According to the corpuscular theory of light, set forward by Sir Isaac Newton, light is made up of small discrete particles called "corpuscles" (little particles). In its contemporary incarnation, the theory of Photons, this idea explains many properties of light, in particular the photoelectric effect. However, it fails to explain other effects, such as interference and diffraction. It was therefore superseded by the wave theory of light, later understood as part of electromagnetism, and eventually supplanted by modern quantum mechanics and the wave–particle duality.
WAVE THEORY
The Huygens–Fresnel principle (named for Dutch physicist Christiaan Huygens, and French physicist Augustin-Jean Fresnel) is a method of analysis applied to problems of wave propagation (both in the far field limit and in near field diffraction). It recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed. This view of wave propagation helps better understand a variety of wave phenomena, such as diffraction.
For example, if two rooms are connected by an open doorway and a sound is produced in a remote corner of one of them, a person in the other room will hear the sound as if it originated at the doorway. As far as the second room is concerned, the vibrating air in the doorway is the source of the sound. The same is true of light passing the edge of an obstacle, but this is not as easily observed because of the short wavelength of visible light.
Huygens principle mathematically follows from the fundamental postulate of quantum electrodynamics – that wavefunctions of every object propagate over any and all allowed (unobstructed) paths from the source to the given point. It is then the result of interference (addition) of all path integrals that defines the amplitude and phase of the wavefunction of the object at this given point, and thus defines the probability of finding the object (say, a photon) at this point. Not only light quanta (photons), but electrons, neutrons, protons, atoms, molecules, and all other objects obey this simple principle.
PROPERTIES OF LIGHTReflection
is the bouncing of light when it is "reflected" from a surface:
- Specular when in a smooth surface
- Diffused reflection if in a rough surface
is the bending of light from one medium to another
Diffraction
Diffraction refers to various phenomena associated with wave propagation, such as the bending, spreading and interference of waves passing by an object or aperture that disrupts the wave. It occurs with any type of wave, including sound waves, water waves, electromagnetic waves such as visible light, x-rays and radio waves. Diffraction also occurs with matter – according to the principles of quantum mechanics, any physical object has wave-like properties. While diffraction always occurs, its effects are generally most noticeable for waves where the wavelength is on the order of the feature size of the diffracting objects or apertures.
Rectilinear Propagation
Rectilinear propagation is a wave property which states that waves propagate (move or spread out) in straight lines. This property applies to both transverse and longitudinal waves. Even though a wave front may be bent (the waves created by a rock hitting a pond) the individual waves are moving in straight lines.
Interference
As most commonly used, the term interference usually refers to the interaction of waves which are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.
Einstein: light quanta
Albert Einstein's mathematical description in 1905 of how the photoelectric effect was caused by absorption of quanta of light (now called photons), was in the paper named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". This paper proposed the simple description of "light quanta," or photons, and showed how they explained such phenomena as the photoelectric effect. His simple explanation in terms of absorption of single quanta of light explained the features of the phenomenon and the characteristic frequency. Einstein's explanation of the photoelectric effect won him the Nobel Prize (in Physics) of 1921.
The idea of light quanta began with Max Planck's published law of black-body radiation ("On the Law of Distribution of Energy in the Normal Spectrum". Annalen der Physik 4 (1901)) by assuming that Hertzian oscillators could only exist at energies E proportional to the frequency f of the oscillator by E = hf, where h is Planck's constant. By assuming that light actually consisted of discrete energy packets, Einstein wrote an equation for the photoelectric effect that fit experiments (it explained why the energy of the photoelectrons was dependent only on the frequency of the incident light and not on its intensity: a low intensity, high frequency source could supply a few high energy photons, whereas a high intensity, low frequency source would supply no photons of sufficient individual energy to dislodge any electrons). This was an enormous theoretical leap but the reality of the light quanta was strongly resisted. The idea of light quanta contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations for electromagnetic behavior and more generally, the assumption of infinite divisibility of energy in physical systems. Even after experiments showed that Einstein's equations for the photoelectric effect were accurate resistance to the idea of photons continued, since it appeared to contradict Maxwell's equations, which were well understood and verified.
Einstein's work predicted that the energy of the ejected electrons increases linearly with the frequency of the light. Perhaps surprisingly, that had not yet been tested. In 1905 it was known that the energy of the photoelectrons increased with increasing frequency of incident light -- and independent of the intensity of the light. However, the manner of the increase was not experimentally determined to be linear until 1915 when Robert Andrews Millikan showed that Einstein was correct.[7]
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