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Chapter 8 Waves Physics 11th Class Punjab Short Questions

Question No. 1 - Differentiate between travelling waves and stationary waves.

Answer No. 1 - A waves which transfer energy by moving away from the source of disturbance, is called a travelling wave. The ripples produced in the water are the examples of travelling waves.
Two waves of equal frequency travelling in opposite direction produce stationary waves. In stationary waves energy cannot flow past the nodes and remains "standing" in the medium between nodes. Waves produced in a stretched string and air column are the examples of stationary waves.

Question No. 2 - Can Doppler Effect be applied to electromagnetic waves? Give an example.

Answer No. 2 - Yes, Doppler effect can be applied to electromagnetic waves. For example, in radar systems, the Doppler effect is used to determine the elevation and speed of aeroplane.

Question No. 3 - Define transverse waves, Give two examples.

Answer No. 3 - Transverse waves are those in which particles of the medium are displaced in a direction perpendicular to the direction of propagation of waves.
Waves produced in a stretched string and ripple produced in water are good examples of transverse waves.

Question No. 4 - Why can micro waves not detect under water objects?

Answer No. 4 - Microwave are strongly absorbed by sea water within feet of their transmission Ultrasonic is a high frequency sound wave. Its is not part of electromagnetic spectrum. They are preferred on microwaves for the use of undersea communication because they can travel longer distance in water.

Question No. 5 - Explain the term "Beats".

Answer No. 5 - Two waves that are travelling in the same direction with a slight difference in frequencies will produce beats. Number of beats per second is equal to the difference in frequencies.

Question No. 6 - Differentiate between longitudinal and transverse waves.

Answer No. 6 - Longitudinal waves: A traveling wave that causes the elements of the medium to move parallel to thee direction of propagation is called a longitudinal wave.

Transverse waves: A traveling wave that causes the elements of the disturbed medium to move perpendicular to the direction of propagation is called a transverse wave.

Question No. 7 - Define Beat and Beat Frequency.

Answer No. 7 - Beat: Two waves of slightly different frequencies and travelling in the same direction produce beats.

Beat Frequency: Number of beats per second is called beat frequency which is equal to the difference in frequencies.

Question No. 8 - Which is richer in harmonic, and why: (A) An open organ pipe (B) A closed organ pipe.

Answer No. 8 - The pipe, which is open at both ends, is richer in harmonics.
At open end molecules of the air are free to move and an antinode is formed while the movement of air molecules is restricted at the closed end and a node is formed.
Therefore, the pipe open at both ends have antinode at each end and is richer in harmonics.

Question No. 9 - How should a source of sound move with.r.t an observer so that the frequency of its sound does not change?

Answer No. 9 - If sound source is moving in circular path with the observer at the center of the circle then relative velocity of the observer with respect to the source of sound is zero, there will be no change in the frequency of sound.

Question No. 10 - What do you mean by the term progressive waves?

Answer No. 10 - A wave which transfer energy by moving away from the source of disturbance is called progressive wave.For example, longitudinal and transverse waves.

Question No. 11 - What is radar?

Answer No. 11 - It is acronym for Radio Detection and Ranging. It is a device which transmits and receive radio waves which are used to determine height and speed of aeroplane.

Question No. 12 - What feature do longitudinal waves have in common with transverse waves?

Answer No. 12 - The common features are:
1. Both are mechanical waves.
2. Both transport energy from one place to another.
3. Both satisfy the equation

Question No. 13 - How stationary waves are produced in a medium.

Answer No. 13 - Theses waves are produced by the superposition of two identical waves traveling in opposite direction. When a stretched string clamped at its two ends is plucked then the stationary waves are produced.

Question No. 14 - Explain the term red shift and blue shift in Doppler Effect.

Answer No. 14 - Stars moving away from the Earth show red shift. The emitted waves have a longer wavelength than if the star had been at rest. So the spectrum is shifted towards longer wavelength.
Stars moving towards the Earth show blue shift. This is because the wavelength of light emitted by the star is shorter than if the star had been at rest. So the spectrum is shifted towards shorter wavelength.

Question No. 15 - Why ultrasonic waves are preferred on radio waves for the use of undersea communication?

