## 2008-07-15

### Electromagnetic Spectrum

When you watch television, listen to the radio or cook something in a microwave oven , you are actually apply the properties of electromagnetic waves. Do you know what is an electromagnetic wave?

Electromagnetic waves are propagating waves that travel in space with both electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation.

Electromagnetic waves carry energy and momentum which may be given when they interact with matter.

Electromagnetic waves comprise of a series of waves whose frequencies and wavelengths extend over a broad range. Waves in the electromagnetic spectrum vary in size from very long radio waves to very short gamma rays.

Visible light waves are the only electromagnetic waves we can see. We see these waves as the colours of the rainbow. Each colour has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. When all the waves are seen together, they produce white light.

When white light shines through a prism or through water vapour, the white light is broken apart into the colours of the visible light spectrum.

The electromagnetic spectrum is the range of frequencies and wavelengths over which electromagnetic waves are propagated.

Sources of Electromagnetic Waves.

Matter is made up of elementary particles called atoms.

Every atom has a nucleus at its centre which is surrounded by orbiting electrons.

Electrons are negatively charged particles and they circle around the nucleus in orbits, each of which is at a specific energy level. When a charged electron travels from an orbit with a particular energy level to one of a lower energy level, electromagnetic waves are emitted.

Electromagnetic waves are also produced when a charged particle (electron or nucleus) oscillates.

Properties of electromagnetic waves

Electromagnetic waves are:

a. Transverse waves

b. Do not require a medium to propagate and can travel in a vacuum.

c. The magnetic and electric field components of the wave oscillate at right angles to each other and to the direction of propagation of the wave.

d. Obey the wave equation c = fλ. c is the velocity of light, f is the frequency of the wave and the λ is the wavelength.

e. In a vacuum , the waves travel at the speed of light c = 3 X 10^8 ms-1.

f. Undergo the same phenomena as light: reflection, refraction, diffraction and interference.

g. The waves are electrically neutral.

h. Show characteristics of polarization.

i. Energy is transferred by the waves.

In a vacuum, c is a constant for all elecgromagnetic waves The formula c = fλ shows that the frequency f is inversely proportional to the wavelength λ. (f = c/λ)

The velocities of an electromagnetic wave in other media are different from its velocity in vacuum.

Detrimental effects of electromagnetic spectrum.

The invisible waves or radiation that are emitted from power lines, cellular phones, radio antenna, could potentially be harmful to our health.

The detrimental effects of excessive exposure of the human body to electromagnetic waves of increasing frequencies:

b. Microwaves: internal heating of body tissue

c. Infrared: skin burns

d. Visible light: increased rates of premature skin aging and skin cancer

e. Ultraviolet: damage to surface cells (including skin cancer) and blindness.

f. X-rays: damage to cells.

g. Gamma rays: cancer, mutation