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Wave Particle Duality

Wave Character: The experiments of Thomas YOUNG with a double slit in the beginning of the 19th century finally lead to the acceptance of the wave character of light.[1] As shown in figure 1, this experiment resulted in an interference pattern on the screen with positive and negative interference.


Figure 1: The double-slit experiment of Thomas YOUNG, with a being the light source, b and c are slits in a screen and d is a screen with an interference pattern.[2]


Particle Character: The theory on the particle character of light was already published in the 17th and 18th century but were neglected after acceptance of the wave character of light in the 19th century. But some effects could not be explained by considering light as a wave. The particle character of light was brought into scientific discussion again by Einstein with his explanation of the photoelectric effect.

When metal plates are irradiated with light, it is possible to remove electrons from these plates. In 1905 Einstein found, that this effect is not coupled to the intensity of the light but the wavelength and thus the energy. His explanation of this observation by using Planck’s relation finally lead to the acceptance of the particle character of light.


Dualism: Electromagnetic radiation thus possess properties of both, a particle and a wave. It is common to associate light with the properties of a wave, e.g. with scattering and diffraction. Despite this, light also shows properties of a particle, e.g. a momentum by which solar “wind mills” can be driven. The correlation between wavelength λ and momentum p can be described mathematically with the de-Broglie-correlation[3]:


For photochemistry, considering light as photons is in line with the amount based description of chemical processes. Conversion of energy based quantities, often used to describe light, can be done with PLANCK-EINSTEIN correlation:



  1. NAUJOKS, W.; WERRES, B.: Metzler Physik (2. A.). Gesamtband. (Lernmaterialien), 1992, Schroedel, ISBN 3507052091.
  2. PRESBYTHERIAN: Ebohr1 IP, https://commons.wikimedia.org/wiki/File:Ebohr1_IP.svg.
  3. ATKINS, P. W.; DE PAULA, J.: Physikalische Chemie, 2013, Wiley VCH Verlag GmbH, ISBN 3527332472.

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