Chapter 8

Electromagnetic Waves

The NCERT Solutions for Class 12 Physics Chapter 8 is now available for download on the MSVgo website. The solutions cover topics under Electromagnetic Waves, including important questions and problems related to the chapter. Students are strongly encouraged to make use of the NCERT Solutions Class 12 Physics to prepare thoroughly for the exams as the solutions provide in-depth explanations, formulas, easy-to-follow illustrations, and detailed answers to numerical problems and exercises. The solutions are curated as per the latest CBSE board syllabus by experts, due to which studying from the solutions ensures that the students’ preparation is aligned with the curriculum. Topics covered in this Chapter 1. Introduction 2. Displacement Current 3. Electromagnetic Waves a. Sources of electromagnetic waves b. Nature of electromagnetic waves 4. Electromagnetic Spectrum a. Radio waves b. Microwaves c. Infrared waves d. Visible rays e. Ultraviolet rays f. X-rays g. Gamma rays

Introduction

In Chapter 4, it was established that an electric current generates a magnetic field and that two current-carrying wires exert a magnetic force on each other. In Chapter 6, it was established that a magnetic field changing with time gives rise to an electric field. Chapter 8, on the other hand, shows that the converse is also true, i.e., an electric field changing with time results in a magnetic field. James Clerk Maxwell (1831-1879) theorised that this was indeed true. While applying Ampere’s circuital law to find the magnetic field at a point outside a charging capacitor connected to a time-varying current, Maxwell found that something was amiss. He theorised the possibility of a new current called the displacement current to fill the gap in Ampere’s circuital law. Maxwell derived a set of equations involving electric and magnetic fields, their sources, the charge and current densities. These equations are known as Maxwell’s equations. Maxwell’s equations led to the discovery of electromagnetic waves other than just the visible light rays. This chapter covers electromagnetic waves, their sources and nature, derivation of displacement current, the electromagnetic spectrum and the types of waves.

Physicist James Clerk Maxwell theorised that a changing electric field must also produce a magnetic field. Using this theory, he noticed that Ampere’s circuital law was missing a quantity. Apply Ampere’s circuital law to some surfaces where a capacitor is charged. Calculated one way, there is a magnetic field at a point. Calculated another way, however, the magnetic field at the same point is zero. This proves that Ampere’s circuital law is missing a quantity. The missing quantity could be something that gives the same magnetic field at the point irrespective of which surface is used. This missing quantity is the displacement current between the capacitor plater, defined in terms of the rate of change of the electric displacement field. Maxwell derived the displacement current, which is given by . Displacement current has the units of electric current density and an associated magnetic field just like actual currents. Ampere (Amp) is the SI unit of displacement current. This section of the NCERT Solutions for Class 12 Physics Chapter 8 covers the derivation of the displacement current.

Electromagnetic waves are generated when a magnetic field comes in contact with an electric field. EM waves are the result of oscillating electric and magnetic fields.

a. Sources of electromagnetic waves

So how are electromagnetic waves formed? A charged particle oscillating at a particular frequency creates an oscillating electric field in space, which in turn creates an oscillating magnetic field. The process repeats itself in the same manner. The oscillating electric and magnetic fields regenerate each other as the wave propagates through space. The frequency f of the electromagnetic wave is the same as the frequency of oscillation of the charged particle. The wavelength λ of an electromagnetic wave is given by λ = c/f, where c is the speed of light.

b. Nature of electromagnetic waves

Electric and magnetic fields in an electromagnetic wave are perpendicular to each other and to the direction of propagation. This is referred to as the transverse nature of electromagnetic waves. They are different from other waves, in the sense that they travel without any material medium. Electromagnetic waves can be diffracted, refracted and polarised. All electromagnetic waves travel at the speed of light c. Electromagnetic waves carry energy and momentum and also exert pressure which is nothing but radiation pressure.

