Classical Electromagnetic Theory
The fundamental idea in electromagnetic theory is that an electromagnetic wave exists when a changing electric field creates a changing magnetic field, which in turn creates another changing electric field, and so on. The concept of a field is very important in physics and is utilized to explain forces that occur in nature in the absence of any physical contact.
Each type of force( electric, magnetic, or gravitational) has its appropriate field, that is a region through which a force may be exerted. A field is thus a kind of medium for transmitting forces through space without any physical medium.
Example: The gravitational, electric and magnetic fields transmit the force that primitively, a field may be considered a continuous entity like fluid that fills the space surrounding the origin of the force.
Electric and magnetic fields of an electromagnetic wave are perpendicular to each other as below figure:
Thus electromagnetic waves are transverse waves. Waves in which electric and magnetic fields are parallel to a pair of perpendicular axes are denoted as linearly polarized. Maxwell calculated the propagation speed of electromagnetic waves and found it to be equal to the speed of light (c). He concluded that light itself is an electromagnetic wave.
Waves have certain characteristics like wavelength, frequency and energy, which define the nature of a particular wave. A wavelength (λ) is the distance between two adjacent wave crests and is often expressed in centimetres. The frequency f represents the number of wave crests that pass a given point in unit time.
It is usually expressed in hertz (cycles/second). These two quantities are related to the speed of light c by the equation:
c = f.λ
Note: The relationship between frequency f and wavelength λ from the above expression:
f = c/λ
∴ λ = c/f
Thus, the frequency is inversely proportional to the wavelength. The longer the wavelength, the lower the frequency and vice-versa. Electromagnetic wave spans an enormous range of wavelengths and frequencies. This range is known as the electromagnetic spectrum.