Electromagnetic spectrum

The electromagnetic spectrum encompasses all types of electromagnetic radiation, ranging from radio waves to gamma rays, each with its own wavelength and frequency. This spectrum is fundamental to numerous technologies and scientific fields. For instance, radio waves are essential for communication systems like television and mobile phones, while microwaves are used in radar and cooking. Infrared radiation is employed in remote controls and thermal imaging, whereas visible light is crucial for human vision and photography. Ultraviolet light, X-rays, and gamma rays have significant roles in medical imaging and treatments. In this article, we will discuss the concept of the Electromagnetic spectrum, and important terms related to it and provide examples for a better understanding

Electromagnetic Spectrum

The electromagnetic spectrum encompasses all types of electromagnetic radiation, from low-frequency radio waves to high-frequency gamma rays. This spectrum is essential for various technologies and scientific fields. Radio waves enable communication systems like radio, television, and mobile phones, while microwaves are used in radar and cooking. Infrared radiation is employed in remote controls and thermal imaging, whereas visible light is crucial for human vision and photography. When we see our surroundings, we see only a visible range of electromagnetic waves. So, the only familiar electromagnetic waves were the visible light waves. But, we now know that electromagnetic waves include visible light waves, X-rays, gamma rays, radio waves, microwaves, ultraviolet and infrared waves. The classification of EM waves according to frequency in the electromagnetic spectrum is shown in the figure given below.

Now we will discuss all these EM waves one by one with the help of the following table

Earth's Atmosphere

The Earth's atmosphere is a complex layer of gases that envelop our planet, playing a crucial role in sustaining life. Composed primarily of nitrogen (78%) and oxygen (21%), along with trace amounts of other gases like carbon dioxide and argon, the atmosphere is divided into several layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. These layers regulate temperature, and weather patterns, and protect us from harmful solar radiation. In everyday life, the atmosphere influences everything from the air we breathe and the weather we experience to long-term climate patterns.

Earth’s atmosphere has the following six layers.

(i) Troposphere

The troposphere is the innermost layer of Earth’s atmosphere. i.e. it is Closest to the surface of the Earth. It is the thermal classification of the atmosphere.“Tropos” means change. This layer gets its name from the weather that is constantly changing. The troposphere is between 8 and 14 kilometers. This layer has the air we breathe and the clouds in the sky.

(ii) Stratosphere

The stratosphere is located above the troposphere and below the mesosphere. It extends between 17-50 Km above the earth's surface. The ozone layer is located in the stratosphere. Ozone layer - It absorbs most of the ultraviolet rays emitted by the sun.

(iii) Mesosphere

The mesosphere is located above the stratosphere and below the thermosphere. It is characterized by temperatures that quickly decrease with increasing height. It extends between 50-80 Km.

(iv) Thermosphere

The thermosphere is located above the mesosphere and below the exosphere. Based on the vertical temperature profile in the atmosphere, the thermosphere is the highest layer, located above the mesosphere.

In the thermosphere, temperature increases with altitude. It extends from about 90 km to between 500 and 1,000 km above our planet.

(v) Ionosphere

It starts at about 75 Km and goes up to 650 Km. It contains ions and free electrons. Aurora occurs in the Ionosphere.

(vi) Exosphere

The outermost layer of the earth's atmosphere. (640 Km - 1280 Km)

Point to remember

1. Polarisation in EM wave - For an EM wave, the direction of polarisation is taken to be the direction of the electric field.

2. Wavelength of EM Wave

$
\lambda=\frac{\lambda_o}{\mu}
$
$\lambda_o=$ Wavelength in vacuum
$\mu=$ Refractive index of the medium (Detail analysis will be studied in Optics)

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Solved Examples Based on Electromagnetic Spectrum

Example 1: An electromagnetic wave of frequency $f=3.0 \mathrm{MHz}$ passes from a vacuum into a dielectric medium with permittivity $\epsilon_r=4.0$. Then

1) wavelength is doubled and the frequency remains unchanged

2) wavelength is doubled and frequency becomes half

3) wavelength is halved and frequency remains unchanged

4) wavelength and frequency both remain unchanged.

Solution:

Wavelength of EM Wave

$\lambda=\frac{\lambda_o}{\mu}$

wherein
$\lambda_o=$ Wavelength in vacuum
$\mu=$ Refractive index of the medium
$
\mu=\sqrt{\frac{\epsilon}{\epsilon_0}}=\sqrt{\epsilon_r}=\sqrt{4}=2
$

Since $\mu \alpha \frac{1}{\lambda}$
$\therefore \quad$ Wavelength is halved

The frequency of electromagnetic waves won't change with the change in a medium,

Hence, the answer is the option (3).

Example 2: A radar sends the waves towards a distant object and receives the signal reflected by an object. These waves are

1) Sound waves

2) Light waves

3) Radio waves

4) Microwaves

Solution:

Application of Radio and Microwaves:

These are used in radio and TV communication.

Nowadays, microwaves are used to locate flying objects by radar.

Hence, the answer is the option (4).

Example 3: Given below in the left column are different modes of communication using the kinds of waves given in the right column.

A. Optical Fibre Communication P. Ultrasound

B. Radar Q. Infrared Light

C. Sonar R. Microwaves

D. Mobile Phones S. Radio Waves

From the options given below, find the most appropriate match between entries in the left and the right column.

1) A-Q, B-S, C-R, D-P

2) A-S, B-Q, C-R, D-P

3) A-Q, B-S, C-P, D-R

4) A-R, B-P, C-S, D-Q

Solution:

Optical fiber communication Infrared light

Radar Radio waves

Sonar Ultrasound

Mobile phones Microwaves

Hence, the answer is the option is (3)

Example 4: What is the name given to that part of the electromagnetic spectrum that is used for taking photographs of Earth under foggy conditions from great heights?

1) U.V. rays

2) Visible rays

3) Infrared rays

4) Microwaves

Solution:

Application of Infrared rays

1. Treat muscular pain.

2. For taking photographs in fog or smoke

3. In weather forecasting

Therefore, Infrared rays are used to take photos of Earth.

Hence, the answer is the option (3).

Example 5: Which of the following rays are used to sterilise the surgical instruments?

1) Infrared rays

2) X - rays

3) U.V. rays

4) None of these

Solution:

Application of UV rays

1. In the study of molecular structure.

2. In sterilizing the surgical instruments

3. In the detection of forged documents, fingerprints

Hence, UV rays are used for sterilizing surgical instruments.

Hence, the answer is the option (3).

Summary

The electromagnetic spectrum spans a range of electromagnetic radiation types, from radio waves to gamma rays, each with unique wavelengths and frequencies. This spectrum is vital to various technologies and scientific fields, enabling communication systems, medical imaging, and environmental monitoring. Understanding the electromagnetic spectrum helps us harness different types of radiation for practical applications, such as using infrared for thermal imaging, microwaves in radar, and ultraviolet rays for sterilizing medical instruments.