Energy and Waves

Energy and Waves



  1. In 1887 Hertz discovered the photoelectric effect, where electrons were emitted from a zinc surface when illuminated with ultra-violet light. These electrons (called ‘photoelectrons’) were only emitted when the frequency of the ultra-violet light was above a certain value (called the ‘threshold frequency’).

  A. Assume that ultra-violet light is falling onto a piece of zinc, and photoelectrons are being emitted from the surface. As the intensity (brightness) of the ultra-violet light is increased, what happened to the kinetic energy of the photoelectrons?
If the intensity of the ultra-violet light is increased, more electrons will be emitted. There will be no change in the kinetic energy of the photoelectrons.

  B. Explain why this discovery was not in agreement with the wave model of light.

According to electromagnetic theory, when light, thought to be composed of waves and falls on electrons bound in an atom, the energy of the liberated electrons ought to be proportional to the intensity of light. Experiments showed that, although the electron current produced depends upon the intensity of the light, the maximum energy of the electrons was not dependent on the intensity. Moreover, classical theory predicted that the photoelectric current should not depend on the frequency of the light and that there should be a time delay between the wave falling on the metal surface and the emission of the electrons.

  C. Describe the new photon model of light, which was proposed as a result of photoelectric effect.

In quantum theory light is composed of individual quanta or wave packets called photon. The energy of each photon being proportional to its frequency according to the equation E=hf, where E is the energy, (f) is the frequency, and (h) is Planck's constant. Each photoelectron ejected is the result of the absorption of one photon. The maximum kinetic energy, KE, that any photoelectron can possess is given by hf= φ...