Quantum Physics Interaction of matter with energy

The Quantum Nature of Radiation Contradictions between classical physics and the photoelectric effect The causes of the failure The dual nature of light : wave and particle The birth of Quanta The origin of Photons Energy quantization is a fundamental property of electromagnetic radiation: the origin of Quantum Physics Note: light and electromagnetic radiation are used interchangeably.

Light-Matter Interaction 1887 Light incident on a clean metal surface: Electrons gain energy and overcome the pull of the nuclei. The metal surface becomes positively charged. An electroscope connected to the metal surface establishes the presence of positive charges. The phenomenon is called the photoelectric effect. How is this phenomenon studied? + + + + + + + +

The Photoelectric Effect Observations Using the results of the figure, find out and discuss observations relating to energy of emitted electrons, current, light intensity, light frequency, voltage. Write down your observations and explain possible reasons?

The Photoelectric Effect – In Class Discussion http://phet.colorado.edu/en/simulation/photoelectric What relationship exists between light intensity and current. How the energy of the electrons is affected by the light intensity. What is the effect of different radiations on the emission electrons.

Puzzles of the Photoelectric Effect The intensity of light does not affect the energy of the electrons. The stopping voltage is independent of the intensity of the incident light. The increase in the current is due to more electrons being emitted, not to the speed of the electrons. Stopping voltage is a function of the frequency of the incident light, and increases with increased frequency. Electrons are emitted with no delay, no dependency on the intensity of light.

Einstein’s Photon 1905 Einstein postulated the existence of quanta or discrete packets of energy and momentum called photons. All electromagnetic radiation energy consists of photons. The energy E of a photon is given by: E = hf h : Planck’s constant = 6.63 × 10-34 J s. f : frequency of the electromagnetic radiation. Photon is known as the particle of light  light is a wave and particle. Note: The expression E = hf was first coined by Max Planck in 1900 as a mathematical tool in solving the Blackbody radiation problem. 1900 E = hf

Einstein Photoelectric Equation Photon energy Ep absorbed by an electron: Ep= hf Ф : energy necessary to remove an electron from the metal surface. Ф is called the work function: Ф = hfc fc is the critical or threshold frequency (no current flow). Why electric current cannot knock off electrons in conductors ?

Einstein Photoelectric Equation The maximum kinetic energy Ek of the free electron is given by: Ek = hf – Ф = h(f – fc ) The kinetic energy of the free electron is also given by the stopping voltage Vs : Ek = ½ mv2 = eVs e, m, and v are the charge, mass, and velocity of the free electron, respectively.

Problems Problem-1 Determine the work function of a metal in Joules if the maximum threshold wavelength is 1.10x10-7 m. Problem-2 Determine the maximum speed of electrons emitted from a zinc surface if they are stopped by a 16 N/C uniform electric field over a distance of 3.0cm. Problem-3 Problem # 6/p.396 (textbook).

Millikan‘s Experiment Two equations to use: Ek = hf – Ф or Vs = (h/e)f – Ф/e Both show a linear relationship with f. The slope of the second equation yield the ratio: h/e In1914, R.C. Millikan reported that the Einstein’s equation was correct. fc R.C. Millikan's experiment