Class 10 Physics – Chapter 12: Waves

Board: Punjab Board
Subject: Physics
Class: 10th

A. Multiple Choice Questions (MCQs)

12.1 The direction of vibration in a longitudinal wave is:

(a) At right angles to the wave direction
(b) In circular motion
(c) Opposite to the wave direction
(d) In the same direction as the wave travels

Correct Answer: (d) In the same direction as the wave travels

12.2 Which part of a transverse wave carries the lowest point?

(a) Crest
(b) Compression
(c) Rarefaction
(d) Trough

Correct Answer: (d) Trough

12.3 Which of the following types of waves requires a medium to travel?

(a) X-rays
(b) Light waves
(c) Radio waves
(d) Sound waves

Correct Answer: (d) Sound waves

12.4 The property of a wave that indicates how much energy it carries is:

(a) Crest
(b) Amplitude
(c) Wavelength
(d) Speed

Correct Answer: (b) Amplitude

12.5 A wavefront is:

(a) The distance between two crests
(b) A region of high pressure in a wave
(c) A line joining points that vibrate in phase
(d) The outer edge of a wave medium

Correct Answer: (c) A line joining points that vibrate in phase

12.6 The wave phenomenon that occurs when waves change direction due to a change in speed is:

(a) Diffraction
(b) Reflection
(c) Absorption
(d) Refraction

Correct Answer: (d) Refraction

12.7 What happens to wave speed and wavelength when a tsunami approaches shallow water?

(a) Speed increases, wavelength increases
(b) Speed decreases, wavelength shortens
(c) Speed remains the same, wavelength shortens
(d) Speed increases, wavelength stays constant

Correct Answer: (b) Speed decreases, wavelength shortens

12.8 Which wave behaviour explains how sound is heard around a corner?

(a) Reflection
(b) Refraction
(c) Interference
(d) Diffraction

Correct Answer: (d) Diffraction

B. Short Answer Questions

12.1 What do waves transfer from one place to another?

Answer:

Waves transfer energy and momentum from one place to another without transferring matter.

12.2 Name any two examples of mechanical waves.

Answer:

Sound waves
Water waves (or waves on a string)

12.3 Which type of wave can travel through a vacuum?

Answer:

Electromagnetic waves, such as light waves, radio waves, and X-rays, can travel through a vacuum because they do not require a material medium.

12.4 What is the difference between transverse and longitudinal waves?

Answer:

Transverse Waves: The particles of the medium vibrate perpendicularly (at \(90^\circ\)) to the direction of wave propagation. Examples include waves on a string and electromagnetic waves.
Longitudinal Waves: The particles of the medium vibrate parallel to the direction of wave propagation, forming compressions and rarefactions. Sound waves are an example.

12.5 What is meant by wavelength?

Answer:

Wavelength (\(\lambda\)) is the distance between two consecutive identical points in phase on a wave, such as the distance between two successive crests or troughs in a transverse wave.

12.6 Write the definition of frequency.

Answer:

Frequency (\(f\)) is the number of complete waves or cycles produced per unit time. Its SI unit is hertz (Hz or \(\text{s}^{-1}\)).

12.7 What causes a tsunami in the ocean?

Answer:

A tsunami is caused by the sudden, large-scale displacement of ocean water, typically resulting from underwater earthquakes, volcanic eruptions, or submarine landslides.

C. Constructed Response Questions

12.1 Why can sound not travel through space, but light can? Explain using the concept of wave types.

Answer:

Sound is a mechanical longitudinal wave that relies on the vibration and interaction of neighbouring material particles (atoms or molecules) to transmit energy. Space is a vacuum containing virtually no particles, so sound cannot propagate through it.
Light is an electromagnetic wave consisting of oscillating electric and magnetic fields. It does not require a material medium and can travel through a vacuum.

12.2 A student shakes one end of a rope and observes waves moving along it. What kind of wave is this, and how do the particles move?

Answer:

Kind of wave: It is a transverse mechanical wave.
Particle motion: The individual particles of the rope vibrate perpendicularly to the direction in which the wave travels along the rope.

12.3 How does the amplitude of a wave relate to the energy it carries? Give an example to support your answer.

Answer:

The energy carried by a wave is directly proportional to the square of its amplitude:

\[ E \propto A^2 \]

When the amplitude of a wave increases, the energy it carries increases significantly.

Example: Small ocean waves carry relatively little energy, whereas large storm waves can carry much more energy and cause significant damage.

12.4 Why do we still hear a person talking even if they are behind a wall? Explain using the concept of diffraction.

Answer:

Sound waves can bend around the edges of obstacles due to diffraction. When sound waves encounter a wall or corner, they spread into the region beyond the obstacle, allowing us to hear someone speaking even when they are not in our direct line of sight.

12.5 If the size of a slit through which a wave passes is much smaller than the wavelength, what will the wave be like after passing through the slit? Explain briefly.

Answer:

When the slit size is much smaller than or comparable to the wavelength, diffraction becomes pronounced. The wave spreads out widely after passing through the narrow opening, producing curved wavefronts.

D. Comprehensive Questions

12.1 Explain how waves transfer energy without transferring matter. Give examples from daily life.

Answer:

Waves represent the propagation of a disturbance. As a wave travels through a medium, individual particles oscillate about their mean positions rather than travelling along with the wave. Energy is transferred from particle to particle, while the medium does not undergo bulk movement with the wave.

Daily-life examples:

Dropping a pebble into water creates ripples that carry energy outward. A floating leaf bobs up and down as the ripples pass.
A sound wave travels across a room and carries sound energy to a listener’s ear without transporting air molecules all the way across the room.

