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 Sub Topics Electromagnetic radiation is composed of oscillating electric and magnetic fields emitted by vibrating charged particles. It transport energy and travels through empty space with a constant velocity c, where c equals the product of wavelength and frequency. The electromagnetic spectrum comprises cosmic rays to long wavelength electrical oscillations.With the exception of objects at absolute zero all objects emit electromagnetic radiation. Objects also reflect radiation that has been emitted by other objects. The most familiar form of electromagnetic radiation is visible light, which forms only a small portion of the full electromagnetic spectrum.Electromagnetic energy is generated by several mechanisms, including changes in the energy levels of electrons, acceleration of electrical charges, decay of radioactive substances and the thermal motion of atoms and molecules.

"Electromagnetic radiation is a phenomenon of nature that is caused by accelerating charged particles. This phenomenon is called electromagnetic radiation because a charged particle has an electric field associated with it."
Light is a result of an accelerating charged particle and because accelerating charged particles have both electric and magnetic fields associated with them, light is called electromagnetic radiation. Electromagnetic radiation however is not just the light we see with our eyes. The light we see with our eyes is only a small fraction of what electromagnetic radiation actually is.

"It is the movement of the energy through space as a combination of electric and magnetic field. It is generated when the velocity of an electrically charged particle is altered."

Electromagnetic radiation consists of electric and magnetic field that oscillates in the direction perpendicular to each other and perpendicular to the direction in which the wave is traveling. All electromagnetic radiations travel with speed of light. It obeys the principle of superimposition.

Electromagnetic radiation is defined by wavelength ($\lambda$) and by radiation frequency ($\nu$) related by a relation of inverse proportionality.

$c = \lambda \nu$

Where
c = velocity of light
$\lambda$ = wavelength
$\nu$ = radiation frequency

The general law of electromagnetic emission was enunciated by Planck in synthetic form.

$e = h \nu$

Where
e = quantum of the energy of radiation
h = Planck quantum
$\nu$ = radiation frequency

The equation points out how the quantity of energy provided by the radiation is directly proportional to the frequency and inversely proportional to the wavelength. The Electromagnetic Radiation Formula is expressed by the equation.

$M_{\lambda} = \frac{2\pi hc^{2}h\lambda^{-5}}{e^{\frac{ch}{\lambda k t}}-1}$

Where $\lambda$ is the radiation wavelength.

The electromagnetic radiation spectrum describes all the various forms of radiation virtually all of them emitted by the sum that exist. The electromagnetic radiation spectrum is a continuum and the energy emitted by the radiation gradually diminishes from gamma rays to radio waves.

The electromagnetic radiation spectrum is a continuum and the energy emitted by the radiation gradually diminishes from gamma rays to radio waves. Visible light is just a small portion of the spectrum and the only thing unique to this portion is our ability to directly sense these wavelengths.

The electromagnetic spectrum is usually represented as a list or diagram showing the types of waves and their frequency.

The types s waves in the electromagnetic spectrum currently used for communication systems are visible light, infra-red, microwaves and radio waves which include: TV, FM radio and AM radio.

Energy in the form of transverse magnetic and electric waves. In a vacuum these waves travel at the speed of light. The acceleration of electric charges gives rise to electromagnetic radiation. Other common examples of electromagnetic radiation are x-rays, microwaves and radio waves.

The examples of electromagnetic radiation are listed below.
1. The radio waves that we hear.
2. The light waves we see.
3. The infrared waves that can take pictures in the dark.
4. The ultraviolet rays that cause sunburn.
5. The gamma rays of the atomic bomb.
6. The cosmic rays that hinder travel in space.

All electromagnetic radiation consists of simultaneous electric and magnetic waves. Electromagnetic radiation travels at the speed of light and each type has its own unique frequency and wavelength. Electromagnetic radiation may appear in the form of visible light, x-rays, infrared or radio waves depending on its energy. The entire band of electromagnetic energies is known as the electromagnetic spectrum.

