Problem & Concept OF Electromagnetic Radiations By-N.K.D. Sir Please -Rate review and share this lesson

Wave characteristics: A wave is a type of disturbance whose origin is some vibrating source. The wave travels outwards as a continuous sequence of alternating crests and troughs. The following are the wave-characteristics I. Wave length ( ): It is the distance between two neighbouring trough or crests. Crest a=Amplitude Trough Distance It is expressed in cm, nanometers or angstrom ( ) units IA = 10"cm= 10 meter - 10 nm * The colour of a beam of visible light is determined by its wave length 2. Frequency (v) :The number of times, a wave passes through a given point in one second is known as the frequency of the wave. It is denoted by v (nu). It is expressed in cycles per second or Hertz (Hz) units 1 Hz= 1 cycle per second (cps)

3. Velocity (c) : The distance traveled by the wave in one second is known as velocity of the wave. It is expressed in cm or meter per second. 'C, is related to v (nu) and (wave length) as follows: or V(nu) All the electromagnetic radiations travel with the same velocity i.e. with the velocity of light i.e., 3 1010 cm sec. 3 108 m sec. or 186000 miles sec-1. Wave number v, (nu bar): The number of wave lengths per cm. is called wave number. It is reciprocal of wave length (x). It is expressed in cm 4. or 5. Amplitude (a) : The height of the crest or depth of the trough of a wave is called its amplitude. It determines the intensity of brightness of the beam of light. The intensity of radiation is proportional to the square of amplitude (a')

ELECTROMAGNETIC SPECTRUM: When the different types of electromagnetic radiations are arranged in the increasing or decreasing order of their wave lengths or frequencies, we get a pattern, known as electromagnetic spectrum. The following is the increasing order of wave length of decreasing order of energy of electromagnetic radiations Cosmic rays, Y-rays X-rays UV radiations -| Violet Red (visible spectrum) | Infra red Microwaves Radiowaves

Black Body Radiations When solids are heated, they emit radiations over a wide range of wavelengths. For example, when an iron rod is heated in a furnance, it first turns a dull red and then progressively becomes more and more red as the temperature increases. On heating further it emits white radiations which turns blue at very high temperature. It is thus, evident radiations emitted goes from a lower frequency (low energy) to higher frequency (high energy) as the temperature increases. The ideal body which emits and absorbs all frequencies is called a black body and the radiations emitted by this body are called black body radiations.

QUANTUM THEORY OF RADIATION: This theory was proposed by German physicist, Max Planck in 1900, to explain the phenomena of black body radiations. According to this theory_ "Energy is emitted or absorbed by a body not continuously but discontinuously in the form of small packets or bundles of energy known as Quanta. The energy of a body can increase or decrease by Quantum of light is called photon. The energy of a photon is given by the expression: E hv Where h Planck's constant, V = frequency of radiation Since v- E-h Thus, the energy of radiation is proportional to its frequency and inversely proportional to wave length.

Ultraviolet light of wavelength 800A and 700A when allowed to fall on hydrogen atoms in their ground state is found to liberate electrons with kinetic energy 1.8 eV and 4.0 eV respectively. Find the value of Planck's constant. Solution: KE, =hv,-W-h--W h.1 Substracting equation (i) from equation (ii), (KE2-KE) hc , orh (4.0eV-1.8eV) x 800 x10-10 700x1 0-10 3x10x (800-700)x10-10

10 (3x10 msx (100x10m) 2.2 x1.6x56x10 J-s -6.57x103MJ-s Calculate the energy per mole of photon of electromagnetic radiations of wavelength 4000A Solution E/ Photon c/ 6.023x102x6.626x 102 x3.0x100 4000x 10 - 2.99 x 101 erg

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