NATURE OF ELECTROMAGNETIC RADIATIONS
WAVE THEORY OF RADIATIONS
Light is an important form of energy. According to Maxwell’s wave theory, light is transmitted in the form of electromagnetic waves. These waves are associated with oscillating electric field and magnetic field. These two fields are perpendicular to each other and also to the direction of propagation as shown in Fig. 5.13.
Fig. 5.13. Electromagnetic wave.
In order to characterize electromagnetic waves following parameters are used:
1. Wavelength. It may be defined as the distance between two neighbouring crests or troughs of the wave as shown in Fig. 5.14.1t is represented by Greek letter’)… (lambda) and is generally measured in Angstrom units (A) or nanometers (nm).
Fig 5.14. Propagatior of wave motion
2. Frequency. It may be defined as the number of waves which pass through a particular point in one second. It is represented by Greek letter v (nu). Its units are:
Cycles per second (cps) or Hertz (Hz).
1 cps= 1 Hz
3. Velocity. The distance travelled by a wave in one second is called velocity of the wave. It is denoted by letter c.
The frequency (v) and wavelength (A.) are related to velocity
(c) by the following relation:
Velocity of all electromagnetic radiations in space or in vacuum is same and is equal to 3 x 108 m/sec.
4. Wave number. It may be defined as the number of wavelengths per unit length. It is equal to the inverse of wavelength. It is denoted by v . Its unit is the reciprocal of wavelength unit. It is generally expressed as cm-1 or m-1.
v = 1/λ =v /c
5.Amplitude. It is the height of a crest or depth of a trough of a wave. It is generally expressed by the letter ‘a’. The amplitude of a wave determines the intensity of radiation.
The different electromagnetic radiations have different wavelengths. The visible light in the presence of which our eye can see contains radiations having wavelengths between 400 nm-750 nm. Different colours in the visible light correspond to radiations of different wavelengths. In addition to visible light there are so many other electromagnetic radiations, such as X-rays, ultraviolet rays, infrared rays, microwaves and radiowaves.
Arrangement of all the electromagnetic radiations in the increasing order of their wavelengths or decreasing order of their frequencies is called electromagnetic spectrum.
Different regions of electromagnetic spectrum are identified by different names. Complete electromagnetic spectrum is shown in Fig. 5.15.
The various types of electromagnetic radiations have different energies and are being used for different purposes as listed in Table 5.2.
Table 5.2. Some Applications of Electromagnetic Waves
Example 5.6 Yellow light emitted from a sodium lamp has a wavelength ( J..) of 580 nm. Calculate frequency (v) and wave number (v) of the yellow light.
Solution. Wavelength of the radiation λ = 580 nm
= 580 x 10-9 m =5.80 x 10-7 m
Velocity of radiation , c = 3 x 108 ms-1
c = vλ
Frequency v = c /λ = 3 x108 ms-1 / 5.80 x 10-7m
= 5.17 x 1014 s-1
Wave number v = 1 / λ =5.80 x 10-7 m
= 1.72 x 106 m-1
Example 5.7. The wave number of a beam of light is 400 cm What is the wavelength of the light in nanometers? Also find out frequency of the light.
Solution. Calculation of wavelength
v = 400 cm-1
v =1 /λ
λ = 1/v = 1/400 cm = 2.5 x 10-3 cm
= 2.5 x 104 nm.
Calculation of frequency
c = vλ
v = c /λ = cv = 3 x 1010 cm s-1 x 400 cm-1
= 1.2 x 1013 s-1
LIMITATIONS OF WAVE THEORY OF RADIATION
Though the wave theory of radiation could successfully explain the phenomena of interference and diffraction, it failed to explain the following phenomena:
(i) Black-body radiation i.e., emission of radiation from hot bodies.
(ii) Photoelectric effect i.e., ejection of electrons from metal surface when radiation of suitable frequency strikes it.