The Wave-Particle Duality of Light Joaquin Riojas Science P.3 February 17, 2014

Early theories By the 1600’s two separate and competing theories of light existed the first by Christian Huygens and the second by Sir Isaac Newton, Newton’s was a corpuscular or particle theory while Huyhen’s was a wave theory.

Thomas Young and Diffraction Newton’s prestige as a physicist made his corpuscular theory the most popular one for a long time; till, diffraction began to be observed in the early nineteenth centaury. Light can be seen behaving as a wave through diffraction as observed in Thomas Young’s double slit experiment.

Huygens, the ether, and Maxwell Since a wave generally has to propagate though some medium, Huygens proposed an ether or a hypothetical substance supposed to occupy all space, which he postulated to account for the propagation of electromagnetic radiation through space. James Clerk Maxwell quantified a set of equations to explain EM radiation as the propagation of waves and assumed such an ether as the method of propagation. Problems regarding the wave theory began after failure to detect the ether and astronomical observations in stellar aberration by James Bradley in 1720 had indicated that ether would have to be stationary relative to a moving Earth.

Heinrich Hertz and the Hertz effect The Photoelectric effect can be observed when a metallic surface is exposed to a form of EM radiation such as light, when done so electrons may be emitted from the surface. Documented in 1887 by Heinrich Hertz as the Hertz effect it is now know as the Photoelectric effect. Problems for the wave theory began to arise when experimental results concerning the photoelectric effect were the exact opposite of what they should have been if light was a wave.

Results of The Photoelectric effect According to Wave Theory Experimental Results The intensity of the radiation should have a proportional relationship with the resulting maximum kinetic energy. The photoelectric effect should occur for any light, regardless of frequency or wavelength. There should be a delay on the order of seconds between the radiation’s contact with the metal and the initial release of photoelectrons. The intensity of the light source had no effect on the maximum kinetic energy of the photoelectrons. Below a certain frequency, the photoelectric effect does not occur at all. There is no significant delay (less than 10-9 s) between the light source activation and the emission of the first photoelectrons.

Einstein and the Photoelectric effect In 1905 (Einstein's wonder year) he published hi Nobel prize winning paper on the photoelectric effect. He explained the experimental results on the photoelectric effect, by describing electro magnetic radiation as quantized bundles of electromagnetic energy, or photons, whose energy was related to their frequency and are always at motion at the speed of light in free space. In his paper Einstein described the effect of a single photon with a single electron, which explains the third phenomenon by implying that the energy from the photon immediately transfers to the electron in order to knock it free. The single photon interaction also explains why below a frequency there is no emission because, the photon’s energy is less than the metal’s work force. He also explains how if the photon exceeds the work force the excess energy is converted to the kinetic energy of the electron; therefore, having the maximum kinetic energy is independent to the intensity but not to the energy of the photon. Increasing the intensity only results in more elections being released.

Conclusion Light, particle or wave? Early on theories and experiments favored the wave; Thomas Young’s double slit experiment explores diffraction which undyingly proves how light behaves as a wave. Yet, the experimental results of the photoelectric effect which can only be explained by Einstein’s photon theory, ultimately proves that light can behave as a particle. The question persists; to be answered by only though quantum mechanics, in which the properties of an object are contradictory until a measurement is made. And so, the wave-particle duality of light and electromagnetic radiation is born.