CBSE 11 Phy Ch-15 Waves (VSQ)

Q.1. What type of mechanical wave exists in air and inside water?

Answer Answer: Longitudinal waves exist in both air and inside the water.

Q.2. What is the phase difference between the wave y = a cos(wt + kx) and y = a sin (wt + kx)?

Answer Answer: Phase difference is 90°

Q.3. What characteristics of a medium determine the speed of sound waves though it?

Answer Answer: The speed of sound waves in a medium is determined (i) elasticity and (ii) density of the medium.

Q.4. What is the effect on the velocity of waves in a string if only 1/4th of the original length is used with the same tension?

Answer Answer:

Q.5. Why should the difference between the frequencies be less than 10, to produce beats?

Answer Answer: Human ear cannot identify any change in intensity of sound in less than 1/10 of a second due to persistence of hearing. So, the difference should be less than 10.

Q.6. Velocity of sound in air at N.T.P. is 332 m/s. What will be the velocity, when pressure is doubled and temperature is kept constant?

Answer Answer:

Q.7. Why is it possible to detect the approach of a train by placing the ear upon the rails of track?

Answer Answer: The speed of sound is more in solids than in air. So, the sound of train approaches faster through rails than air.

Q.8. Is the phenomenon of beats observable in case of two light waves of nearly equal frequencies?

Answer Answer: No, as the phase difference of two independent light sources changes randomly and round only at a given position.

Q.9. Why is sound heard in water more intense in comparison to sound heard in air?

Answer Answer: This is because intensity of sound increases with increase in density of the medium and the density of liquid is more than that of air.

Q.10. How is the vibration of the air column in a flute different from that of a string?

Answer Answer: The nodes in a sitar string are replaced by the antinodes in a flute.

Q.11. What is an echo ? What should be the minimum distance between the source of sound and the reflector for hearing a distinct echo ?

Answer Answer: Echo is the phenomenon of repetition of sound due to its reflection from the surface of a large obstacle. If ‘s’ be the distance between the source and reflector, ‘v’ the velocity of sound and ‘t’ be the total time taken by sound to reach the listener after the reflection, then
2s = vt or s=vt/2= 340/20 = 17 m

Q.12. Sometimes, in a stringed instrument, a thick wire is wrapped by a thin wire. Why?

Answer Answer: This increases mass per unit length and hence helps in obtaining a desired low frequency.

Q.13. If oil of density higher than the density of water is used in a resonance tube, how will the frequency change?

Answer Answer: The frequency of vibration depends on the length of the air column. The liquid surface only causes the reflection of waves. Hence frequency will change if oil of density higher than that of water is used in the resonance tube.

Q.14. Why are there so many holes in a flute?

Answer Answer: The flute is basically an open organ pipe. The location of the open end can be changed by keeping the one hole open and closing the other holes. Thus the frequency of the note produced by the flute can be changed.

Q.15. An open pipe makes a good musical instrument, in comparison to a closed pipe. Why?

Answer Answer: In open pipe, all harmonics are present. So, the sound is of higher quality than a closed pipe where only odd harmonics are present.

Q.16. What is the difference between a tone and a note?

Answer Answer: A sound of single frequency is called a tone. A combination of tones of different frequencies is called a note.

Q.17. As in sound, can beats be observed by two light sources?

Answer Answer: No, to observe beats by two light-sources the phase difference between the sources should change regularly. In light sources, phase difference changes at random because the light-source consists of a large number of atoms and each atom emits wave independently, so we cannot observe the beats.

Q.18. A violin note and a sitar note may have the same frequency and yet we can distinguish between the two notes. Explain, why it is so.

Answer Answer: Two instruments of same fundamental frequency produce different overtones and hence have different sounds. Therefore, we can differentiate between the Sitar and Violin notes.

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