Electromagnetic Radiation

We can see different objects with the help of light. We can also communicate with each other through ‘smart-phones’ over large distances with the help of radiations. But have we ever wondered what this ‘radiation’ or ‘light’ actually is? What is it composed of?.This ‘radiation’ or ‘light’ is actually ‘electromagnetic radiation’. The term ‘electromagnetic’ is a combination of two words ‘electric’ and ‘magnetic’. We can also conclude that the term ‘electromagnetic’ indicates the relation between an ‘Electric Field’ and a ‘Magnetic Field’.The term ‘radiation’ can be considered as a ‘wave’. A ‘wave’ is a physical phenomenon that is a disturbance in medium such as air. The medium can be solid, liquid, gas, electric or a magnetic field.

We shall first discuss the nature of an ‘Electric Field’. It is a type of force that is experienced by a charged particle placed in the territory of another charged particle. The charged particle can either be ‘positive’ or ‘negative’.An example of charged particle placed in the field of another charged particle is shown here. A charged particle experiences a force produced by another charged particle. This force is termed as ‘Electric Field’ and is experienced by the ‘positive’ or ‘negative’ charges. When a charged particle accelerates it produces a change in the ‘electric field’. This change in the electric field gives rise to a ‘magnetic field’. The magnetic field is also a force like the electric field.

Changing magnetic field also gives rise to electric field. We can say that electric and magnetic fields are coherent in nature. These changing electric and magnetic fields give rise to ‘electromagnetic radiation’.‘Electromagnetic radiation’ is a type of energy that propagates through space or material medium in the form of ‘electromagnetic wave’ which consists of changing ‘Electric’ and ‘Magnetic’ fields. The coherent nature of both fields produce electromagnetic waves. In such waves, the electric field is perpendicular to magnetic field. They travel in the form of ‘transverse wave’. In simple words we can say that electromagnetic radiation is the energy that propagates in the form of advancing electric and magnetic field disturbances.

We shall now discuss the properties of ‘Electromagnetic Radiation’. Electromagnetic radiation exhibits dual nature. It shows particulate nature and it shows wave like properties. With regard to the wave nature, it travels in the form of a transverse wave.A transverse wave exhibits particular characteristics. Wavelength is the distance between two adjacent top sections of the wave. We can also measure this by measuring the distances of two adjacent bottom sections. These are called crests and troughs of wave forms. This figure illustrates the wavelength of a wave. This wavelength varies for different electromagnetic waves. The length of a wave also determines frequency of a radiation.

Frequency is the number of waves passing through a fixed point in unit time.For instance, consider the box in the figure as a unit area and one wave is passing through it in one second. So it’s frequency is 1 Hz.In the second example, two waves are passing through a unit area in one second. So it’s frequency is 2 Hz.Frequency is measured in Hertz which is abbreviated as Hz. It means number of cycles per second. In simple words we can say that it is the number of waves passing through a unit area in one second.

The mathematical expression for frequency is as shown.In this expression, ‘v’ represents ‘frequency’, ‘c’ represents ‘speed of electromagnetic radiation’ and ‘λ’ represents ‘wavelength’.From this expression we can conclude that frequency ‘v’ is inversely proportional to wavelength ‘λ’. This means that greater the wavelength, smaller will be the frequency.Let us understand this with the help of an example.

In the illustration, we can see that wavelength is larger at first when it is passing through the unit area. Time taken for waves to pass through the unit area is 1 second. We can observe that only one large wave is passing through unit area in one second so the ‘frequency’ is 1 Hz.But when we decrease the length of the wave passing through the unit area, we can observe that two waves are passing through the unit area in one second. The frequency is now 2 Hz. So we have decreased the wavelength and frequency has been increased from 1 Hz to 2 Hz.

In the shown expression let’s see what ‘c’ is. It represents the speed of ‘electromagnetic’ radiation. It is constant. Speed of any electromagnetic radiation is 299,792,458 m/s which is approximately 3×10⁸ m/s. Light is considered electromagnetic radiation. The speed of electromagnetic radiation is not affected by change in wavelength or frequency. It is not changed by the change in the energy of electromagnetic radiation.

The speed of ‘electromagnetic radiation’ is used as a unit to measure the ‘interstellar distances’. For example, the time it takes for radiations from sun to reach the earth is almost 8 minutes. We can say that distance between ‘sun’ and ‘earth’ is 8 light minutes. ‘1 light year’ is the distance that light or ‘electromagnetic radiation’ travels in 1 year.

