# 2 questions on Wave Phase Differences [Diagram Included]!

1. Nov 15, 2009

### qazxsw11111

2 URGENT questions on Wave Phase Differences [Diagram Included]!

Two microwave sources S( and S, are situated as shown in Fig. 5.1. The waves emitted by the two sources are in phase and are polarised in the same plane.

http://img190.imageshack.us/img190/3924/wave1f.jpg [Broken]

A microwave detector is placed on a line XY which is parallel to, and 3.2 m from, the line joining S, and S2. M is the midpoint of the line joining S( and S2. The line from M perpendicular to the line S,S2 meets XY at O. The detector produces an output which is proportional to the displacement of the wave.
With only S, switched on, the change with time of the detector output measured at P, a distance of 5.0 cm from O, is as shown in Fig.
The waveform detected at P for S2 only is also shown on Fig.

Determine phase difference between the waves at P.

My solution:
According to my knowledge, phase difference= Δt/T x 2π
Hence, looking at the distance between the two peaks or two troughs, the time lag is about 0.2 (x10s) Dividing by period of 8.3 (x10s) you get around π/2. Is this correct? Is there a better method to do this?

Second part (Still same question):
S1 and S2 are switched on together, with the emitted waves in phase. The detector is moved from P along the line OY, in the direction away from O. State and explain the approximate distance that the detector must be moved before the intensity is a maximum, given that there is no maximum between O and P.

My solution:
Since constructive interference occurs, path difference=nλ. Since it is first maximum, n=1. But I do not know how to carry on. Anyone please help?

Another Question:

http://img687.imageshack.us/img687/446/wave2.jpg [Broken]

Fig shows a displacement-distance graph for two sound waves A and B. of the same frequency and amplitude. WaveA travelling to the right and wave B is travelling to the left.

(ii) The period of each wave is T. Determine the maximum displacement of the resultant of the two waves

1. at the instant shown.
2. at the instant shown +1/8T,
3. at the instant shown +3/8T.

Well guys, Im really unsure on how to do this question, with all the 1/8T and 3/8T which are confusing me. Anyone help?

Thanks.

Last edited by a moderator: May 4, 2017
2. Nov 16, 2009

### qazxsw11111

Re: 2 URGENT questions on Wave Phase Differences [Diagram Included]!

Bumpy?

3. Nov 16, 2009

### qazxsw11111

Re: 2 URGENT questions on Wave Phase Differences [Diagram Included]!

Anyone?

4. Nov 17, 2009

### qazxsw11111

Re: 2 URGENT questions on Wave Phase Differences [Diagram Included]!

Anyone can help? I need understanding about wave phases. And my exam is only a day away.

5. Nov 17, 2009

### qazxsw11111

Re: 2 URGENT questions on Wave Phase Differences [Diagram Included]!

Anyone?

6. Nov 18, 2009

### qazxsw11111

Re: 2 URGENT questions on Wave Phase Differences [Diagram Included]!

7. Nov 18, 2009

### ehild

Re: 2 URGENT questions on Wave Phase Differences [Diagram Included]!

First problem: Although you made a mistake in the period (it is 0.833 x 10-10 s) the method is all right, but a bit less accurate than to get the phase difference by calculation.

The phase of a wave is

$$2\pi(\frac{t}{T}-\frac{x}{\lambda})+\psi$$

The two sources emit in the same phase so the phase constant can be taken zero. The phase difference between the waves emerging from S1 and S2 is caused by the difference in length of path they travel from their source to the point P. Calculate the path difference, determine the wavelength and you get the phase difference.
In the second part, you know the phase difference, find the arrangement where the path difference is equal to the wavelength.

ehild