# Assertion and Reason Questions for Class 11 Physics Chapter 15 Waves

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Assertion and Reason Questions for Class 11 Physics Chapter 15 Waves

Directions:
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.

Q.1. Assertion : Two persons on the surface of moon cannot talk to each other.
Reason : There is no atmosphere on moon.

Answer Answer: (a) Sound waves require material medium to travel. As there is no atmosphere (vacuum) on the surface of moon, therefore the sound waves cannot reach from one person to another.

Q.2. Assertion : The velocity of sound increases with increase in humidity.
Reason : Velocity of sound does not depend upon the medium.

Q.3. Assertion : Ocean waves hitting a beach are always found to be nearly normal to the shore.
Reason : Ocean waves are longitudinal waves.

Answer Answer: (c) Ocean waves are transverse waves travelling in concentric circles of ever increasing radius. When they hit the shore, their radius of curvature is so large that they can be treated as plane waves. Hence they hit the shore nearly normal to the shore.

Q.4. Assertion : Transverse waves are not produced in liquids and gases.
Reason : Light waves are transverse waves.

Answer Answer: (b) Transverse waves travel in the form of crests and troughs involving change in shape of the medium. As liquids and gases do not possess the elasticity of shape, therefore, transverse waves cannot be produced in liquid and gases. Also light wave is one example of transverse wave.

Q.5. Assertion : Compression and rarefaction involve changes in density and pressure.
Reason : When particles are compressed, density of medium increases and when they are rarefied, density of medium decreases.

Answer Answer: (a) A compression is a region of medium in which particles come closer i.e., distance between the particles becomes less than the normal distance between them. Thus there is a temporary decrease in volume and a consequent increase in density of medium. Similarly in rarefaction, particle get farther apart and a consequent decrease in density.

Q.6. Assertion : Sound would travel faster on a hot summer day than on a cold winter day.
Reason : Velocity of sound is directly proportional to the square of its absolute temperature.

Q.7. Assertion : The basic of Laplace correction was that, exchange of heat between the region of compression and rarefaction in air is not possible.
Reason : Air is a bad conductor of heat and velocity of sound in air is large.

Answer Answer: (c) According to Laplace, the changes in pressure and volume of a gas, when sound waves propagated through it, are not isothermal, but adiabatic. A gas is a bad conductor of heat. It does not allow the free exchange of heat between compressed layer, rarefied layer and surrounding.

Q.8. Assertion : The flash of lightening is seen before the sound of thunder is heard.
Reason : Speed of sound is greater than speed of light

Answer Answer: (c) Speed of light is greater than that of sound, hence flash of lightening is seen before the sound of thunder.

Q.9. Assertion : When we start filling an empty bucket with water, the pitch of sound produced goes on decreasing.
Reason : The frequency of man voice is usually higher than that of woman.

Q.10. Assertion : A tuning fork is made of an alloy of steel, nickel and chromium.
Reason : The alloy of steel, nickel and chromium is called elinvar.

Answer Answer: (b) A tuning fork is made of a material for which elasticity does not change. Since the alloy of nickel, steel and chromium (elinvar) has constant elasticity, therefore it is used for the preparation of tuning fork.

Q.11. Assertion : The change in air pressure effect the speed of sound.
Reason : The speed of sound in a gas is inversely proportional to square root of pressure.

Q.12. Assertion : Solids can support both longitudinal and transverse waves but only longitudinal waves can propagate in gases.
Reason : For the propagation of transverse waves, medium must also necessarily have the property of rigidity.

Answer Answer: (a) For the propagation of transverse waves, medium must have the property of rigidity. Because gases have no rigidity, (they do not posses shear elasticity), hence transverse waves cannot be produced is gases. On the other hand, the solids possess both volume and shear elasticity and likewise both the longitudinal and transverse waves can be transmitted through them.

Q.13. Assertion : Under given conditions of pressure and temperature, sound travels faster in a monoatomic gas than in diatomic gas.
Reason : Opposition for wave to travel is more in diatomic gas than monoatomic gas.

Q.14. Assertion : The speed of sound in solids is maximum though their density is large.
Reason : The coefficient of elasticity of solid is large.

