Characteristics of good tax system. (Canons of taxation)

Nature and sources of light:

Light is a form of energy which on entering our eyes gives the sensation of sight, and enables us to see various objects.

Various theories have been purposed for true nature of light. Some theories are discussed below”

1.     Newton’s Corpuscular theory (1678)

Newton assumed that light is emitted from luminous objects in the form of tiny particles known as corpuscles and travel with the speed of light. He also assumed that these particles travels in straight line and when entered in our retina cause the sensation of vision and different colors are due to the different size of corpuscles.

The phenomenon of refraction was explained by assuming that the corpuscles are attracted by the particles of denser medium and are accelerated and hence velocity of light is more in denser medium and this results change in direction.

This result is contrary to the fact that velocity of light is less in denser medium. Also other phenomena like interference, diffraction and polarization of light could not be explained on the basis of this theory. So the corpuscular theory was discarded as an incomplete and inaccurate theory.

2.     Huygens wave theory: (Christen Huygens , 1690, Dutch Physicist):

According to Huygens theory, light travels in the form of mechanical wave considering the hypothetical medium ether in the atmosphere. He thought that, light was a longitudinal wave and ether medium is massless, colorless, having high elasticity and no density.

This wave theory successfully explained many of the observed phenomena including the slower speed of light in denser medium. However, Huygens theory could not prove the presence of ether so this theory was also discarded as incomplete.

3.     Electromagnetic wave theory (Maxwell, 1860):

According to this theory light waves are electromagnetic waves with changing electric and magnetic field which are mutually perpendicular to each other and to the direction of propagation of wave. Thus light waves are transverse in nature. The electromagnetic waves can propagate through the space in the absence of any medium and thus the difficulty of ether was removed.

4.     Quantum theory (Einstein 1905)

Purposed by Einstein, in 1905, light is transmitted as tiny bundles of energy called as photons in order to explain the photoelectric effect. A photon is considered as massless bundle of electromagnetic energy. The energy E of a photon depends upon its frequency f and is given by . Where h is plancks constant. Photons exist while moving but does not have rest mass.

5.     Dual nature of light:

From the discussion of the various theory explained it is clear that light has particle nature as well as wave nature. The particle nature could describe the photoelectric effect but could not define interference, diffraction etc. which are described by wave nature. So the light has dual nature i.e. particle nature and wave nature.   is the relation which relates the wave and particle nature of light as formulated by French physicist, Louis De Broglie (1924).

When the light is emitted or absorbed it is done in quantas or particle form or packets of energy in a discontinuous way, but when light is propagated it does so in electromagnetic wave form travelling with speed 3 x 10⁸ m/s.

Electromagnetic spectrum:

The orderly distribution of electromagnetic radiations according to their wavelength or frequency is called the electromagnetic spectrum. The usual classification of electromagnetic spectrum is explained below:

1.     Radio waves: These waves are generated by electronic circuits, mainly oscillating circuits having inductor and capacitor. They are used in radio and television broadcasting system and generally have wavelength of several meter to one meter.

2.     Microwaves: These waves are produced by oscillating electrons in a cavity. The commonly used oscillators are klystron and magnetron. They have wavelength generally greater than 1 mm to 30 cm. they are used in radar communication, aircraft navigation, atomic and molecular research and also in cooking and warming food.

3.     Infra- red rays: These rays are produced by excitation of atoms and molecules also hot bodies radiate infra-red rays. They have wavelength of range 10^-3 m to 700 nm. They are used in night vision devices, green houses and also used in revealing the secret writings on the ancient wall.

4.     Visible light: Visible light is emitted when an electron jumps from higher energy level to lower energy level. The range of visible light is 400 nm to 700 nm. They are used in photography, microscopy, and astronomy and for the visibility to our sense of sight.

5.     Ultra-violet rays: (U-V rays): U-V rays are produced from sun, and also produced from the spark of wielding. They have wavelength of 400 nm to 60 nm. They are harmful for living organisms. They are used in killing bacteria in food, water, in surgical instruments and also in the study of structure of molecules.

6.     X- rays: x- rays are produced by bombarding moving electrons on target of high atomic number elements. They have wavelength of 60 nm to 0.001 nm. They can penetrate through human flesh but bones or metallic materials block these rays. They are used in medical diagnosis, radio therapy, curing cancer, also in engineering and by detectives (to detect gold, silver, diamonds etc.).

7.     Gamma rays (ϒ rays): ϒ- rays are produced during radioactive decay of nuclei and nuclear reactions. They have high penetrating power. Their wavelength varies from 0.1 nm to 10^-5 nm. They are used in treatment of cancer and tumors, to examine the thick materials for structural flaws, nuclear reactions and to preserve food for long time by soft ϒ rays.