12.2 Describe the difference between mechanical and electromagnetic waves with suitable examples.

Answer:

Feature	Mechanical Waves	Electromagnetic Waves
Medium requirement	Require a material medium (solid, liquid, or gas) to travel.	Can travel through a vacuum and do not require a material medium.
Nature of vibration	Involve oscillations of material particles.	Involve oscillating electric and magnetic fields.
Speed	Depends on the medium. For example, sound travels at approximately \(340\ \text{m s}^{-1}\) in air.	Travel at the speed of light, \(3 \times 10^8\ \text{m s}^{-1}\), in a vacuum.
Examples	Sound waves, water waves, and seismic waves.	Light waves, radio waves, microwaves, and X-rays.

12.3 What are transverse and longitudinal waves? Explain each with the help of diagrams and examples.

Answer:

1. Transverse Waves

Description: Transverse waves are waves in which particle displacement is perpendicular to the direction of wave travel.
They consist of alternating crests (highest points) and troughs (lowest points).
Example: Waves on a stretched string.

2. Longitudinal Waves

Description: Longitudinal waves are waves in which particle displacement is parallel to the direction of wave travel.
They consist of alternating compressions, where particles are close together, and rarefactions, where particles are spread apart.
Example: Sound waves travelling through air.

12.4 Discuss the properties of waves, such as reflection, refraction, and diffraction. Support your answer with examples.

Answer:

1. Reflection

Reflection occurs when a wave strikes a barrier or boundary and bounces back into the same medium. It follows the law of reflection:

\[ \text{Angle of incidence} = \text{Angle of reflection} \]

Example: An echo produced when sound reflects from a distant cliff.

2. Refraction

Refraction is the bending of a wave when it passes from one medium into another at an angle due to a change in its speed and wavelength.

Example: Light bending as it enters a glass prism or a pool of water.

3. Diffraction

Diffraction is the bending or spreading of waves around the edges of obstacles or through narrow openings.

Example: Hearing someone speaking from around a hallway corner.

12.5 What is diffraction? How do wavelength and slit size affect the diffraction of waves?

Answer:

Diffraction is the bending of waves around obstacles or the spreading of waves as they pass through an opening or slit.

Effect of wavelength and slit size:

Slit size comparable to or smaller than the wavelength: Diffraction is pronounced, and the wave spreads out into wide curved wavefronts after passing through the slit.
Slit size much larger than the wavelength: Diffraction is less pronounced, and the wave continues mainly in a straight path, with slight bending at the edges.

E. Numerical Problems

12.1 The wavelength of a wave is 2 metres and its frequency is 5 Hz. What is the speed of the wave?

Given Data:

Wavelength: \(\lambda = 2\ \text{m}\)
Frequency: \(f = 5\ \text{Hz}\)

To Find:

Speed of the wave, \(v = ?\)

Formula:

\[ v = f\lambda \]

Calculation:

\[ v = 5 \times 2 \]

\[ v = 10\ \text{m s}^{-1} \]

Result: The speed of the wave is \(10\ \text{m s}^{-1}\).

12.2 A wave travels at a speed of 300 m s⁻¹ and has a wavelength of 3 metres. Calculate its frequency.

Given Data:

Speed: \(v = 300\ \text{m s}^{-1}\)
Wavelength: \(\lambda = 3\ \text{m}\)

To Find:

Frequency, \(f = ?\)

Formula:

\[ v = f\lambda \]

Rearranging:

\[ f = \frac{v}{\lambda} \]

Calculation:

\[ f = \frac{300}{3} \]

\[ f = 100\ \text{Hz} \]

Result: The frequency of the wave is \(100\ \text{Hz}\).

12.3 A sound wave has a frequency of 250 Hz and a speed of 340 m s⁻¹. Find its wavelength.

Given Data:

Frequency: \(f = 250\ \text{Hz}\)
Speed: \(v = 340\ \text{m s}^{-1}\)

To Find:

Wavelength, \(\lambda = ?\)

Formula:

\[ v = f\lambda \]

Rearranging:

\[ \lambda = \frac{v}{f} \]

Calculation:

\[ \lambda = \frac{340}{250} \]

\[ \lambda = 1.36\ \text{m} \]

Result: The wavelength of the sound wave is \(1.36\ \text{m}\).

12.4 A wave has a frequency of 50 Hz. How much time does it take to complete one wave?

Given Data:

– Frequency: \(f = 50\ \text{Hz}\)

To Find:

Time period, \(T = ?\)

Formula:

\[ T = \frac{1}{f} \]

Calculation:

\[ T = \frac{1}{50} \]

\[ T = 0.02\ \text{s} \]

Result: The time taken to complete one wave is \(0.02\ \text{s}\).

12.5 A wave has a wavelength of 0.5 m and a frequency of 200 Hz. Calculate the speed of the wave.

Given Data:

Wavelength: \(\lambda = 0.5\ \text{m}\)
Frequency: \(f = 200\ \text{Hz}\)

To Find:

Speed, \(v = ?\)

Formula:

\[ v = f\lambda \]

Calculation:

\[ v = 200 \times 0.5 \]

\[ v = 100\ \text{m s}^{-1} \]

Result: The speed of the wave is \(100\ \text{m s}^{-1}\).

12.6 The time period of a wave is 0.02 seconds. Calculate the frequency of the wave.

Given Data:

– Time period: \(T = 0.02\ \text{s}\)

To Find:

Frequency, \(f = ?\)

Formula:

\[ f = \frac{1}{T} \]

Calculation:

\[ f = \frac{1}{0.02} \]

\[ f = 50\ \text{Hz} \]

Result: The frequency of the wave is \(50\ \text{Hz}\).

Notes by Sir Dilawar

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