CBSE Class 12 Physics (2026–27)
Chapter 8: Electromagnetic Waves
20 Important Questions and Answers
Q1. What are electromagnetic waves? How are they produced?
Answer:
Electromagnetic (EM) waves are waves consisting of oscillating electric and magnetic fields that propagate through space without requiring a material medium. They are produced when an electric charge accelerates or vibrates. According to Maxwell’s theory, a changing electric field generates a magnetic field, and a changing magnetic field generates an electric field. These mutually perpendicular fields sustain each other and travel through space as electromagnetic waves. The electric field, magnetic field, and direction of propagation are all perpendicular to one another. Examples include radio waves, microwaves, visible light, X-rays, and gamma rays. EM waves travel at the speed of light in a vacuum, approximately (3 \times 10^8) m/s.
Q2. State the main characteristics of electromagnetic waves.
Answer:
Electromagnetic waves possess several important characteristics. They are transverse waves in which electric and magnetic fields oscillate perpendicular to each other and to the direction of propagation. They do not require any material medium and can travel through vacuum. All electromagnetic waves travel with the speed of light in vacuum, which is (3 \times 10^8) m/s. They carry energy and momentum from one place to another. Electromagnetic waves exhibit phenomena such as reflection, refraction, interference, diffraction, and polarization. Their wavelength and frequency determine their position in the electromagnetic spectrum. They are generated by accelerating charges and obey Maxwell’s equations.
Q3. Why are electromagnetic waves called transverse waves?
Answer:
Electromagnetic waves are called transverse waves because the oscillations of their electric and magnetic fields occur perpendicular to the direction in which the wave travels. If an electromagnetic wave propagates along the x-axis, the electric field may oscillate along the y-axis while the magnetic field oscillates along the z-axis. Thus, both fields are at right angles to each other and to the direction of propagation. This behavior is a defining property of transverse waves. Unlike longitudinal waves, where particle vibrations occur parallel to the direction of propagation, electromagnetic waves show perpendicular field oscillations, making them purely transverse in nature.
Q4. What is the relation between the electric field and magnetic field in an electromagnetic wave?
Answer:
In an electromagnetic wave, the electric field (E) and magnetic field (B) are mutually perpendicular and vary sinusoidally with time. Their magnitudes are related by the expression:
[E = cB]
where (c) is the speed of light in vacuum ((3 \times 10^8) m/s). This relation shows that the electric field strength is much greater than the magnetic field strength. Both fields reach their maximum and minimum values simultaneously and are always in phase. The energy carried by the electromagnetic wave is equally distributed between the electric and magnetic fields. This relationship was derived by Maxwell while developing the electromagnetic theory of light.
Q5. Why can electromagnetic waves travel through vacuum?
Answer:
Electromagnetic waves can travel through vacuum because they do not depend on the vibrations of material particles. Instead, they consist of oscillating electric and magnetic fields. A changing electric field produces a magnetic field, while a changing magnetic field generates an electric field. This continuous interaction allows the wave to propagate without any medium. Mechanical waves such as sound require particles of a medium to transfer energy, but electromagnetic waves do not. This is why sunlight and radio signals can travel through the empty space between the Sun and Earth. The ability to propagate through vacuum is one of the most important properties of electromagnetic waves.
Q6. State Maxwell’s prediction regarding electromagnetic waves.
Answer:
James Clerk Maxwell predicted that changing electric and magnetic fields can propagate through space as waves. He mathematically showed that these waves travel with a speed equal to the speed of light. This led him to conclude that light itself is an electromagnetic wave. Maxwell’s theory unified electricity, magnetism, and optics into a single framework. According to his prediction, electromagnetic waves consist of oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation. Later, Heinrich Hertz experimentally verified the existence of electromagnetic waves, confirming Maxwell’s prediction and laying the foundation for modern communication technology.
Q7. What is the electromagnetic spectrum?