Q.15. Assertion : On a rainy day sound travel slower than on a dry day.
Reason : When moisture is present in air the density of air increases.

Answer Answer: (d) When moisture is present in air, the density of air decreases. It is because the density of water vapours is less than that of dry air. The velocity of sound is inversely proportional to the square root of density, hence sound travel faster in moist air than in the dry air. Therefore, on a rainy day sound travels faster than on a dry day.

Q.16. Assertion : To hear distinct beats, difference in frequencies of two sources should be less than 10.
Reason : More the number of beats per sec more difficult to hear them.

Q.17. Assertion : Sound produced by an open organ pipe is richer than the sound produced by a closed organ pipe.
Reason : Outside air can enter the pipe from both ends, in case of open organ pipe.

Answer Answer: (b) Sound produced by an open organ pipe is richer because it contains all harmonics and frequency of fundamental note in an open organ pipe is twice the fundamental frequency in a closed organ pipe of same length.
Reason is also correct, but it is not explaining the assertion.

Q.18. Assertion : It is not possible to have interference between the waves produced by two violins.
Reason : For interference of two waves the phase difference between the waves must remain constant.

Answer Answer: (a) Since the initial phase difference between the two waves coming from different violins changes, therefore, the waves produced by two different violins does not interfere because two waves interfere only when the phase difference between them remain constant throughout.

Q.19. Assertion : Beats can also be observed by two light sources as in sound.
Reason : Light sources have constant phase deference.

Answer Answer: (d) As emission of light from atom is a random and rapid phenomenon. The phase at a point due to two independent light source will change rapidly and randomly. Therefore, instead of beats, we shall get uniform intensity. However if light sources are LASER beams of nearly equal frequencies, it may possible to observe the phenomenon of beats in light.

Q.20. Assertion : In the case of a stationary wave, a person hear a loud sound at the nodes as compared to the antinodes.
Reason : In a stationary wave all the particles of the medium vibrate in phase.

Answer Answer: (c) The person will hear the loud sound at nodes than at antinodes. We know that at anti-nodes the displacement is maximum and pressure change is minimum while at nodes the displacement is zero and pressure change is maximum. The sound is heard due to variation of pressure.
Also in stationary waves particles in two different segment vibrates in opposite phase.

Q.21. Assertion : Velocity of particles, while crossing mean position (in stationary waves) varies from maximum at antinodes to zero at nodes.
Reason : Amplitude of vibration at antinodes is maximum and at nodes, the amplitude is zero, And all particles between two successive nodes cross the mean position together.

Q.22. Assertion : Where two vibrating tuning forks having frequencies 256 Hz and 512 Hz are held near each other, beats cannot be heard.
Reason : The principle of superposition is valid only if the frequencies of the oscillators are nearly equal.

Answer Answer: (c) The principle of superposition does not state that the frequencies of the oscillation should be nearly equal. For beats to be heard the condition is that difference in frequencies of the two oscillations should not be more than 10 times per seconds for a normal human ear to recognize it. Hence we cannot hear beats in the case of two tuning forks vibrating at frequencies 256 Hz and 512 Hz respectively.

Q.23. Assertion : The fundamental frequency of an open organ pipe increases as the temperature is increased.
Reason : As the temperature increases, the velocity of sound increases more rapidly than length of the pipe.

Q.24. Assertion : Sound waves cannot propagate through vacuum but light waves can.
Reason : Sound waves cannot be polarised but light waves can be polarised.

Answer Answer: (b) Sound waves cannot propagate through vacuum because sound waves are mechanical waves. Light waves can propagate through vacuum because light waves are electromagnetic waves. Since sound waves are longitudinal waves, the particles moves in the direction of propagation, therefore these waves cannot be polarised.

Q.25. Assertion : Sound travel faster in solids than gases.
Reason : Solid possess greater density than gases.

Q.26. Assertion : Like sound, light can not propagate in vacuum.
Reason : Sound is a square wave. It propagates in a medium by a virtue of damping oscillation .

Q.27. Assertion : When a beetle moves along the sand with in a few tens of centimeters of a sand scorpion the scorpion immediately turn towards the beetle and dashes to it
Reason : When a beetle disturbs the sand, it sends pulses along the sands surface one set of pulses is longitudinal while other set is transverse.