Nature of Light:
What is light? This question has been considered by philosophers and scientists ever since the study of light started. Some of the important theories concerning the nature of light and presented from time to time have been briefly described below:
(1) Corpuscular Theory- The corpuscular theory of light also known as the particle theory of light was formulated by Newton in 1675. According to this theory, a source of light emits small particles called corpuscles, which fly off into space in all directions at very high speeds. When these particles strike the retina of the eye, a sensation of vision is produced.
In order to explain the colors of light, Newton assumed that corpuscles have different sizes. Rectilinear propagation of light was easily explained since these particles were supposed to travel at such high speeds that the earth’s gravity would hardly cause any measurable deviation. Reflection was explained by assuming that a force of repulsion acts on the corpuscles due to the reflecting boundary. However, in order to explain the phenomena of refraction, the boundary of separation was assumed to attract these corpuscles.
Newton’s corpuscular theory met great difficulty in explaining the simultaneous reflection and refraction of light. It failed to explain the phenomenon of interference and diffraction later discovered at the beginning of the 19th century. Although Newton could explain the phenomenon of refraction of light, an important consequence of the theory that light should travel faster in a denser medium proved to be wrong when the results of Focault’s measurement of the speed of light in water were known.
(2) Wave Theory- In 1678, Newton’s contemporary, the Dutch physicist Huygens suggested that light consists of periodic disturbances transmitted through the medium in the form of waves. Huygens assumed that the whole universe with all matter and space is permeated with a luminiferous ether. A source of light was supposed to create disturbance in the ether which travelled as a wave through the ether. Huygens supposed that light waves were longitudinal.
Huygens’s theory could explain the reflection and refraction of light. However, Huygens could not explain the phenomenon of polarization discovered by himself nor could he explain the rectilinear propagation of light.
The wave theory of light was, therefore, not readily accepted. A major step in favor of this theory was taken in 1801 when Thomas Young demonstrated an interference pattern between two light beams and measured the wavelength of light. From such experiments, it was found that light waves have very small wavelengths. This fact was later utilized by Fresnel to explain the rectilinear propagation of light and diffraction effects. Fresnel made an important modification to the Huygens’ theory. He declared that the light waves are transverse in nature and ether behaves like an elastic solid.
The wave theory of light could explain successfully the rectilinear propagation, reflection, refraction, interference, diffraction, and polarization effects of light. According to this theory, light should travel slower in a denser medium- this result was just opposite to the result obtained by the corpuscular theory. The results of Focault’s experiment on the velocity of light in water confirmed the prediction of the wave theory.
(3) Electromagnetic Theory- An electric charge in motion produces a magnetic field and a time-changing magnetic field produces an electric field. This strange relationship between electric and magnetic fields was studied by Clark Maxwell who arrived at important results in 1873.
According to Maxwell, an oscillating current flowing in a circuit produces oscillating electric and magnetic fields in the space surrounding the circuit. Thus if an oscillating current is flowing in a circuit kept at A, then at any point B in space an oscillating electric as well as magnetic fields are produced. The electric and magnetic fields are perpendicular to the line AB and also perpendicular to one another. The frequency of oscillations at B is the same as that at A, but they are not in the same phase showing that oscillations do not jump but travel to point B in the form of waves. The oscillating electric and magnetic fields propagating in space are called electromagnetic waves.
Maxwell calculates the speed of electromagnetic waves from theoretical considerations. It was found equal to the speed of light in a vacuum. Maxwell, therefore, had a strong reason to believe that light is an electromagnetic phenomenon. In 1887 Hertz using electromagnetic waves showed that they possessed all the properties of light waves except that light waves are too small as compared to the waves used by Hertz in his experiments. Since the electromagnetic theory of light did not contradict any of the results of the wave theory it was supposed to be an extension of the wave theory- with the advantage that it covers all the waves which can travel in a vacuum- from the shortest gamma rays t the longest radio waves.
(4) Quantum Theory- The electromagnetic theory failed to account for several phenomena like black body radiation, the origin of the spectrum, the photoelectric effect, etc. All these phenomena are related to the emission and absorption of light.
In 1900, Max Planck while explaining the laws of black body radiation suggested that radiant energy may be given out or taken in not continuously but in small packets. A packet of energy is called a quantum (plural quanta). The amount of energy in one quantum is given by hν, where h is a constant known as Planck’s constant and ν is frequency. The quantum of electromagnetic radiation is now called a photon. Considering light as the streams of photons instead of waves, Einstein in 1905 explained the photoelectric effect. The quantum ideas were further applied by Niels Bohr in 1913 to give the well-known model of the atom known as the Bohr model.