Reflection of Waves - UNSOLVED PRACTICE SET

Q7. Explain what happens when a transverse wave pulse on a string reflects from:

(i) A fixed end

(ii) A free end

Draw diagrams showing the incident and reflected pulses.

Q8. Why does a wave reflected from a fixed end undergo a phase change of π, while reflection from a free end involves no phase change?

Q9. Define echo. What are the conditions necessary for hearing a distinct echo?

Q10. A person stands 85 m from a cliff and claps. Calculate the time after which he hears the echo. (Speed of sound = 340 m/s)

Q11. Explain the difference between reflection of sound from a hard surface and a soft surface. Which surface produces a clearer echo?

Q12. A sonar device sends a sound pulse underwater and receives the reflected pulse after 2 seconds. Calculate the depth of the water if the speed of sound in water is 1500 m/s.

Q13. With the help of clear diagrams, explain the reflection of waves from:

(i) A rigid boundary (fixed end)

(ii) A free boundary (free end)

For each case, discuss:

The direction of the reflected wave

The phase change (if any)

The amplitude of the reflected wave

The formation of nodes and antinodes when incident and reflected waves superpose

Q14. Discuss the laws of reflection of waves. Show that:

(i) The angle of incidence equals the angle of reflection

(ii) The incident wave, reflected wave, and the normal to the surface all lie in the same plane

Apply these laws to explain:

(a) Why a concave surface focuses sound

(b) Why whispering galleries work

(c) Why megaphones are shaped like cones

Q15. A student sets up an experiment to study reflection of sound waves. She uses two tubes arranged at an angle to a reflecting surface.

(i) Describe how she can verify the laws of reflection of sound.

(ii) How would she demonstrate that sound follows the same laws of reflection as light?

(iii) She places a soft cushion at the reflecting surface. What happens to the reflected sound? Explain.

(iv) Design an experiment to measure the speed of sound using reflection (echo method).

Q16. A sound wave of frequency 500 Hz is sent towards a wall and the echo is heard after 0.4 s.

(i) Calculate the distance between the source and the wall.

(ii) Calculate the wavelength of the sound wave. (Speed of sound = 340 m/s)

(iii) If the temperature increases by 20°C, how does the time to hear the echo change?

(iv) A person starts walking towards the wall at 1 m/s at the instant the sound is produced. After how much time will he hear the echo?

(v) At what minimum distance from the wall should he stand to hear a distinct echo?

Q17. A submarine uses sonar to detect objects underwater. It emits a sound pulse of frequency 40 kHz.

(i) Calculate the wavelength of the sound in water. (Speed of sound in water = 1500 m/s)

(ii) The echo is received after 3 seconds. Calculate the distance to the object.

(iii) If the object is moving towards the submarine at 10 m/s, explain how the frequency of the reflected pulse changes.

(iv) Calculate the apparent frequency of the reflected pulse received by the submarine. (Use Doppler effect formula)

Q18. In a school auditorium, a student stands at one end and speaks. The auditorium is 50 m long, 20 m wide, and 10 m high. The walls are made of concrete, the floor is marble, and the ceiling has acoustic panels.

(i) Calculate the time for an echo from the back wall. (Speed of sound = 340 m/s)

(ii) Calculate the time for an echo from the side wall.

(iii) Explain why multiple echoes are heard in the auditorium.

(iv) The acoustic panels on the ceiling are made of porous material. Explain how they reduce echoes.

(v) A teacher suggests hanging curtains on the side walls. How would this affect the echoes? Explain using the concept of reflection coefficient.