Wave Motion

11. The flash of lightning is seen before the thunderstorm is heard. It ve

The flash of lightning is seen before the thunderstorm is heard. It verifies that

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sound travels much faster than light
light travels much faster than sound
light and sound both travel with same speed
intensity of flash of lightning is very high during thunderstorm
This question was previously asked in
UPSC NDA-2 – 2023
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The fact that the flash of lightning is seen before the thunderstorm is heard verifies that light travels much faster than sound.
– Lightning and thunder occur simultaneously as a result of an electrical discharge in the atmosphere.
– Light (the flash) and sound (the thunder) are both produced by the same event.
– We see the light almost instantly because the speed of light in air is extremely high (approximately 3 x 10^8 m/s).
– We hear the thunder later because the speed of sound in air is much lower (approximately 343 m/s at 20°C).
– The time delay between seeing the flash and hearing the thunder is directly proportional to the distance of the thunderstorm.
For every 3 seconds delay between the flash and the thunder, the thunderstorm is approximately 1 kilometer (or 5 seconds per mile) away. This difference in speeds makes it possible to estimate the distance to a thunderstorm.

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12. Which of the following are the characteristics of electromagnetic wave

Which of the following are the characteristics of electromagnetic waves ?

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  • They are elastic waves.
  • They can also move in vacuum.
  • They have electric and magnetic components which are mutually perpendicular.
  • They move with a speed equal to 3 lakh meters per second.

Select the correct answer using the code given below :

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1, 2, 3 and 4
1, 2 and 4 only
2 and 3 only
3 and 4 only
This question was previously asked in
UPSC NDA-2 – 2019
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Statement 1 is incorrect because electromagnetic waves are not elastic waves; they do not require a medium for propagation. Statement 4 is incorrect as electromagnetic waves move with a speed of approximately 3 x 10^8 meters per second (speed of light), which is 3 lakh *kilometres* per second, not 3 lakh meters per second. Statements 2 and 3 correctly describe characteristics of electromagnetic waves: they can travel through a vacuum, and their electric and magnetic field components are mutually perpendicular.
Electromagnetic waves are transverse waves consisting of oscillating electric and magnetic fields that propagate through space at the speed of light, even in a vacuum. They do not require a material medium, unlike mechanical waves (like sound or elastic waves). The electric and magnetic fields are perpendicular to each other and to the direction of wave propagation.
The speed of electromagnetic waves in a vacuum is a fundamental constant, denoted by ‘c’, approximately 299,792,458 meters per second, often rounded to 3 x 10^8 m/s or 300,000 km/s (3 lakh km/s). The spectrum of electromagnetic waves includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays, all differing in wavelength and frequency but sharing these fundamental characteristics.

13. Which one of the following waves does not belong to the category of th

Which one of the following waves does not belong to the category of the other three ?

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X-rays
Microwaves
Radio waves
Sound waves
This question was previously asked in
UPSC NDA-2 – 2017
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The correct answer is Sound waves.
X-rays, microwaves, and radio waves are all forms of electromagnetic waves. They are part of the electromagnetic spectrum, travel at the speed of light in a vacuum, and do not require a medium for propagation. Sound waves, on the other hand, are mechanical waves. They are vibrations that travel through a medium (like air, water, or solids) and require a medium to propagate.
Electromagnetic waves are transverse waves caused by oscillations of electric and magnetic fields. Mechanical waves like sound waves can be longitudinal or transverse and are caused by the vibration of particles in a medium. This fundamental difference in nature and propagation makes sound waves distinct from the other three types of waves listed.

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14. Which one among the following waves carries the maximum energy per pho

Which one among the following waves carries the maximum energy per photon ?