It is now established that electromagnetic waves include not only visible light but also X-rays, gamma rays, radio waves, microwaves, ultraviolet and infrared waves. The electromagnetic spectrum is the spectrum of EM waves classified based on their frequency. There is no clear-cut distinction between the two kinds of waves. The classification is based approximately on how the waves are generated and detected. The types of waves in the electromagnetic spectrum are given below.

a. Radio Waves

Radio waves are generated by the increasing motion of charges in conducting wires. Their frequencies range from 500kHz to 1000MHz. Radio and television communication systems use radio waves to function. The AM (amplitude modulated) band ranges from 530kHz to 1710kHz. Television waves operate at frequencies ranging from 54MHz to 890MHz. The FM (frequency modulated) radio band ranges from 88MHz to 108MHz. Radio waves are used in cellular phones to transmit voice waves in the ultrahigh frequency (UHF) band.

b. Microwaves

Microwaves are short-wavelength radio waves whose frequencies are in the gigahertz (GHz) range. They are produced by special vacuum tubes. Their short wavelengths make them useful in radar systems that are used in aircraft navigation. Microwave ovens happen to be one of the most common applications of microwaves. In microwave ovens, the frequency of the microwaves is set in such a way that it matches the resonant frequency of water molecules. All food items contain some amount of water molecules. When an object is heated, the motion of its molecules increases and the molecules vibrate with higher energies. The frequency of the motion of water molecules in food is around 3GHz. These water molecules absorb microwave radiation at this frequency, which heats them up. These molecules then transfer this energy to the adjacent food molecules, thereby heating up the food.

c. Infrared Waves

Produced by hot bodies and molecules, Infrared waves are rays that are found adjacent to the low-frequency end of the visible spectrum. Infrared rays are sometimes referred to as heat waves because water molecules present in some materials absorb infrared waves. After absorption, the rays heat up and in turn heat their surroundings. The earth’s temperature is dependent on infrared radiation and the greenhouse effect to be stable. Visible light from the sun is absorbed by the earth’s surface and is then transmitted back into the atmosphere as infrared radiations, which have longer wavelengths than visible light. Greenhouse gases such as carbon dioxide then trap the re-transmitted infrared radiation. Satellites use infrared detectors for military purposes as well as to observe the growth of crops. Household electronic devices such as TV sets rely on infrared rays emitted by the remote.

d. Visible Rays

Visible rays are the only part of the electromagnetic spectrum that can be detected by the human eye. The frequency of visible light ranges from 4 × 1014 Hz to 7 × 1014 Hz, with wavelengths ranging from 700 – 400nm. Visible light is the rays emitted or reflected from objects, helping humans see things in the physical world. Different animals are sensitive to different ranges of wavelengths, and the human eye is sensitive to the range of wavelengths of visible light. For example, snakes are known to detect infrared waves, and many insects are known to be able to detect rays well into the ultraviolet range.

e. Ultraviolet Rays

Ultraviolet rays have wavelengths ranging from 4 × 10–7 m (400 nm) to 6 × 10–10m (0.6 nm). UV radiation is produced by special lamps and extremely hot bodies. The sun is a good example of a hot body that produces ultraviolet light. Most of the UV radiation generated by the sun is absorbed by the ozone layer in the atmosphere approximately 50km above the Earth’s surface. Overexposure to UV radiation can be harmful to humans. UV radiation is the reason humans get tanned in sunlight as it induces the production of melanin. Humans can protect themselves from UV radiation behind a layer of glass as glass absorbs UV rays.

f. X-Rays

The X-ray region is found beyond the UV region of the electromagnetic spectrum. Their wavelengths range from 10–8m (10nm) to 10–13m (10–4nm). X-rays can be produced by bombarding a metal target with high energy electrons. X-rays are commonly used in medicine as a diagnostic tool and to treat cancer. X-rays harm living tissues; therefore, they must be dealt with carefully to avoid overexposure.

g. Gamma Rays

Gamma rays are found in the higher frequency range of the electromagnetic spectrum. They have high frequencies and small wavelengths. Their wavelengths range from 10–10m to less than 10–14m. High-frequency radiation such as gamma rays can be found as a result of nuclear reactions. They are very effective in destroying cancer cells.

1. How does the NCERT Solutions for Class 12 Physics Chapter 8 help score full marks?

Using the NCERT Solutions for Class 12 Physics Chapter 8 to study for the exam ensures a high score in the exam as the solutions cover all the important topics and questions required for the exam. The solutions provide step-by-step answers to problems and detailed explanations for concepts, making it easy to understand and boosting the students’ confidence in the chapter.

 

2. Why is the ozone layer crucial for human survival?

The ozone layer absorbs harmful ultraviolet (UV) and other radiations, preventing them from reaching the Earth’s surface. Without the ozone layer, the harmful UV radiations reaching the earth’s surface can cause skin cancer in humans.

 

3. What is an electromagnetic spectrum?

The electromagnetic spectrum is the spectrum of EM waves classified based on their frequency. The spectrum includes radio waves, microwaves, infrared rays, visible light, ultraviolet rays, x-rays and gamma rays, starting from low-frequency waves to ultra-high-frequency waves.


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