Answer:
The electromagnetic spectrum is the complete range of electromagnetic radiations arranged according to their wavelengths or frequencies. It includes radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. Radio waves have the longest wavelengths and lowest frequencies, while gamma rays have the shortest wavelengths and highest frequencies. All these radiations travel at the same speed in vacuum but differ in their energy, wavelength, and frequency. The electromagnetic spectrum has numerous applications in communication, medicine, astronomy, industry, and scientific research. Different regions of the spectrum interact with matter in different ways.
Q8. Differentiate between radio waves and microwaves.
Answer:
Radio waves and microwaves are both electromagnetic waves but differ in wavelength and applications. Radio waves have longer wavelengths ranging from a few centimeters to several kilometers and lower frequencies. They are mainly used in radio broadcasting, television transmission, and long-distance communication. Microwaves have shorter wavelengths, generally between 1 mm and 30 cm, and higher frequencies. They are widely used in radar systems, satellite communication, mobile phones, and microwave ovens. Due to their shorter wavelength, microwaves can carry more information and are suitable for high-frequency communication systems. Both travel at the speed of light in vacuum.
Q9. What are infrared rays? Mention their applications.
Answer:
Infrared rays are electromagnetic waves with wavelengths longer than visible red light but shorter than microwaves. They are emitted by hot objects and are often associated with heat radiation. Infrared rays are invisible to the human eye but can be detected as heat. They are used in remote controls for televisions and air conditioners, thermal imaging cameras, night vision devices, and medical physiotherapy treatments. Infrared radiation is also used in weather forecasting and satellite imaging. Since these rays are absorbed by water molecules, they play an important role in atmospheric heating and greenhouse effects.
Q10. What are ultraviolet rays? State their uses.
Answer:
Ultraviolet (UV) rays are electromagnetic waves with wavelengths shorter than visible violet light but longer than X-rays. They possess higher energy than visible light and can cause fluorescence in certain materials. UV rays are used for sterilizing medical instruments, purifying drinking water, and detecting counterfeit currency notes. They also help the human body produce vitamin D when exposed to sunlight in moderate amounts. However, excessive exposure can damage skin cells and increase the risk of skin cancer. The ozone layer in Earth’s atmosphere absorbs most harmful ultraviolet radiation from the Sun, protecting living organisms.
Q11. What are X-rays? Mention two applications.
Answer:
X-rays are high-frequency electromagnetic waves with very short wavelengths. They possess high penetrating power and can pass through soft tissues while being absorbed by dense materials such as bones. This property makes them extremely useful in medical diagnosis. X-rays are widely used to detect bone fractures, dental problems, and internal injuries. They are also employed in airport security scanners to inspect luggage and identify concealed objects. In industry, X-rays are used to detect cracks and defects in metal structures. Due to their ionizing nature, excessive exposure should be avoided to prevent damage to living tissues.
Q12. What are gamma rays and how are they produced?
Answer:
Gamma rays are the highest-energy electromagnetic waves with extremely short wavelengths and very high frequencies. They are produced during radioactive decay and nuclear reactions. Unlike X-rays, which originate from electronic transitions, gamma rays arise from changes within the atomic nucleus. Because of their high energy and strong penetrating power, they can pass through thick materials. Gamma rays are used in cancer treatment through radiotherapy, sterilization of medical equipment, and scientific research. However, excessive exposure can be harmful to living cells because gamma rays are highly ionizing and may cause genetic mutations.
Q13. Arrange the electromagnetic waves in increasing order of frequency.
Answer:
The electromagnetic spectrum can be arranged in increasing order of frequency as follows:
Radio waves → Microwaves → Infrared rays → Visible light → Ultraviolet rays → X-rays → Gamma rays.
As frequency increases, wavelength decreases according to the relation:
[c = \nu \lambda]
where (c) is the speed of light, (\nu) is frequency, and (\lambda) is wavelength. Therefore, radio waves have the lowest frequency and longest wavelength, while gamma rays have the highest frequency and shortest wavelength. The energy of electromagnetic radiation also increases with frequency. This sequence is important for understanding the properties and applications of different regions of the electromagnetic spectrum.
Q14. Why are microwaves used in radar systems?
