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Lecture VII: Feb 15, 2021) Lecture VII: Feb 15, 2021) Photo Electric & Compton Effect: NOBEL PRIZES for Quantum Revolution, now known as “Old Quantum Theory” • Max Planck: 1918 • Albert Einstein: 1921. • Neil Bohr: 1922 • Arthur Compton: 1927 • Louis be Broglie: 1929 I. FRENCH PHYSICIST: LOUIS DE BROGLIE - WAVE- PARTICLE DUALITY Prince Louis-Victor Pierre Raymond de Broglie was a history major. In 1913 he switched to physics as his brother Maurice De Broglie was an experimental physicist brought him all the exciting news folding in the quantum physics. He won Nobel prize in 1929. 1 Matter Waves or De Broglie Waves Compton experiment confirmed that electro- magnetic waves behave like particles. The scattering of these waves by electrons changes the color or frequency of waves and this can be explained by assuming that the scattering of waves and electrons is very similar to collision of two billiards. NOTE: Compton scattering experiment was done with x-rays- very short wave length: Scattering angle with be almost zero for light waves. That means, change in the wave length of radiation after scattering will be almost zero. After learning about Compton experiment Louis de Broglie argued that if electromagnetic waves behave like particles then particles can also behave like waves. If particles have wave properties, what is the wave length of a particle of mass m, moving with a velocity v?? The following two equations describe the de-Broglie hypothesis: h E λ = ; f = (1) p h where p is the momentum of the particle. p = mv (2) For photons, which has m = 0, p = E=c. WHEN IS the de Broglie wave length meaningful ??? Calculate de Broglie wave length of a runner and of electrons inside the atom. NOTE: 3 m/s is approximately 10 km per hour Speed of electrons inside the atom is about 1 percent the speed of light c = 3 × 10−8m=sec. h = 6:6 × 10−34m2kg=s 2 −30 Mass of electron me ≈ 10 kilograms II. EXPERIMENTS CONFIRM DE BROGLIE’S THEORY • A graduate student Walter Elsasser from Gotinged¨ University in Germany pointed out that if de Broglie was right, electrons should show interference pattern when scattered from a crystal.. crystal is like a multi-slit version of double slit. If one accelerates electrons by 150 volts, their de Broglie wave length should be of the order of spacing between the crystals and one should see interference pattern. • Einstein who read the de Broglie’s thesis told Elsasser “ Young man, you are sitting on a gold mine”. • In 1926, at a conference in Oxford – annual conference of the British Association for the Advancement of Science, two experimentalist Clinton Davisson ( Bell Lab) and George Thomson ( son of J. J. Thompson who proved that electron was a particle ), English physicist from Aberdeen discussed the possibility of experimental test of De Broglie. These two experimentalists independently saw the interference pattern when electrons scatter from a crystal. They got Nobel prize in 1937. De Broglie got the Nobel in 1929. 3.
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