Early Quantum Theory-College Physics B-Lecture 15 Slides-Physics, Slides of Physics

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PHY 2054C – College Physics B

Fall 2004

Electricity, Magnetism, Light Optics and Modern Physics Dr. Ingo Wiedenhöver Dr. A. Volya Today:

1. Early Quantum Theory

Early Particle Physics

Today we are re-enacting the early discoveries of Quantum Physics. Unlike Einstein's Theory of Relativity, the development of Quantum Mechanics was driven by experimental observations on particles and atoms. Where Einstein taught us the structure of space and time, Quantum Mechanics is describing the structure of atoms and sub-atomic particles.

The Electron

J.J. Thompson measured the q/m ratio of “Cathode Rays”

1. measure bending radius due to B
2. balance deflection with electric field E e v B = mv 2 r ⇒ e m = v B r e v B = e E ⇒ v = E B e m

E

B

2 r

Light: Wave or Particle?

1. If light is a wave, the power transmitted is prop. to its amplitude 2 . Bright light – large amplitude – large power Dim light – small amplitude – small power It can deliver any energy value, large or small to a particle it hits. A

Experiment:Photo-Electric Effect

Light which strikes metal surfaces knocks out electrons. Any electron requires a certain amount of energy in order to leave the metal surface, W 0 . For light of different frequency f, we measure the KE of electrons. It shows that with h = 6.63 10

• J s (Planck's constant ) The electron KE is independent of light intensity! KE f

KE

max = hf − W 0

Question

The fact that the measured electron KE is independent of light intensity, but depends on the light's frequency (color) shows 1.) That light is a wave 2.) that light is a particle 3.) that electrons are negatively charged 4.) that light travels at the speed of light

Compton Effect

 (^) Scattering of light by electrons  (^) Behaves like an elastic collision between photons and electrons (energy of photons changes with their outgoing direction).  (^) Wave theory would predict that the energy of the outgoing light does not depend on the direction. θ λ λ’ φ λ’ = λ + h/m o c * (1 - cosθ) The Compton effect shows that light has momentum, is a particle!

Wave-Particle Duality

 (^) Light is a wave. Light is a particle. Now what? It is both. You have to live with it.  (^) “normal” particles can behave like waves, too!  (^) The principle of complementarity  (^) Sometimes light (matter) behaves like a wave.  (^) Sometimes light (matter) behaves like a particle.  (^) Which concept applies depends on the experiment!  (^) This is Heisenberg's uncertainty principle at work!

Question

Single photons are directed, one by one, toward a double slit. The distribution pattern of impacts that make it through to a detector behind the slits is identical to an interference pattern. We now repeat this experiment, but block slit 1 for the first half of the experiment and slit 2 for the second half. The distribution of impacts in the second experiment is

1. the same as in the first experiment, but with half the intensity
   2. the pattern of a “single slit” experiment
   3. neither of the above.

Summary

Waves can interfere with other waves (double slit) particles carry small packets of energy (photo-effect) We have seen examples that light and matter can both behave like waves and particles in different circumstances. Is nature made of waves or of particles? The answer is an affirmative “yes” Waves of probability to find a particle!