Electromagnetic Waves- Waves and their applications - science lessons for life

## Monday, December 12, 2016

Electromagnetic Waves
The figure shown here is a photograph of a radio telescope. The antenna of this telescope receives radio waves emitted at by very distant stars. Understanding the information contained in those waves helps us to understand more about the history of the universe. Radio waves are electromagnetic waves. Now let us consider more about electromagnetic waves.

The participation of material particles of a medium is not required for the propagation of electromagnetic waves. While electromagnetic waves consist of electric fields and magnetic fields that oscillate in directions perpendicular to each other, the wave propagates in a direction perpendicular to the directions of both the electric and magnetic fields as shown below Figure. Therefore electromagnetic waves belong the class of transverse waves.
Electric and magnetic fields of an electromagnetic wave.
All electromagnetic waves propagate with the speed of 2.988 × 10^8 ms^-1 in a vacuum (It is often taken as 3 × 10^8 ms^-1 in calculations). Speed of electromagnetic waves in material media is less than the speed in a vacuum and accordingly the wavelength also changes. For electromagnetic waves, the speed c is given by the relationship c = f Î» where f is the frequency and Î» is the wavelength.

Characteristics of electromagnetic waves

• Electromagnetic waves are not affected by external electric or magnetic fields.
• They do not require a material medium for propagation.
• They travel at a speed of 3 × 10^8 ms^-1 in a vacuum.

Electromagnetic Spectrum
The characteristics of electromagnetic waves vary significantly in various frequency ranges. Various frequency ranges identified by such characteristics are known as the electromagnetic spectrum. Main types of electromagnetic waves belonging to the electromagnetic spectrum are listed in the table below.

Electromagnetic spectrum
Applications of Electromagnetic Waves

Visible Light
Visible light is the range of the electromagnetic spectrum to which our eyes are sensitive. It is only a narrow band in the electromagnetic spectrum. The frequencies of the visible light range from 4.28 × 10^14 Hz to 7.69 × 10^14 Hz corresponding to a wavelength range from 690 nm to 400 nm. The region containing the lowest wavelength (highest frequency) in this range appears violet to the human eye.When the wavelength increases (frequency decreases) gradually the color changes to indigo, blue up to red. These are the colors that we identify as the seven colors in the rainbow.

Gamma rays
Gamma rays are a type of waves emitted by radioactive elements. Frequencies of gamma rays are extremely high and so are the energies possessed by them. Gamma rays have the ability even to penetrate thick sheets of steel as well as concrete slabs. Since gamma rays can destroy living cells they are used to destroy cancer cells.
Gamma rays are also used to sterilize utensils used for food and surgical instruments.
An instance where gamma rays are used

X - rays
X–rays are mostly used to take photographs of internal organs of the human body. Although X-rays travel quite easily through the soft tissues in the body, their intensity decreases rapidly when traveling through the bones of the body. When the X–ray generator is turned on, X–rays propagate through the relevant part of the body of the person being photographed and thus forms an image of that part of the
body. Excessive exposure to X–rays can cause cancers.
X – ray imaging
X–rays are generated by bombarding high speed electrons on metal targets. Then part of the kinetic energy of the electrons gets converted to X–rays. X–rays are also used to examine the baggage of airline passengers and cargo inside containers transported by ships, without opening them.

Ultraviolet means ‘above violet’. Violet is the color having the highest frequency out of the seven colors that form the visible spectrum. Ultraviolet radiation is a type of rays having a frequency range above that of violet and is invisible to the human eye. Although ultraviolet rays are invisible to the human eye, it has been found that insects like bees can detect ultraviolet rays. Sunlight contains a small amount of ultraviolet rays. Ultraviolet rays are also produced in electric discharge and from mercury vapor lamps. Since these rays produce vitamin D in the human body, it is useful to be exposed to sunlight to some extent. However, over exposure to ultraviolet rays can cause cataract in the eye and cancers in the skin.

An instance of generating ultraviolet rays
Ultraviolet radiation is used in hospitals to kill germs. Certain chemical substances show a glitter
when exposed to ultraviolet radiation. This phenomenon is used in places like banks to check hidden symbols in currency notes. Such chemicals are also added to some washing powders. Clothes washed with such washing powders show a brightness when exposed to sunlight.

The range of frequencies below the red color that is not visible to us is known as infrared radiation. Because infrared radiation is emitted by heated bodies and we feel a warm sensation when infrared radiation falls on our skin, infrared radiation is often referred to as heat rays.
A heat photograph
Infrared radiation is emitted by our bodies too. Heat photographs are taken with the aid of heat rays emitted by body organs. Certain diseases can be identified using such photographs.

Infrared radiation is used to send signals to television sets from remote controls. Most of the cameras in mobile telephones and computers are sensitive to infrared radiation. Infrared radiation is also used for physiotherapy treatments.
A remot control
An infrared camera
Microwaves
The range of frequencies below the infrared frequencies is known as microwaves. Microwaves are used in RADAR systems, mobile telephones, and microwave ovens.

Microwaves too can cause adverse effects on our bodies. Generally microwave ovens are produced so that microwaves do not leak out from the oven. However as a precautionary measure it is better not to stay too close to microwave ovens when they are in operation. It is suspected that the excessive use of mobile telephones can cause harm to the brain.
A microwave oven