Download The Crisis in Physics: From Classical to Quantum Mechanics and more Study notes Physics in PDF only on Docsity! PHY102 1 Major Concepts in Physics Lecture 16. Prof Simon Catterall Office 309 Physics, x 5978 smc@physics.syr.edu http://physics/courses/PHY102.08Spring PHY102 So far … Prior to 20th century: Mechanics (Newton) Electromagnetism (Faraday, Maxwell) Thermodynamics (Boltzmann, Gibbs) classical physics … 20th century saw 2 dramatic revolutions Relativity Quantum Mechanics – experiments on atoms
Relative intensity
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A 3000, kK, Classical theory before Planck
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Frequency (10!4 Hz)
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PHY102 Ultraviolet catastrophe … Unfortunately classical physics was not able to account for the shape of these curves f E(f) E(f)=cf2 Total radiation emitted = infinite! PHY102 To correctly explain the shape of the blackbody spectrum Planck (1900) proposed that the energy absorbed or emitted by oscillating charges came in discrete bundles called quanta. The energy of the quanta are hfE =0 where h=6.626×10-34 J s is called Planck’s constant. The quantum of EM radiation is the photon.
| Fig. 27.4
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Fig. 27.6
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PHY102 Photoelectric effect demo Note: see electric current as emitted electrons collected by anode and flow in circuit Add battery: serves only to collect more electrons – not responsible for current. Current increases with visible light Still non zero with blue light BUT DISAPPEARS with red ! PHY102 Problem for classical physics Classical wave theory would predict that electron energy should just depend on light intensity, not frequency. There should be no minimum frequency and no maximum to electron energy PHY102 Einstein’s explanation (1905) Planck says: EM radiation may only be emitted/absorbed in discrete amounts hf Einstein: Perhaps EM radiation in vacuum consists of only these discrete packets of energy – photons Light is not a wave at all but a stream of photons !! But we just spent 8 weeks arguing that it was a wave !! PHY102 What is the energy of a photon of red light (wavelengh 670 nm) ? A: 3x10-19 J B: 3x10-15 J C: 1.5 J D: 6x10-15 J PHY102 Atoms Seen earlier that light emitted by single element contains only a discrete set of possible frequencies. This is also a puzzle for classical physics. An electron orbiting a nucleus could radiate light at a frequency equal to its orbital frequency. But that is unrestricted so the emitted radiation should also be at all frequencies PHY102 Worse still … Actually this loss of energy in the form of EM waves should lead to a decaying electron orbit – energy is continuously being lost If one works out how fast before the electron decays into the nucleus one finds a very short time – 10-8 s !! So matter is not even stable classically … PHY102 Summary Classical physics fails completely to describe what happens in atoms Infinite radiation energies, photoelectric effect, line spectra, … Partial explanation due to Planck/Einstein. Treat light (a wave) as a particle the photon with energy E=hf Conversely discrete line spectra look like wave phenomena – for the electron (particle)