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X-rays
Radio waves
Light waves
Microwaves
This question was previously asked in
UPSC NDA-2 – 2017
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The energy carried by a single photon is directly proportional to the frequency of the electromagnetic wave. The relationship is given by the equation E = hf, where E is the energy of the photon, h is Planck’s constant, and f is the frequency of the wave. Comparing the given options based on their typical frequencies in the electromagnetic spectrum (from lowest frequency/energy to highest): Radio waves < Microwaves < Light waves (visible light) < X-rays. Therefore, X-rays have the highest frequency and carry the maximum energy per photon among the listed options.
– Photon energy is directly proportional to wave frequency (E = hf).
– X-rays have higher frequency than radio waves, microwaves, and visible light.
– Higher frequency means higher energy per photon.
The order of the electromagnetic spectrum by increasing frequency and energy is: Radio waves, Microwaves, Infrared radiation, Visible light, Ultraviolet radiation, X-rays, Gamma rays. This spectrum spans a vast range of frequencies and energies, each part having different properties and applications.

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15. Which one of the following waves is used for detecting forgery in curr

Which one of the following waves is used for detecting forgery in currency notes ?

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Ultraviolet waves
Infrared waves
Radio waves
Microwaves
This question was previously asked in
UPSC NDA-2 – 2017
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Many modern currency notes incorporate security features that are visible under ultraviolet (UV) light. These features often include fluorescent threads, watermarks, or specific inks that glow or change color when exposed to UV radiation. Checking currency notes under UV light is a common method used to detect counterfeits that lack these specialized security features.
– Ultraviolet (UV) light is used to reveal fluorescent security features in currency notes.
– These features are difficult to replicate by counterfeiters.
Infrared (IR) waves are also used in some advanced currency verification systems, as some inks have specific absorption or reflection properties in the IR spectrum. However, UV light inspection is a more common and basic method for detecting forgery in widely circulated currency. Radio waves and Microwaves are not typically used for visual security checks on currency notes.

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16. X-rays are electromagnetic radiation whose wavelengths are of the orde

X-rays are electromagnetic radiation whose wavelengths are of the order of:

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1 metre
10^-7 metre
10^-10 metre
10^-15 metre
This question was previously asked in
UPSC NDA-2 – 2015
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X-rays are a form of electromagnetic radiation with wavelengths ranging approximately from 10 picometers ($10^{-11}$ m) to 10 nanometers ($10^{-8}$ m). The option $10^{-10}$ metre falls squarely within this range.
X-rays occupy a position in the electromagnetic spectrum between ultraviolet radiation and gamma rays, characterized by wavelengths in the range of approximately $10^{-11}$ m to $10^{-8}$ m.
The electromagnetic spectrum spans a wide range of wavelengths and frequencies. Radio waves have wavelengths of metres to kilometres. Microwaves are around centimetres to metres. Infrared radiation is roughly from $7 \times 10^{-7}$ m to $10^{-3}$ m. Visible light is from $4 \times 10^{-7}$ m (violet) to $7 \times 10^{-7}$ m (red). Ultraviolet radiation is from $10^{-8}$ m to $4 \times 10^{-7}$ m. X-rays are from $10^{-11}$ m to $10^{-8}$ m. Gamma rays have wavelengths less than $10^{-12}$ m.

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17. Which one of the following statements about X-rays is not true ?

Which one of the following statements about X-rays is not true ?

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They have wavelengths of about 1 Å.
These can be generated by bombarding a metal target by high energy electrons.
Due to their wavelengths being shorter, these can be used for radar systems.
These are also used for the treatment of certain forms of cancer.
This question was previously asked in
UPSC NDA-1 – 2023
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Let’s analyze each statement about X-rays:
A) They have wavelengths of about 1 Å. X-rays are part of the electromagnetic spectrum with wavelengths typically ranging from 0.01 to 10 nanometers (0.1 to 100 Å). So, a wavelength of 1 Å falls within this range. This statement is true.
B) These can be generated by bombarding a metal target by high energy electrons. This is the standard method for producing X-rays in an X-ray tube (using bremsstrahlung radiation and characteristic X-ray emission). This statement is true.
C) Due to their wavelengths being shorter, these can be used for radar systems. Radar systems work by transmitting electromagnetic waves and detecting the reflected signals. They typically use radio waves or microwaves (wavelengths from centimeters to meters). X-rays have much shorter wavelengths and high energy, which makes them highly penetrating. They are not suitable for radar applications, which rely on reflection from objects in the atmosphere or space. This statement is NOT true.
D) These are also used for the treatment of certain forms of cancer. High-energy X-rays are a common form of radiation therapy used to kill cancer cells. This statement is true.
Therefore, the statement that is not true about X-rays is C.
This question tests knowledge about the properties and applications of X-rays, which are part of the electromagnetic spectrum.
X-rays are used in various fields including medical imaging (radiography, CT scans), industrial inspection, material analysis (X-ray crystallography), and security screening. Their penetrating power makes them useful for imaging internal structures.

