Radiation is the form of energy, which can be transferred from one point to another point in space. Radiations are considered to be transmitted by wave motion. A wave consist of crests and troughs.
The distance between two consecutive troughs or consecutive crests is called as the wavelength (λ). its S.I. unit is m. and practical unit is angstrom (Å).
The number of waves passing through any point in unit time (1 sec) is known as frequency (υ). It is Expressed in cycles per second (cps) or Hz.
Distance travelled by the wave in one second is called the velocity of the wave. denoted by ‘c’. Expressed in m/sec.
Electromagnetic Nature of Radiations:
Electromagnetic nature of radiations is explained by James Maxwell (1870). He suggested that when electrically charged particles move with an acceleration alternating electrical and magnetic fields are produced and transmitted. These fields are transmitted in the forms of waves called electromagnetic waves or electromagnetic radiation.
The oscillating electric and magnetic fields produced by oscillating charged particles are perpendicular to each other and both are perpendicular to the direction of propagation of the wave.
The arrangement of different electromagnetic radiations in order of increasing wavelength is called electromagnetic spectrum.
Unlike sound waves or water waves, electromagnetic waves do not require medium and can move in vacuum.
Particle Nature of Light or Planck’s Quantum Theory:
Quantum theory was given by Max Planck in 1900. Its important postulates are
Absorption and emission of radiant energy does not takes place continuously but it takes place in the form of packets of energy called quanta. Quantum of light is called a photon.
Each quanta has definite amount of energy which depends upon frequency of radiation. The relation is E = hυ, Where E is energy of photon, h is Planck’s constant and υis the frequency of the radiation.
Quantum is the smallest denomination of energy. The quantum of energy is always an integer.
Energy less than quantum can never be absorbed or emitted. It can be emitted as whole number multiple of quantum I.e. 1hυ, 2hυ, 3hυetc