Electromagnetic Radiations.

(1)     Light and other forms of radiant energy propagate without any medium in the space in the form of waves are known as electromagnetic radiations. These waves can be produced by a charged body moving in a magnetic field or a magnet in a electric field. e.g. α−rays, γ−rays, cosmic rays, ordinary light rays etc.

(2)     Characteristics : (i) All electromagnetic radiations travel with the velocity of light. (ii) These consist of electric and magnetic fields components that oscillate in directions perpendicular to each other and perpendicular to the direction in which the wave is travelling.

(3)     A wave is always characterized by the following five characteristics: 

(i)     Wavelength : The distance between two nearest crests or nearest troughs is called the wavelength. It is denoted by l (lambda) and is measured is terms of centimeter (cm), angstrom(Å), micron(μ) or nanometre (nm).

                                     

(ii)    Frequency : It is defined as the number of waves which pass through a point in one second. It is denoted by the symbol v (nu) and is expressed in terms of cycles (or waves) per second (cps) or hertz (Hz).

λv = distance travelled in one second = velocity = c

(iii)   Velocity : It is defined as the distance covered in one second by the wave. It is denoted by the letter ‘c’. All electromagnetic waves travel with the same velocity, i.e., 3 × 10¹⁰ cm/sec.

c = λv = 3 × 10¹⁰ cm/sec

Thus, a wave of higher frequency has a shorter wavelength while a wave of lower frequency has a longer wavelength.

(iv)   Wave number : This is the reciprocal of wavelength, i.e., the number of wavelengths per centimetre. It is denoted by the symbol (nu bar).  It is expressed in cm⁻¹ or m⁻¹.

(v)    Amplitude : It is defined as the height of the crest or depth of the trough of a wave. It is denoted by the letter ‘A’. It determines the intensity of the radiation.

The arrangement of various types of electromagnetic radiations in the order of their increasing or decreasing wavelengths or frequencies is known as electromagnetic spectrum.

 

Name	Wavelength (Å)	Frequency (Hz)	Source
Radio wave	3 × 1014 – 3 × 107	1 × 105 – 1 × 109	Alternating current of high frequency
Microwave	3 × 107 – 6 × 106	1 × 109 – 5 × 1011	Klystron tube
Infrared (IR)	6 × 106 − 7600	5 × 1011 −  3.95 × 1016	Incandescent objects
Visible	7600 − 3800	3.95 × 1016 – 7.9 × 1014	Electric bulbs, sun rays
Ultraviolet (UV)	3800 − 150	7.9 × 1014 – 2 × 1016	Sun rays, arc lamps with mercury vapours
X-Rays	150 – 0.1	2 × 1016 – 3 × 1019	Cathode rays striking metal plate
λ−Rays	0.1 – 0.01	3 × 1019 – 3 × 1020	Secondary effect of radioactive decay
Cosmic Rays	0.01- zero	3 × 1020 − infinity	Outer space

 

Atomic spectrum – Hydrogen spectrum.

Atomic spectrum

(1)     Spectrum is the impression produced on a photographic film when the radiation (s) of particular wavelength (s) is (are) analysed through a prism or diffraction grating. It is of two types, emission and absorption.

(2)     Emission spectrum : A substance gets excited on heating at a very high temperature or by giving energy and radiations are emitted. These radiations when analysed with the help of spectroscope, spectral lines are obtained. A substance may be excited, by heating at a higher temperature, by passing electric current at a very low pressure in a discharge tube filled with gas and passing electric current into metallic filament.       Emission spectra is of two types,

(i)     Continuous spectrum : When sunlight is passed through a prism, it gets dispersed into continuous bands of different colours. If the light of an incandescent object resolved through prism or spectroscope, it also gives continuous spectrum of colours.

(ii)    Line spectrum : If the radiations obtained by the excitation of a substance are analysed with help of a spectroscope a series of thin bright lines of specific colours are obtained. There is dark space in between two consecutive lines. This type of spectrum is called line spectrum or atomic spectrum.

(3)     Absorption spectrum : When the white light of an incandescent substance is passed through any substance, this substance absorbs the radiations of certain wavelength from the white light. On analysing the transmitted light we obtain a spectrum in which dark lines of specific wavelengths are observed. These lines constitute the absorption spectrum. The wavelength of the dark lines correspond to the wavelength of light absorbed.

 

Hydrogen spectrum

(1)     Hydrogen spectrum is an example of line emission spectrum or atomic emission spectrum.

(2)     When an electric discharge is passed through hydrogen gas at low pressure, a bluish light is emitted.

(3)     This light shows discontinuous line spectrum of several isolated sharp lines through prism.

(4)     All these lines of H-spectrum have Lyman, Balmer, Paschen, Bracket, Pound and Humphrey series. These spectral series were named by the name of scientist discovered them.

(5)     To evaluate wavelength of various H-lines Ritz introduced the following expression,

Where R is universal constant known as Rydberg’s constant its value is 109, 678 cm⁻¹.

 

Planck’s Quantum theory and Photoelectric effect

Planck’s Quantum theory

(1)    Max Planck (1900) to explain the phenomena of ‘Black body radiation’ and ‘Photoelectric effect’ gave quantum theory. This theory extended by Einstein (1905).

(2)     If the substance being heated is a black body (which is a perfect absorber and perfect radiator of energy) the radiation emitted is called black body radiation.

(3)     Main points

(i)     The radiant energy which is emitted or absorbed by the black body is not continuous but discontinuous in the form of small discrete packets of energy, each such packet of energy is called a ‘quantum‘. In case of light, the quantum of energy is called a ‘photon‘.

(ii)    The energy of each quantum is directly proportional to the frequency (v) of the radiation, i.e.

where, h = Planck’s constant = 6.62 × 10^–27 erg. sec. or 6.62 × 10⁻³⁴  Joules sec.

(iii)   The total amount of energy emitted or absorbed by a body will be some whole number quanta. Hence E = nhv where n is an integer.

(iv)   The greater the frequency (i.e. shorter the wavelength) the greater is the energy of the radiation.

thus,

(v)    Also E = E₁ + E₂ hence, 

Photoelectric effect

(1)     When radiations with certain minimum frequency (v_o) strike the surface of a metal, the electrons are ejected from the surface of the metal. This phenomenon is called photoelectric effect and the electrons emitted are called photo-electrons. The current constituted by photoelectrons is known as photoelectric current.

(2)     The electrons are ejected only if the radiation striking the surface of the metal has at least a minimum frequency (v_o) called Threshold frequency. The minimum potential at which the plate photoelectric current becomes zero is called stopping potential.

(3)     The velocity or kinetic energy of the electron ejected depend upon the frequency of the incident radiation and is independent of its intensity.

(4)     The number of photoelectrons ejected is proportional to the intensity of incident radiation.

(5)     Einstein’s photoelectric effect equation: According to Einstein, Maximum kinetic energy of the ejected electron = absorbed energy – threshold energy

where, v_o and l_o are threshold frequency and threshold wavelength.

Note :    Nearly all metals emit photoelectrons when exposed to u.v. light. But alkali metals like lithium, sodium, potassium, rubidium and caesium emit photoelectrons even when exposed to visible light.

• Caesium (Cs) with lowest ionisation energy among alkali metals is used in photoelectric cell.