 Electromagnetic radiation is one of the essential forms of energy. When you cook food in the oven or listen to the radio, you use these electromagnetic radiations. Microwaves, radio waves, gamma rays, television waves, radar waves, and X-rays are some examples of electromagnetic radiation.

According to classical theory, the flow of energy occurs at the speed of light in free space or through a material medium, in the form of the magnetic and electric fields that build electromagnetic waves like visible light, gamma rays, and radio waves.

According to quantum theory, electromagnetic radiation is the flow of photons or quanta through space. Photons are discrete packets of energy (hν) that move at the speed of light.

Where,

h – Planck’s constant

ν – frequency of the electromagnetic wave of classical theory

Clerk Maxwell in 1873, through the unified theory of electromagnetism, showed that magnetism and electricity are not separate entities. He explained how the electric charges interact, and also how they interact with the magnetic field.

• Magnetic poles come in pairs that repel and attract each other, just like electric charges do.
• The force of repulsion or attraction between two electric charges is inversely proportional to the square of the distance between the particles.
• An electric field in motion produces a magnetic field.
• A wire with an electric current generates a magnetic field. The directions of these fields depend on the direction of the current.

Electromagnetic radiation features a huge range of frequencies and wavelengths known as the electromagnetic spectrum. They are visible light, radio waves, X-rays, infrared (IR), ultraviolet (UV) light, microwaves, and gamma-rays. These waves travel at the speed of light in a vacuum. The electromagnetic spectrum is categorised into seven regions.

Since these radiations radiate from electrically charged particles, they are known as electromagnetic radiations. They travel through empty space as well as through air and other substances.

1. Choose the correct answer: Is visible light a type of electromagnetic wave?

1. YES
2. NO

Explanation: Visible light is a part of the electromagnetic spectrum and is a type of electromagnetic wave.

2. How are electromagnetic waves created?

Electromagnetic waves are produced due to periodic changes in the electric or magnetic fields. The waves generated have synchronised oscillations of electric and magnetic fields.

3. What is meant by the atmospheric window?

The electromagnetic radiation with a certain wavelength that can pass through the Earth’s atmosphere is known as the atmospheric window.

4. Choose YES or NO: Can electromagnetic waves travel through a vacuum?

1. YES
2. No

Explanation: Electromagnetic waves travel in a vacuum at the speed of light.

5. A short-wavelength indicates that the frequency will be _____.

1. Zero
2. Lesser
3. Higher
4. None of the above options

Explanation: A short wavelength indicates that the frequency will be higher since one cycle can pass in a shorter amount of time.

6. Within the electromagnetic spectrum, the position of an electromagnetic wave can be characterised using which parameter?

1. Time
2. Wavelength
3. Frequency of oscillation
4. Option b) and c)

Answer: d) Option b) and c)

7. Photons have _____ mass.

1. Less
2. More
3. No
4. None of the above options

Explanation: Photons have no mass since they travel at the speed of light.

8. Unified theory of electromagnetism was explained by _____.

1. Albert Einstein
2. James Clerk Maxwell
4. Kelper

Explanation: in 1873, James Clerk Maxwell explained the unified theory of electromagnetism, and presented the relationships between the motion of charged particles in electric and magnetic fields and the behaviour of electric and magnetic fields.

9. The formula of wave equation is:

1. $$\begin{array}{l}c=\lambda + f\end{array}$$
2. $$\begin{array}{l}c=\lambda – f\end{array}$$
3. $$\begin{array}{l}c=\lambda * 2f\end{array}$$
4. $$\begin{array}{l}c=\lambda f\end{array}$$

$$\begin{array}{l}c=\lambda f\end{array}$$

Explanation: Wave equation is given by the formula,

$$\begin{array}{l}c=\lambda f\end{array}$$
.

10. As per quantum mechanics, electromagnetic radiations behave like discrete particles known as _____

1. Electron
2. Photons
3. Atom
4. Molecules

Explanation: Photons always move with the universal speed of light according to quantum mechanics; electromagnetic radiation is defined as the flow of photons.

## Practice Questions 