The spectrum of electromagnetic radiation ranges continuously from low energy radio waves to very high energy gamma rays. The forms of electromagnetic radiation comes in packets of energy called photons. The electromagnetic radiation chart in order of increasing photon energy is given below.

The boundary between each type of radiation is not a set value. For example, the highest energy microwave photons merge into the lowest energy infrared photons.

The types of electromagnetic radiation and the effects of electromagnetic radiation and electromagnetic radiation protection are listed below in the table.

 S.No Types of radiation Dangers to exposure Electromagnetic Radiation Protection 1 Microwaves Absorbed by water in living cells. Causes the cells to heat up and be killed or damaged. Make sure that the seals around the door are good. 2 Infrared radiation Absorbed by the skin and felt as heat. Do not stay in the sun too long. 3 Ultraviolet radiation Passes through the skin to deeper tissues. High doses can cause cells to become cancerous. Darker skins have natural UV protection. Wearing UV filter sun lotions can help reduce the risks of sunburn and damage to skin. 4 X-rays Mostly go straight through the skin and soft tissues, but some absorbed by cells. High doses can kill cells. Low doses can cause cells to become cancerous. If taking X-rays stay well away from the source. Patients should not be X-rayed too often. 5 Gamma rays Mostly go straight through the skin and soft tissues, but some absorbed by cells. High doses can kill cells. Low doses can cause cells to become cancerous. Enclose the source in a thick lead lined container.

Frequency is the number of complete wave cycles that pass a given point in a second (s). The unit is the hertz (Hz), in reciprocal seconds (s-1). It is designated by the Greek letter nu ($\nu$) or the lower case "f". Frequency is used to describe the part of the electromagnetic spectrum including radio frequency, microwaves and extremely low frequency.

In addition to having wave properties electromagnetic radiation also behaves like discrete packets of energy called photons. Photons energy is given by

$Q = h \nu$

Where h is Plancks constant, the product h$\nu$ is an indivisible unit of energy for radiation of frequency $|nu$.

The sun, Earth and other bodies radiate electromagnetic energy of varying wavelengths. The similarity of these radiations lies in the fact that all transport energy and travel with the same speed. All electromagnetic radiations pass through space at the speed of light (300 million meters per second) in the form of sinusoidal waves.

The frequency is the time interval between passing peaks. Since they all move at the same speed the frequency of oscillation is related to the wavelength is vice versa.

$Frequency = \frac{Speed\ of\ EM\ radiation}{Wavelength}$
$Wavelength = \frac{speed\ of\ Em\ radiation}{Frequency}$

The speed of visible light is depicted below in terms of wavelength and corresponding frequency.

The general properties of electromagnetic radiation are listed below.
1. They have no mass or weight. There is no electrical charge.
2. They usually travel at the speed of light that is 186000 miles per second.
3. They are made up of energy and travel both as particle and wave. All transfer energy in the form of quanta.
4. They generate an electric field at right angle to the path they travel.
5. In free space they travel in straight line and also obey inverse square law.
6. Electromagnetic radiation consists of electric and magnetic field that oscillates in the direction perpendicular to each other and perpendicular to the direction in which the wave is traveling.

Some of the solved problems based on electromagnetic radiation is given below.

### Solved Examples

Question 1: A quantum of electromagnetic radiation has an energy of 1.77eV. What is the associated wavelength?
Solution:

Use Plancks relation

hc = 6.63 $\times$ 10-34 Js 3.0 $\times$ 108

= 19.89 $\times$ 10-26 Jm $\frac{1eV\ 1nm}{1.6 \times 10^{-19} J 10^{-9}m}$

= 1243 eV nm

Question 2: A major visible line in an atomic emission spectrum occurs at 450nm. How much does the energy of an electron decreases as this photon is emitted? h = 6.626 $\times$ 10-34 J-sec
Solution:

E = h$\nu$ = $\frac{hc}{\lambda}$

= $\frac{6.626 \times 10^{-34}J-sec) \times (3.0 \times 10^{8} m sec^{-1}}{450 \times 10^{-9}m}$

= 4.417 $\times$ 10-19J