Answer:
Microwaves are used in radar systems because of their short wavelength and high frequency. These properties allow them to travel long distances with minimal atmospheric absorption and to be reflected effectively by objects such as aircraft, ships, and vehicles. Radar systems emit microwave pulses and analyze the reflected signals to determine the position, speed, and direction of objects. Microwaves can also penetrate clouds, fog, and light rain, making them suitable for weather monitoring and navigation. Their directional nature enables accurate detection and tracking of targets, which is essential in aviation, defense, and meteorological applications.
Q15. Explain the role of electromagnetic waves in communication.
Answer:
Electromagnetic waves form the basis of modern communication systems. Information such as voice, video, and data is converted into electrical signals and transmitted using electromagnetic waves. Radio waves are used for broadcasting radio and television programs. Microwaves are employed in satellite communication and mobile phone networks. Optical fibers use visible and infrared light for high-speed internet communication. Since electromagnetic waves travel at the speed of light and do not require a medium, they can efficiently transmit information over long distances. Their ability to carry large amounts of data has revolutionized global communication and connectivity.
Q16. Why is visible light considered a small part of the electromagnetic spectrum?
Answer:
Visible light occupies only a narrow range of wavelengths, approximately 400 nm to 700 nm, within the vast electromagnetic spectrum. Human eyes are sensitive only to this small region, enabling us to perceive different colors from violet to red. Beyond this range lie infrared and ultraviolet radiations, which are invisible to the human eye but still exist as electromagnetic waves. Although visible light represents a tiny portion of the spectrum, it plays a crucial role in vision, photography, and optical technologies. The rest of the spectrum is equally important and finds applications in communication, medicine, and scientific research.
Q17. What is displacement current?
Answer:
Displacement current is the current associated with a changing electric field. Maxwell introduced this concept to modify Ampere’s law and explain the continuity of current in capacitors. When a capacitor is charging, no actual conduction current flows through the dielectric gap, yet a changing electric field exists between the plates. Maxwell proposed that this changing electric field behaves like a current and produces a magnetic field. This displacement current completes the circuit conceptually and plays a crucial role in the generation and propagation of electromagnetic waves. Its introduction led to the successful prediction of electromagnetic radiation.
Q18. How do electromagnetic waves carry energy?
Answer:
Electromagnetic waves carry energy through their oscillating electric and magnetic fields. As these fields propagate through space, they transport energy from the source to distant locations. The energy is shared equally between the electric and magnetic components of the wave. When electromagnetic waves interact with matter, this energy can be absorbed, reflected, or transmitted. For example, sunlight transfers energy from the Sun to Earth, enabling photosynthesis and maintaining life. Radio waves carry information in communication systems, while microwaves transfer energy for heating food. Thus, electromagnetic waves serve as efficient carriers of energy across vast distances.
Q19. Why are X-rays more penetrating than visible light?
Answer:
X-rays have much shorter wavelengths and higher frequencies than visible light. According to quantum theory, higher frequency corresponds to greater energy. Due to this high energy, X-rays can penetrate materials that visible light cannot pass through. Soft tissues absorb X-rays weakly, while dense materials such as bones absorb them strongly. This difference creates contrast in X-ray images used for medical diagnosis. Visible light, having lower energy and longer wavelength, is easily absorbed or scattered by many materials. Therefore, the greater penetrating power of X-rays makes them valuable in medicine, industry, and security applications.
Q20. What are the harmful effects of ultraviolet rays and X-rays?
Answer:
Ultraviolet rays and X-rays can be harmful because they possess relatively high energy. Excessive exposure to ultraviolet radiation may cause sunburn, premature skin aging, eye damage, and skin cancer. X-rays are ionizing radiations that can damage living cells and DNA. Prolonged or uncontrolled exposure increases the risk of genetic mutations and cancer. Therefore, protective measures such as sunscreen, UV-blocking glasses, and lead shielding during X-ray examinations are essential. Despite these risks, both ultraviolet rays and X-rays have important beneficial applications in medicine, sterilization, and scientific research when used under controlled conditions.