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18. A sound wave has a frequency of 1 kHz and wavelength 50 cm. How long w

A sound wave has a frequency of 1 kHz and wavelength 50 cm. How long will it take to travel 1 km?

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5 s
4 s
3 s
2 s
This question was previously asked in
UPSC NDA-1 – 2022
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The correct answer is 2 seconds. The time taken to travel a certain distance can be calculated if the velocity of the wave is known.
– The relationship between the velocity (v), frequency (f), and wavelength (λ) of a wave is given by the formula v = fλ.
– Given frequency f = 1 kHz = 1000 Hz.
– Given wavelength λ = 50 cm = 0.50 m.
– Velocity v = (1000 Hz) × (0.50 m) = 500 m/s.
– Distance to be travelled = 1 km = 1000 m.
– Time taken (t) = Distance / Velocity.
– t = 1000 m / 500 m/s = 2 seconds.
Frequency is the number of wave cycles passing a point per second, measured in Hertz (Hz). Wavelength is the distance between two consecutive corresponding points on a wave. Velocity is the speed at which the wave propagates through the medium.

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19. ‘Beats’ is a phenomenon that occurs when frequencies of two harmonic w

‘Beats’ is a phenomenon that occurs when frequencies of two harmonic waves are

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equal.
far apart.
multiples of each other.
nearly same.
This question was previously asked in
UPSC NDA-1 – 2021
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The correct answer is (D) nearly same.
‘Beats’ is a phenomenon that occurs when two waves of slightly different frequencies interfere with each other. This interference results in periodic variations in the intensity of the combined wave, heard as a waxing and waning of sound (or seen as flickering in light).
The beat frequency is the difference between the frequencies of the two individual waves (f_beat = |f1 – f2|). For distinct beats to be perceived, the frequency difference must be relatively small (typically less than about 7-10 Hz for sound waves). If the frequencies are exactly equal, they interfere constructively or destructively to produce a wave of constant amplitude. If the frequencies are far apart, the rapid variations in intensity are not perceived as beats.

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20. Which of the following statements about electromagnetic waves, sound w

Which of the following statements about electromagnetic waves, sound waves and water waves is/are correct ?

  • 1. They exhibit reflection
  • 2. They carry energy
  • 3. They exert pressure
  • 4. They can travel in vacuum

Select the correct answer using the code given below :

1, 2 and 3
2 and 4
1 and 3 only
1 only
This question was previously asked in
UPSC NDA-1 – 2018
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The correct answer is A.
Let’s examine each statement for electromagnetic waves, sound waves, and water waves:
1. They exhibit reflection: Yes, all three types of waves can be reflected when they encounter a boundary. (e.g., light reflection from a mirror, sound reflection creating an echo, water waves reflecting off a wall). Correct.
2. They carry energy: Yes, all waves are disturbances that transfer energy from one place to another. Correct.
3. They exert pressure: Electromagnetic waves exert radiation pressure. Sound waves are essentially pressure variations in a medium. Water waves exert pressure on objects they encounter. Correct.
4. They can travel in vacuum: Electromagnetic waves (like light, radio waves) can travel through a vacuum. However, sound waves require a material medium (like air, water, or solids) to propagate and cannot travel in a vacuum. Water waves also require water as a medium. Incorrect.

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Therefore, statements 1, 2, and 3 are correct for all three types of waves, while statement 4 is only correct for electromagnetic waves.

Electromagnetic waves are transverse waves. Sound waves are longitudinal waves (in fluids) or can be both longitudinal and transverse (in solids). Water waves are complex, often a combination of transverse and longitudinal motions, particularly surface waves. The ability to travel in vacuum is a key distinction between electromagnetic waves and mechanical waves (like sound and water waves), which require a medium.

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