Electromagnetic radiation in the field of physics refers to the waves of the electromagnetic field. These waves propagate through space and carry electromagnetic radiant energy. These waves include different types of waves. Some are microwaves, radio waves, visible light, X-rays, ultraviolet rays, and gamma rays.

The electromagnetic waves are oscillations which are synchronized with perpendicular electric and magnetic fields. The velocity of the electromagnetic waves is equal to the speed of light c. In media that are isotropic and homogenous, the propagation of a wave is perpendicular to the fields. These fields are perpendicular to each other like a transverse wave.

If a point source emits electromagnetic waves, its wavefront appears to be a sphere. The electromagnetic spectrum characterizes the electromagnetic waves with respect to their frequency of oscillation and sometimes wavelength. There are different names for the electromagnetic waves of varying frequencies depending upon their sources and effects on the surroundings.

Electromagnetism is a physical phenomenon which elaborates the theory of electrodynamics. The principle of superposition is followed by the electric and magnetic fields. A time-varying electric or magnetic field or a field produced by any specific particle adds to the fields present in the vicinity due to other causes.

The magnetic and electric fields are vector fields, thus they follow vector addition. When two or more coherent light waves interact, they form a constructive or destructive interference pattern. This resultant irradiance deviates from the sum of the constituent irradiances of the individual overlapping light waves.

When the electromagnetic field travels through static and magnetic fields in a medium that is linear such as vacuum, they remain unaffected. In the case of non-linear media, there is an occurrence of some interaction. In media like some crystals, the light and the static magnetic or electric fields interact through Faraday effect and Kerr effect.

During refraction of a light wave, it crosses from one medium to another medium of different density. This results in the alteration of its speed and the direction of the light wave. The degree of change in the path is determined by the ratio of the refractive indices of the two media, known as Snell’s law.

Natural sunlight consists of composite wavelengths. When it passes through a prism, it disperses into a spectrum of visible light. This is due to the fact that the refractive index of the prism depends on the wavelength of the light wave passing through it. Thus the components of the sunlight bend by different amounts and form a spectrum.

### Wave Model

Electromagnetic radiation travel in forms of transverse waves in isotropic, homogenous media. Transverse wave suggests that the direction of energy transfer is perpendicular to the oscillations of the wave. The Maxwell equations specify the production of magnetic part from the electric part and vice versa. Thus, the strengths of these fields remain in a fixed ratio to satisfy Maxwell’s equations.

The electric and the magnetic fields are in the same phase in a lossless medium. This means that both reach maxima and minima at the same points. There is a common misconception that both the fields are out of phase. This is because one is produced by the other. As the fields are sinusoidal functions, this can create a phase difference between the electric and magnetic fields. This is true for near-field antennas.

For far-field electromagnetic radiation, the time-change in one type of field comes out to be proportional to the space-change in the other type of field. This fact requires both the fields to be in-phase. An electromagnetic wave of single frequency generally known as a monochromatic wave contains successive crests and troughs. The distance between two consecutive troughs or crests is the wavelength. The electromagnetic waves vary in size and thus have different wavelengths. The wavelength of an electromagnetic wave is inversely proportional to the frequency:

$$\nu =f\lambda$$

where,

$$\nu$$ denotes the frequency of the wave

$$\lambda$$ is the wavelength of the wave

f denotes the frequency of the wave

The solutions of Maxwell’s electromagnetic wave equation gives us the electromagnetic waves.  The solutions can be categorized as plane waves and spherical waves. The limiting case of spherical waves for a very long distance from the source appears to be plane waves.

### Particle Model

The black body radiation contradicted the wave model of the light. A scientist named Max Planck proposed a theory of black-body radiation in 1900. This theory explained the observed spectrum. The theory is based on the idea that the emission of light takes place in discrete packets of energy. Theses bundles of energy are quanta of energy.

The light quanta were compared with original particles by Albert Einstein in 1905. Finally, the light particle was named photon in correspondence with the other prevalent particles around the time that were electrons and protons. The energy of a photon is proportional to the frequency of the wave representing it.

E=hf=$$\frac{hc}{\lambda}$$

Where,

h denotes the Planck’s constant

f is the frequency of the wave

$$\lambda$$ is the wavelength

E denoted the energy of the photon

The photoelectric effect was the anomaly which could not be explained by the wave nature of the light. This experiment led Einstein to state that the light was composed of particles. In this particular effect, electrons are ejected from the metal surface when the light of a particular frequency strikes it. This causes an electric current to flow when a voltage is applied across.

The calculations of this experiment showed that the kinetic energy of the ejected electrons was proportional to the frequency of the light. It was independent of the intensity of the incoming light. No current was observed, when the value of the frequency is less than a particular value, regardless of the intensity of the light. All these conclusions contradict the wave theory.

Q.1.What is wave-particle duality?

Answer. The particle and wave model of the light are contradictory in nature. Some phenomena are explainable by the concept of wave nature and others by particle nature. Explicitly, the theory states that a particle exhibits the properties of both a wave and a particle. The particle having large mass is easily conceived as particles and microscopic masses exhibit wave nature predominantly. Louis de Broglie also came up with the de Broglie wavelength that led to the realization of waver-particle duality.

Q.2. How do Maxwell’s equations explain electromagnetic radiation?

Maxwell’s equations state that if a spatially varying electric field exists in an area, a magnetic field is always associated with it. This magnetic field changes over a period of time. For an electromagnetic wave, whenever the electric field changes, a wave creates in the magnetic field in a particular direction and vice versa. The existence of electric and magnetic fields in electromagnetic radiation is similar to the occurrence of time and space. Thus they are interlinked in special relativity. These fields form a propagating electromagnetic wave that moves in space.

Q. 3. What are the daily-life applications of electromagnetic radiation?

Answer. Electromagnetic radiation is an integral part as light is an electromagnetic wave. The other applications are:

• Transmission of wireless signals and energies over telephone and TV.
• Transmission of long and short AM and Fm radio waves.
• Responsible for the transmission of electromagnetic energy in the form of infrared light, ultraviolet light, gamma rays etc.
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