Answer No. 15 - Ultrasonic is a high frequency sound wave. It is not a part of electromagnetic spectrum. Ultrasonic waves transmit energy from one place to another using elastic properties of matter. They are preferred on radio waves for the use of undersea communication because they can travel longer distances in water.

Question No. 16 - Why "stationary waves" are called standing waves?

Answer No. 16 - In stationary waves energy cannot flow past the nodes and remains "standing" in the medium between nodes. Therefore stationary waves are called standing waves.

Question No. 17 - What are the factors on which speed of sound in air depends?

Answer No. 17 - 1. Speed of sound is inversely proportional to the square
root of densities of gases.
2. Speed of sound increases with the increase in
temperature
3. Speed of sound is not affected by a variation in the
pressure of the gas.

Question No. 18 - What happens when a jet plane like a Concorde files faster than the speed of sound?

Answer No. 18 - A conical surface of concentrated sound energy sweeps over the ground as a supersonic plane passes overhead. It is known as sonic boom.

Question No. 19 - How are beats useful in tuning musical instruments?

Answer No. 19 - Beats are used in tuning musical instruments. One can use beats to tune a string of musical instrument such as plane by beating a not against a note oh known frequency.The string is then adjusted to the desired frequency by tightening or loosening it until no beats are heard.

Question No. 20 - What is meant by sonar? Explain.

Answer No. 20 - Sonar is an acronym derived from "sound navigation and ranging". Sonar is the name of technique for detecting the presence of objects under water by acoustical echo.
It employs the Doppler Effect, in which an apparent change in frequency occurs when the source and the observer are in relative motion, Its applications are detection of submarines, mine hunting and depth measurement of sea.

Question No. 21 - Give any two applications of Doppler Effect.

Answer No. 21 - Doppler effect is used:
1. In radar systems, the Doppler effect is used to
determine the elevation and speed of aero plane.
2. Astronomers use the Doppler Effect to calculate the
speed of distant stars and galaxies.

Question No. 22 - Define Mechanical and Electromagnetic waves.Give examples of each.

Answer No. 22 - Mechanical Waves: The waves which require any medium for their propagation by the oscillation of material particles are called mechanical waves e.g sound waves, water waves etc.

Electromagnetic waves: The waves which do not require any medium for their propagation are called electromagnetic waves. Fro example, visible light, radio waves, television signals, and x-rays.

Question No. 23 - Is it possible for two identical waves travelling in the same direction along the string to give rise to a stationary wave?

Answer No. 23 - No, it is not possible for two identical waves travelling in the same direction along a string to give rise to stationary waves, For stationary waves, no identical waves must travel in opposite direction.

Question No. 24 - What do you observe in the collective effect of dots in the form of a picture?

Answer No. 24 - We observe that the picture is made up of many closely spaced dots.
In case of mechanical waves, it is actually the effects of cooperative oscillations of a very large number of the particles of the medium through which the wave is passing.

Question No. 25 - Define Doppler Effect.

Answer No. 25 - The apparent change in the frequency of sound due to relative motion between the observer and source of sound is called Doppler Effect. If the observer and source of sound are approaching then the frequency of sound will increase and vice versa.

Question No. 26 - As the result of distant explosion an observer senses a ground tremor and then hears the explosions. Explain the time difference.

Answer No. 26 - The waves produced by the explosion reach the observer quickly through the ground as compared to the sound waves reaching through the air. This is due to the reason that sound travels faster in solid than gases.

Question No. 27 - A wave is produce along a stretched string but some of its particles permanently show zero displacement. What type of wave is it?

Answer No. 27 - A wave is produced along a stretched string but some of its particles permanently show zero displacement. It is a stationary wave and points at zero displacement are called nodes.

Question No. 28 - What is progressive wave? Give the name of two progressive waves.

Answer No. 28 - A wave which transfer energy by moving away from the source of disturbance is called progressive wave.

For Example, longitudinal and transverse waves.

Question No. 29 - Explain the terms Crest, Trough, Node and Anti-node.

Answer No. 29 - Crest: The portion of the wave above the mean level is called crest.
Trough: The portion of the wave below the mean level is called trough.
Nodes: The points of zero displacement in stationary waves are called nodes.
Anti-nodes: The points of maximum displacement in stationary waves are called antinodes.

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