Electromagnetic Spectrum

A changing electric field produces a changing magnetic field and vice versa which gives rise to a transverse wave known as electromagnetic waves or spectrum. The time-varying electric field and magnetic field mutually perpendicular to each other also perpendicular to the direction of propagation. Thus electromagnetic waves consist of sinusoidal time-varying electric and magnetic fields acting at right angles to each other as well as at right angles to the direction of propagation.  

History of Electromagnetic Waves

In the year 1865, Maxwell predicted the electromagnetic waves theoretically. According to him, accelerated charge sets up a magnetic field in this neighborhood. In 1887, Hertz produced and detected electromagnetic waves of wavelength in the range 5mm to 25mm. Seven years later, J.C bose became successful in producing electromagnetic waves of wavelength in the range 5mm to 25mm. In 1896, Marconi discovered that if one of the spark gap terminals is connected to an antenna and the other terminal is earthed, the electromagnetic waves radiated could go up to several kilometers. 

Hertz Experiment

IN 1888, the hertz demonstrated the production of electromagnetic waves by oscillating charge. His experimental apparatus is shown schematically. An induction coil is connected to two spherical electrodes with a narrow gap between them. It acts as a transmitter. The coil provides short voltages surges to the sphere making one end positive and the other negative. A spark is generated between the sphere when the voltage between them reaches the breakdown voltage for air. As the air in the gap is ionized, it conducts more rapidly and the discharge between spheres becomes oscillatory. This experimental arrangement is equivalent to the LC circuit, where the inductance is that loop and the capacitance is due to spherical electrodes. These electromagnetic waves are radiated at a high frequency of about 100MHz as a result of the oscillation of free charges in the loop. Hertz was able to detect these waves using a single loop of wire with its own spark gap.

Properties of Electromagnetic Waves

1. The electric and magnetic fields satisfy the following wave equations, which can be obtained from Maxwell’s third and fourth equations.

2. Electromagnetic waves travel through a vacuum with the speed of light c.

3. The electric and magnetic field of electromagnetic waves is perpendicular to each other and also the direction of propagation of waves. 

4. The instantaneous magnitude of both electric and magnetic fields in an electromagnetic wave are related by the expression.

5. Electromagnetic waves carry energy. The rate of flow of energy crossing a unit area is described by the pointing vector.

6. Electromagnetic waves carry momentum and hence can exert pressure on a surface which is known as radiation pressure. 

7. The electric and magnetic field of a sinusoidal plane electromagnetic wave propagating in the positive x-direction.

8. Fundamental sources of electromagnetic waves are accelerating electric charges. For example, radio waves emitted by an antenna arise from the continuous oscillation of charges within the antenna structure. 

9. Electromagnetic waves obey the principle of superposition.

10. The electric vector of an electromagnetic field is responsible for all optical effects.

Various Parts of the Electromagnetic Spectrum

Different Electromagnetic are discussed below:

1. Gamma rays

This ray was discovered by Henry Becquerel and Marie Curie. They mainly originate due to the decay of radioactive nuclei. Their wavelength ranges from (10)-14m to (10)-10m. 

Properties of Gamma rays

They have a high penetrating power.

They are uncharged.

They have low ionizing power.

Application

Gives information on nuclear structure.

Help in medical treatment.

2. X-rays

This ray was discovered by Roentgen. They mainly originate due to collisions of high energy electrons with heavy targets. 

Properties of X-rays

They have a low penetrating power.

Their other properties are similar to gamma rays except for their wavelength.

Application

Helps in medical diagnosis and treatment.

Helps in the study of the crystal structure.

Useful in industrial radiography.

3. Ultraviolet rays

This ray was discovered by Ritter. They are mainly originated by ionized gases, sum lamp spark, etc. 

Properties of Ultraviolet rays

They are present in all properties of light.

Present in the photoelectric effect.

Application

Helps in the detection of adulteration, writing, and signature.

Helpful in the sterilization of water due to its destructive action of bacteria.

4. Visible light

Visible lights have different subparts, they are violet, blue, green, yellow, orange, red, etc. It was discovered by Newton. They originate from outer orbit electron transition in atoms, gas discharge tubes, incandescent solids, and liquid. 

Properties

Sensitive to the human eye.

Application

Helpful in seeing objects.

Helpful in studying molecular structure.

5. Infra-red Waves

This ray was discovered by William Herschel. Change of molecular vibrational and rotational energies lead to their formation. 

Properties

They have thermal effects also.

All properties are similar to those of light except wavelength.

Application

Used in industry, medicine, and astronomy.

Used from fog or haze photography.

Elucidating molecular structure. 

6. Microwave

This ray was discovered by Hertz. It originated from special electronic devices such as the Klystron tube. 

Properties

Seen in the reflection, refraction, and diffraction process.

Application

Present in radar and telecommunication.

Analysis of fine details of molecular structure.

7. Radiowave 

Radio Waves have different subparts like super-high frequency, Ultra high frequency, Very high frequency. It was discovered by Marconi. It originated from oscillating circuits. 

Properties of Radio Wave

They are present in radar, radio, and satellite communication.

Application

Useful in marine and navigation.

Useful in long-range and long-distance communication.