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