Electromagnetic radiation can also be described in terms of energy particles called photons. Photons are quantized packets of energy. It means they only exist in discrete amount of energy.Their energy is expressed as shown. In this expression E represents ‘energy’ of a photon, h stands for ‘Planck constant’ and v represents ‘frequency’.

We learned the relationship between ‘frequency’, ‘speed of electromagnetic radiation’ and ‘wavelength’. We can substitute the value of frequency ‘v’ in the expression of ‘Energy’. Once we do that we get the equation shown. E is the energy of a photon. C is the speed of EM radiation. h is the Planck constant.The energy of a photon is measured in Joules.

As we can see from the expression, energy is inversely proportional to wavelength. This means that the shorter the wavelength of electromagnetic radiation, the greater will be its’ energy.We can also say that high energy photons have shorter wavelength and higher frequency as compared to low energy photons.

Due to difference in ‘wavelength’, ‘frequency’ and ‘energies’ of electromagnetic radiation they can be classified into different types. These different types of ‘electromagnetic radiation’ are expressed in form of the ‘Electromagnetic spectrum’.The ‘Electromagnetic spectrum’ shows these various ‘electromagnetic radiations’. They range from a large wavelength of 10³m to a short wavelength of 10⁻⁵nm. These long electromagnetic radiations are radio waves. Short ones are gamma rays. Therefore electromagnetic radiation ranges from ‘Radio waves’ to ‘Gamma rays’.

Electromagnetic radiations are classified into following regions.‘Gamma’ rays are very high energy electromagnetic radiations having shortest wavelength. They have frequency above a 30 × 10¹⁸Hz. Due to very high energy, they have very strong penetration in matter. These radiations are created by nuclear decay.Gama rays are also used in the process of ‘irradiation’. ‘Irradiation’ is the killing of living microorganisms and preserving the food from bacteria.

‘X-rays’ have frequency range from 30 × 10¹⁵ Hz to 30 × 10¹⁸Hz. They have high energy than ‘Ultraviolet radiations’ but are lower in energy as compared to Gamma rays. Gamma rays are emitted by ‘atomic nucleus’ and X-rays are emitted by electrons.‘X-rays’ are very high energy photons. Because of this they can excite the electrons in an atom and ionize it. They are also known as ‘ionizing radiations’. These radiations can also cause changes in ‘chemical bonds’.An advantage of ‘ionizing radiations’ is that they can be used for medical diagnosis because they can penetrate inside things. An example is using them to ‘X-ray’ the upper body.

Ionizing radiation can be used for treating cancer. Because these radiations are of high energy and can change chemical bonds, they can be used to kill cancerous cells. A disadvantage of Ionizing radiation is their harmful effect on the human body. Because of the ionizing nature they can cause serious damage to genetic material of living organisms. Serious harm of these radiations on DNA is shown in the figure. As we can observe ionizing radiations have direct and indirect effect on DNA. They can directly ionize atoms in DNA and cause serious harm. On the other hand these radiations can also ionize water and convert it into OH⁻ ion and H₃O⁺ ions which can cause harm to the DNA.

Ultraviolet radiations have lower energy than ‘X-rays’. They have frequency range between 8 × 10¹⁴ to 3 × 10¹⁶Hz. ‘Ultraviolet radiations’ come from sun and they can also cause serious damage to skin. But most of these radiations are filtered out by ‘ozone’ layer in our atmosphere. X-rays, Gamma rays and UV rays are invisible to human eye. This is because we can only see radiations in the wavelength range of 300nm – 400nm.One of the advantage of UV radiations to human body is that these radiations cause the body to produce ‘Vitamin D’.

‘Visible rays’ have wavelength range from 380nm to 750nm. Because these radiations are visible we can see different colors of these radiations based on their wavelength.The color range of visible spectrum is illustrated here. These radiations can be used in ‘fiber optic communication’ and in ‘photography’. Moreover these radiations, when coming from distant stars, are used to measure nature of stars by astrophysics.

Infrared radiations have wavelength range larger than those of visible radiation. They are invisible to human eye. ‘Infrared radiations can be absorbed or emitted by molecules. Therefore they can be used to determine the structure of molecules. These radiations are used in ‘night vision’ and in ‘astronomy’ to observe nature of distant stars and objects. These radiations can be sensed because they produce heat.

Microwaves have wavelength range between 30 cm to 1mm. They have very low frequency. Due to their heating properties, they are used in microwave ovens to cook food.Radio wave’ have longest wavelength in ‘Electromagnetic spectrum’. They are used in ‘navigation’ and ‘radio broadcasting’. They have a wavelength range between 1mm to 100 km.