DEVELOPMENT OF QUANTUM THEORY

WAVE MODEL OF LIGHT

model of light originally thought to follow particle theory developed by Sir Isaac Newton (late 1600’s)

in early 1800’s Christopher Hyugen’s proposed the wave model of light

Thomas Young (1801) performed diffraction experiments that further supported the wave model

allowed scientists to represent light as a wave with wavelength and frequency

Einstein’s work (1900’s) with light absorption and emission suggested that a particle model of light with wave properties

light now said to have wave and particle properties: “wave-particle duality of light”

ELECTROMAGNETIC SPECTRUM

J.C. Maxwell (1865) demonstrated that light is considered to be part of an electromagnetic spectrum consisting of oscillating electric and magnetic waves

includes gamma rays, x-rays, UV light, Visible light, IR light, microwaves and radiowaves

PLANCK’S CONTRIBUTION

all objects emit radiation: at room temperature mainly IR, while at higher temperatures increasing amounts of UV

Kirchhoff (1859) observed behaviour of light emitted by a blackbody at increasing temperatures appeared as follows:

Max Planck (1901) made various attempts to explain the shape of the curve

through mathematical modeling techniques he was able to show that representing the energies given off by the oscillating atoms as multiples of small quantity of energy determined by the frequency of the vibration provided the best curve fit, i.e. energies emitted were NOT a continuous stream of energy

called the quantization of energy

PHOTOELECTRIC EFFECT

Heinrich Hertz (1887) used the apparatus below to study the ejection of electrons from a metal (or other material) surface when exposed to light – photoelectric effect

Hertz observed that the brightness (intensity) of the light did NOT determine the kinetic energy of the liberated electrons as originally assumed

Instead Hertz showed that the frequency of light was the most important factor

Einstein (1905) explained the photoelectric effect observed by Hertz using the quantization of light by Planck

Suggested that light behaves like a stream of energy packets – quanta or photons; this energy is determined by the frequency of the light

When light hits the metal surface the energy of the photon is transferred to the electron

  • A minimum amount of energy is required before the electron can be ejected – dependent on the frequency of light
  • any excess is transferred to the kinetic energy of the electron
  • analogy is the marble rolling in a cup or bowl