiment y theory of relativity Black body radiation quantum theory Two foundations of modern physics Revolution of Q-theory: (1900-1926)>now?
Revolution of classical physics 2 World was well explained except a few puzzles? M-M experiment theory of relativity Black body radiation quantum theory Revolution of Q-theory: (1900 – 1926) → now? Two foundations of modern physics “two dark clouds in the sky of physics
Blackbody radiation All objects emit radiation -> thermal radiation 1)Total intensity of radiation oc T4 2)Continuous spectrum of wavelength Blackbody: absorbs all the radiation falling on it Idealized model Blackbody radiation -easiest
Blackbody radiation 3 All objects emit radiation → thermal radiation 1) Total intensity of radiation ∝ T4 2) Continuous spectrum of wavelength Blackbody: absorbs all the radiation falling on it Idealized model Blackbody radiation → easiest
Classical theories Wien's law nt=2.90x10'm K Experimen Intensity O Rayleigh-Jeans 6000K Wien Planck 3000K 1000 IR 2000 Wavelength(nm) Wavelength
Classical theories 4 Wien’s law: 3 2.90 10 PT m K Experiment Intensity Wavelength Wien Rayleigh-Jeans Planck
Planck,'s quantum hypothesis Planck formula(1900) 2hc25 hc/nk Max planck Completely fit the data Nobel 1918) Planck's constant: h=6.626x10-34. S The energy of any molecular vibration could be only some whole number multiply of hf
Planck’s quantum hypothesis 5 Planck’ formula (1900): Max Planck (Nobel 1918) 2 5 / 2 ( , ) 1 hc kT hc I T e Completely fit the data! 34 h 6.626 10 J s Planck’s constant: The energy of any molecular vibration could be only some whole number multiply of hf
Concept of quantum The energy of any molecular vibration could be only some whole number multiply of hf. h=6.626×10-34J.s E=n·hf f: frequency of oscillation Quantum - discrete amount /not continuous hf: quantum of energy (a) n: quantum number continuous discrete
Concept of quantum 6 The energy of any molecular vibration could be only some whole number multiply of hf. E n hf 34 h 6.626 10 J s f : frequency of oscillation Quantum → discrete amount / not continuous hf : quantum of energy n : quantum number continuous (a) discrete (b)
Photon theory of light Little attention to quantum idea Until Einsteins theory of light Molecular vibration radiation Albert einstein (Nobel 1921) hc E=hf → quantum of radiation The light ought to be emitted, transported, and absorbed as tiny particles, or photons
Photon theory of light 7 Little attention to quantum idea Albert Einstein (Nobel 1921) Until Einstein’s theory of light hc E hf Molecular vibration The light ought to be emitted, transported, and absorbed as tiny particles, or photons. → radiation → quantum of radiation
Energy of photon Examplel: Calculate the energy of a photon with n=450nm (blue light Solution: E hc 44×10-9J=27e Example2: Estimate the number of visible light photons per sec in radiation of 50w light bulb Solution: Average wavelength: Ax550nm hc 1. 4x10 invisible light photons
Energy of photon 8 Solution: Example1: Calculate the energy of a photon with 450nm (blue light). 19 4.4 10 hc E J 2.7eV Example2: Estimate the number of visible light photons per sec in radiation of 50W light bulb. Solution: Average wavelength: 550nm hc n E 20 1.410 invisible light photons?
Photoelectric effect Photoelectric effect: electron emitted under light If voltage v changes Source photocurrent I also changes Light Saturated photocurrent Stopping potential /voltage A E k max
Photoelectric effect 9 Photoelectric effect: electron emitted under light Stopping potential / voltage: 2 max 0 1 2 Ek mv eV If voltage V changes photocurrent I also changes Saturated photocurrent
Experimental results 1)Ekmax is independent of the intensity of light 2)Ekmax changes over the frequency of light 3)If f<fo(cutoff frequency ), no photoelectrons High intensit Low intensity keep f!
Experimental results 10 1) Ekmax is independent of the intensity of light 2) Ekmax changes over the frequency of light 3) If f < f0 (cutoff frequency), no photoelectrons