Electromagnetic Spectrum

Introduction to Electromagnetic Spectrum

Electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. It includes visible light, ultra-violet rays, microwaves, X-rays, radio waves and gamma rays. Electromagnetic waves are produced when electrically charged particles oscillate or change energy in some way. The waves travel perpendicularly to both electric and magnetic fields. 

Wavelength, frequency and energy of electromagnetic waves.

X-rays and gamma rays are usually described in terms of wavelength and radio waves in terms of frequency.

The electromagnetic spectrum

It is divided into seven major regions or bands. A band consists of a range of frequencies in the spectrum in terms of frequencies i.e. radio, microwaves, infra-red.

Properties of electromagnetic waves

Common properties

1. They do not require a material medium and can travel through a vacuum.
2. They undergo reflection, refraction and diffraction.
3. All electromagnetic waves travel at the speed of light i.e. 3 * 108 ms-1.
4. They carry no electric charge
5. They transfer energy from a source to a receiver in the form of oscillating electric and magnetic.

Examples

1. A VHF radio transmitter broadcasts radio waves at a frequency of 30 M Hz. What is their wavelength?

Solution
v = f ? => then ? = v / f = 3.0 * 108/ 300 * 106
= 1.00 m.

2. Calculate the frequency of a radio wave of wavelength 150 m.
Solution

v = f ? =>f = v / ? = 2.0 * 106
= 2 M Hz.

Unique properties

1. Radio waves – they are further divided into long waves (LW), medium waves (MW) and short waves (SW). They are produced by electrical circuits called oscillators and they can be controlled accurately. They are easily diffracted by small objects like houses but not by large objects like hills.
2. Microwaves – they are produced by oscillation of charges in special aerials mounted on dishes. They are detected by special receivers which convert wave energy to sound i.e. ‘RADAR’ / Radio Detection and Raging
3. Infra-red radiation – infra-red radiations close to microwaves are thermal (produce heat) i.e. sun, fire but those closer to the visible light have no thermal properties i.e. TV remote control system. Detectors of infra-red radiation are the human skin, photographic film etc.
4. Optical spectrum (visible light) – they form a tiny part of the electromagnetic spectrum. Sources include the sun, electricity, candles etc. these have wavelengths visible to the human eye and includes the optical spectrum (ROYGBIV). It is detected through the eyes, photographic films and photocells.
5. Ultra-violet rays (UV) – has shorter wavelength than visible light. It is emitted by very hotobjectsi.e. the sun, welding machines etc. Exposure to UV rays may cause skin cancer and cataracts. They can be detected through photographic film.
6. X-rays – they have very short wavelength but are high energy waves. They are produced in Xray tubes when high speed electrons are stopped by a metallic object. They are detected by the use of a photographic film or a fluorescent screen.
7. Gamma rays – produced by some radioactive materials when large changes of energy occur inside their nuclei. They can be detected by the use of photographic films, Geiger Muller tube or a cloud chamber.

Applications of electromagnetic radiation

1. Radio waves

• They are used in radio, TV and cellular mobile communications.
• Used in military communications (satellite imagery) to form an image of the ground even when there are clouds.

2. Microwaves

• used in radar communications by giving direction and distance
• Used in speed guns by the police to detect over speeding.
• Used in microwave ovens to warm food. The food becomes warm by absorbing energy.
• Used reliably for communication (telephone and computer data).

3. Infra-red radiation

• used to produce images of hot objects through the colours
• Produced by the amount of heat dissipated by an object.
• Images produced by satellites give important information on vegetation cover in all areas of the globe. They can also detect fires.
• They are used in hospitals to detect illnesses (diagnosis)
• Used in warfare missiles and burglar alarm systems
• Used in green houses to grow crops

4. Visible light

• used by plants in remote sensing and humans in the identification of things
• Used by plants in the process of photosynthesis

5. Ultra-violet (UV) radiation

• used to make reflective materials which absorb light and re-emit it as visible light.
• Used in banks to detect fake currency

6. X-rays

• used in hospitals to detect fractures, broken bones and in treatment of cancer (radiotherapy).
• Used to detect foreign materials in the body i.e. metals
• Used to detect invisible cracks in metal castings and welding joints

7. Gamma rays

• used to sterilize medical instruments
• Used to kill weevils in grain
• Used to take photographs same way like X-rays.