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The Dual Nature of Light

Dual Nature of Light

Does the light behave like a particle or a wave ? The short answer is light has a dual nature – It  behaves as a wave as well as a stream of corpuscles as has been demonstrated by a number of experiment. The Young and Fresnel’s experiment on interference demonstrate the wave nature of light that can only be explained if we assume light – a wave. However, Einstein’s Photo Electric Effect and the Compton Effect establishes the particle nature of the light.

Background and Details

Way back in the 17th Century light was assumed to be a stream of corpuscles, emitted by light sources and traveled outward from the source in a straight line. To explain the fact light could penetrate glass and other transparent media, it was assumed that the corpuscles could penetrate the transparent materials and wer reflected from surfaces of opaque materials. When these enter eye, they caused the sense of light.

But the turn of the middle of the 17 th Century idea started gathering that the light might be a wave. In the year of 1670 Huygens showed that the laws of reflection and refraction could be explained if we assume light – a wave. Huygen’s theory gave simple explanation to the phenomenon of double refraction. There were immediate reaction and questions were raised as to why light will not bend around the corner if it were a wave. It was explained that the wavelength of the light is so small that the bending around the corner will not be possible. The bending of light does happen to some extent ( called diffraction ) and was noted by Grimaldi in 1663 and was interpreted by Hooke in 1665 in terms of wave picture. Its significance, however, was not recognized at that time.

Shortly after 1827, Young and Fresnel’s experiment on interference and measurement of the velocity of light in liquids by Foucault demonstrated phenomenon which could not be satisfactorily explained by corpuscles  theory and could only be explained by the wave theory. Young was able to measure the wavelength of light using his experiment. Fresnel was able to explain the rectilinear propagation by light as also the diffraction effects that were observed by Grimaldi and others.

In 1873 Maxwell, theoretically showed that and oscillating electrical circuit should radiate electromagnetic waves that will propagate with the velocity of light. This gave the suggestion that light consisted of electromagnetic waves that had very short wavelength. Hertz was able to produce short wavelength waves of electromagnetic origin and showed that they posessed all the properties of light waves including reflection, refraction and polarization. This justified the Maxwell’s electromagnetic theory of light.

The electromagnetic theory, on the other hand, failed to justify and account for the phenomenon of photoelectric emission. In photoelectric effect, the electrons eject from the surface of conductor, when light is incident on it. In 1905 Einstien adopted Plank’s quantum hypothesis and postulated that the energy in a light beam, instead of being distriuted through the space in the electric and magnetic fields of an electromagnetic wave, is concentrated in small packets, or photons. The energy of the photons is proportional to the frequency of the light. In photoelectric effect the energy of the photons transfer to electrons. Millikan conducted an experiment which concluded that the kinetic energy of electrons were in exact agreement with the Einstien’s formula.

Photon nature of light was further confirmed by Compton effect in 1921. Compton showed that the photons and electrons “collision” behave in a manner similar to the collision between two mechanical balls. The collision between photon and electrons result in the transfer of kinetic energy and momentum. The photoelectric effect and the Compton effect demanded the return of the corpuscular theory of light.

At present Physicists have no option but to accept that light is dual in nature. The phenomenon of propagation of light is explained the electromagnetic waves, while the interaction of light with electrons can only be explained when we assume light to consist of photons.

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