1.2 Characteristics of Electro-Magnetic Radiation

1.2 Characteristics of Electro-Magnetic Radiation

Electro-magnetic radiation is a carrier of electro-magnetic energy by transmitting the oscillation of the electro-magnetic field through space or matter. The transmission of electro-magnetic radiation is derived from the Maxwell equations. Electro-magnetic radiation has the characteristics of both wave motion and particle motion.

(1) Characteristics as wave motion
Electro-magnetic radiation can be considered as a transverse wave with an electric field and a magnetic field. A plane wave for an example as shown in Figure 1.2.1 has its electric field and magnetic field in the perpendicular plane to the transmission direction. The two fields are located at right angles to each other. The wavelength , frequency and the velocity have the following relation.

Electro-magnetic radiation is transmitted in a vacuum of free space with the velocity of light c, ( = 2.998 x 108 m/sec) and in the atmosphere with a reduced but similar velocity to that in a vacuum. The frequency n is expressed as a unit of hertz (Hz), that is the number of waves which are transmitted in a second.

(2) Characteristics as particle motion
Electro-magnetic can be treated as a photon or a light quantum. The energy E is expressed as follow.
E = h
where h : Plank's constant
: frequency

The photoelectric effect can be explained by considering the electro-magnetic radiation as composed of particles. Electro-magnetic radiation has four elements of frequency (or wavelength), transmission direction, amplitude and plane of polarization. The amplitude is the magnitude of oscillating electric field. The square of the amplitude is proportional to the energy transmitted by electro-magnetic radiation. The energy radiated from an object is called radiant energy. A plane including electric field is called a plane of polarization. When the plane of polarization forms a uniform plane, it is called linear polarization.

The four elements of electro-magnetic radiation are related to different information content as shown in Figure 1.2.2. Frequency (or wavelength) corresponds to the color of an object in the visible region which is given by a unique characteristic curve relating the wavelength and the radiant energy. In the microwave region, information about objects is obtained using the Doppler shift effect in frequency, that is generated by a relative motion between an object and a platform. The spatial location and shape of objects are given by the linearity of the transmission direction, as well as by the amplitude. The plane of polarization is influenced by the geometric shape of objects in the case of reflection or scattering in the microwave region. In the case of radar, horizontal polarization and vertical polarization have different responses on a radar image.