Electromagnetic Spectrum

Maxwell gave the idea of e.m. waves while Hertz, J.C. Bose, Marconi and others successfully produced such waves of different wavelengths experimentally. However, in all the methods, the source of e.m. waves is the accelerated charge.

Electromagnetic waves are classified according to the method of their generation and are named accordingly. Overlapping in certain parts of the spectrum by different classes of e.m waves is also observed. The e.m. waves of wavelengths in the overlapping region can be produced by two different methods.

The physical properties of e.m. waves are determined by the frequencies or wavelengths and not by the method of their generation. A suitable classification of e.m. waves is called the electromagnetic spectrum.

There is no sharp dividing point between one class of e.m. waves and the next. 

Low Frequency Radiation

v = 60 Hz to 50 Hz

λ = 5 x 106 m 10 6 x 106 m

Generated from a.c. circuits, these are classified as power frequencies or power waves or electric power utility e.m. waves. These weaves have the lowest frequency.

Radio Waves

λ = 0.3 m to 106 m

v = 109 Hz 300 Hz

Radio waves are generated when charges are accelerated through conducting wires. They are generated in such electronic devices as LC oscillators and are used extensively in radio and television communications.

Microwaves

λ = 10–3 m to 0.3 m

v = 1011 Hz 109 Hz

These are generated by oscillating currents in special vacuum tubes. Because of their short wavelengths, they are well suited for the radar system used in aircraft navigation, T.V. communication and for studying the atomic and molecular properties of matter.

Microwave ovens use these radiations as heat waves.

Infra-red Waves

λ = 7 x 10–7 m to 10–3 m

v = 4.3 x 1014 Hz to 3 x 1011 Hz

Infra-red waves, also called heat waves, are produced by hot bodies and molecules. These are readily absorbed by most materials. The temperature of the body, which absorbs these radiations, rises.

Infrared radiations have many practical and scientific applications including physical therapy, infrared photography, etc. These are detected by a thermopile.

Visible Light

λ = 4 x 10–7 m to 7 x 10–7 m

v = 7.5 x 1014 Hz to 4.3 x 1014 Hz

These are the e.m. waves that human eye can detect or to which the human retina is sensitive. It forms a very small portion of the whole electromagnetic spectrum.

These waves are produced by the rearrangement of electrons in atoms and molecules. When an electron jumps from outer orbit to inner orbit of lower energy, the balance of energy is radiated in the form of visible radiation.

The various wavelengths of visible lights are classified with colors, ranging from violet (λ = 4 × 10–7m) to red (λ = 7 × 10–7m).

Human eye is most sensitive to yellow-green light (λ = 5 × 10–7m).

Ultraviolet

λ = 3 x 10–9 m to 4 x 10–7 m

v = 10-17 Hz to 7.5 x 1014 Hz

Sun is the important source of ultraviolet radiations, which is the main cause of suntans. Most of the ultraviolet light from Sun is absorbed by atoms in the upper atmosphere (stratosphere), which contains ozone gas.

This ozone layer then radiates out the absorbed energy as heat radiations. Thus, the lethal (harmful to living beings) radiations get converted into useful heat radiations by the ozone gas, which warms the stratosphere.

The ultraviolet rays are used in killing the bacteria in drinking water, in sterilization of operation theatres and also in checking the forgery of documents.

X-Rays

λ = 4 x 10–13 m to 4 x 10–8 m

v = 7.5 x 1020 Hz to 7.5 x 1015 Hz

These are produced when high energy electrons bombard a metal target (with high melting point) such as tungsten.

X-rays find their important applications in medical diagnostics and as a treatment for certain forms of cancer. Because, they destroy living tissues, care must be taken to avoid over-exposure of body parts.

X-rays are also used in study of crystal-structure. They are detected by photographic plates.

Gamma Rays

λ = 6 x 10–17 m to 10–10 m

v = 5 x 1024 Hz to 3 x 1018 Hz

These are emitted by radioactive nuclei such as cobalt (60) and ceasium (137) and also during certain nuclear reactions in nuclear reactors.

These are highly penetrating and cause serious damage when absorbed by living tissues. Thick sheets of lead are used to shield the objects from the lethal effects of gamma rays.

Gamma rays are used to detect metal flaws in metal castings. They are detected by Geiger tube or scintillation counter.

Energy of em Waves

The energy (E) of e.m. waves is directly proportional to their frequency v (E = hv) and inversely proportional to their wavelength (λ).

Thus gamma rays are the most energetic and penetrating e.m. waves, while the power frequencies, and the A.M. radio waves are the weakest radiations.

Depending on the medium, various types of radiations in the spectrum will show different characteristic behaviors. For example, while whole of the human body is opaque to visible light, human tissues are transparent to X-rays but the bones are relatively opaque. Similarly Earth’s atmosphere behaves differently for different types of radiations.