#### Nature of Light, Physics tutorial

The Corpuscular Model:

You should have read in your school physics course that corpuscular model is because of the reason of Newton. In contrast to this popular belief, the credit must be given to Descartes; however the earliest speculations regarding light are attributed to the Pythagoras.

The speed of propagation of light has been evaluated through a variety of means. The earliest measurement made through Roemer in the year 1676, who made use of observations of the motion of the moons of Jupiter and apparent variations in the periods of their orbits resultant from the finite speed of propagation of light from Jupiter to earth. The initial fully terrestrial measurement of the speed of light was made via Fizeau in the year 1849.

The corpuscular model is possibly the simplest of the models of light. According to it, light comprises of minute invisible stream of particles termed as corpuscles.  A luminous body sends corpuscles out in all the directions. Such particles travel devoid of being influenced by gravitation of earth. Newton highlighted that corpuscle of various sizes stimulate sensation of various colors at the retina of our eye.

The two most significant experimental evidences are as follows:

1) The light travels in straight lines. This rectilinear propagation of light is accountable for the formation of sharp (that is, completely dark) shadows. Whenever we illuminate a barrier in front of the white screen, the area of screen behind the barrier is fully dark and the area outside the barrier is totally lit. This recommends that light doesn't go around corners.

2) Light can propagate via vacuum, that is, light doesn't need any material medium, as does sound, for propagation. We can as well forecast the correct form of the laws of reflection and refraction by using the corpuscular model. Though, a serious flaw in this theory is encountered in respect of the speed of light. Corpuscular model forecasts that light travels faster in a denser medium. This, as you now be familiar with, contradicts the experimental findings of Fizeau. Do you anticipate the speed of light to base on the nature of the source or the medium in which the light propagate? Apparently, this is a property of the medium. This signifies that the speed of light consists of a definite value for each and every medium.

The other severe flaw in the corpuscular model came in the form of experimental inspections such as interference (that is, re-distribution of energy in the form of dark and bright or colored fringes), diffraction (that is, bending around sharp edges) and polarization.

The Wave Model:

The initial systematic theory of light was put forward through a contemporary of Newton, Christian Huygens. By employing the wave model, Huygens was capable to describe the laws of reflection and refraction.  Though, the authority and eminence of Newton was so great that no one reposed faith in the Huygens' proposition. However, wave model was renewed and shaped through Young via his interference experiments.

Young exhibited that the wavelength of visible light lies in the range of 4000 Å to 7000 Å (general values of wavelength for sound range from 15 cm for a high-pitched whistle to 3 m for a deep male voice.) This describes why the wave character of light goes unobserved (that is, on a human scale).

Interference fringes can be observed only if the spacing among the two light sources is of the order of the wavelength of light. That is as well why diffraction effects are small and light is stated to approximately travel in the straight lines. (A ray is stated as the path of energy propagation in the limit of λ → 0). A satisfactory description of diffraction of light was given by Fresnel on the basis of the wave model. A significant portion in establishing wave model was played through polarization that is, a subtle property of light. This established that light is a transverse wave; the oscillations are perpendicular to the path of propagation. However what is it that oscillates?

The answer was given by the Maxwell who presented the real physical significance and sound pedestal to the wave theory. Maxwell recognized light by electromagnetic waves. A light wave is related by changing the electric and magnetic fields.

Light as an Electromagnetic Wave:

A varying electric field gives increase to a time and space varying magnetic field and vice-versa. This interaction of coupled electric and magnetic field's outcomes in the propagation of three-dimensional electromagnetic waves. To represent this, we initially recall the Maxwell's field equations:

∇.D = ρ

∇.B = 0

∇ x E + ∂B/∂t = 0

∇ x H = J + ∂B/∂t

Here, 'ρ' and 'J' symbolize the free charge density and the conduction current density, correspondingly. E, D, B, and H correspondingly stand for the electric field, electric displacement, magnetic induction and the magnetic field. These are joined via the given constitutive relations:

D = ε E

B = μ H

J = σ E

Here, 'ε', 'µ' and 'σ' correspondingly represent the dielectric permittivity, magnetic permeability and the electrical conductivity of the medium.

For ease, we assume that the field equations in vacuum so that ρ = 0 and J = 0. Then, if we employ connecting relations written above, reduce to:

∇.E = 0

∇.H = 0

∇ x E - μo ∂H/∂t

∇ x H = εo ∂E/∂t

Energy Transfer: The Poynting Vector

A general trait or feature of wave motion is: Wave carries energy, not matter. Is it true even for the electromagnetic waves? To identify the answer, you must again assume the two field vectors (E and H) and compute the divergence of their cross product. You can state it as:

∇. (E x H) = H. (∇ x E) - E. (∇ x H)

As, ∇. (A x B) = B. (∇ x A) - A. (∇ x B)

The Electromagnetic Spectrum:

In a little while after Hertz illustrated the existence of electromagnetic waves in the year 1888, intense interest and activity got produced. In the year 1895, J.C. Bose, working at Calcutta, India generated electromagnetic waves of wavelengths in the range 25 mm to 5 m. (In the year 1901, Marconi succeeded in transmitting the electromagnetic waves across the Atlantic Ocean. This made public sensation. However, this pioneering work marked the starting of the era of communication by utilizing electromagnetic waves.) X-rays, discovered in the year 1898 by Roentgen, were represented in the year 1906 to be E.M. waves of wavelength much smaller than the wavelength of light waves.

The knowledge of E.M. waves of different wavelengths has grown-up constantly since then.

The range of wavelengths (and their applications in the modern technologies) is extremely broad. Though, the boundaries of different areas are not sharply stated. The visible light is imprisoned to a very limited part of the spectrum from around 4000 oA to 7000 oA. As we are familiar that, different wavelengths corresponds to various colors. The red is at the long wavelength-end of visible region and the violet at the short wavelength-end. For centuries our mere information concerning the universe beyond earth has come from the visible light. All the electromagnetic waves from 1 m to 106 m are termed to as radio waves. These are employed in the transmission of radio and television signals. The ordinary AM radio corresponds to waves having λ = 100 m, while FM radio corresponds to 1m. The microwaves are employed for radar and satellite communications (λ ~ 0.5m - 10-3 m).

As radio waves and visible light lies in the infrared area or region. Beyond the visible region we encounter the ultraviolet rays, X-rays and gamma rays. You should persuade yourself that all the phenomena from radio waves to gamma rays are basically the same; they are all electromagnetic waves that differ simply in wavelength (or frequency). You might now be persuading to enquire: Why do we aspect various nomenclature to different parts of the electromagnetic spectrum? The distinction is a mere convenience whereas recognizing their practical applications.

In our solar system, the sun is the main source of E.M. waves. If you closely inspect the solar spectrum received on the earth, you will view wide continuous spectrum crossed through Fraunhofer dark absorption lines.

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