physics wave

1. The largest angle of a is 90 degrees, under which the refracted angle equals to the critical angle c, say. After reflection at the plane mirror, the angle incident onto the liquid-gas boundary also equals to c. This gives an emergent ray with angle 90 degrees.

Since angle a cannot be greater than 90 deg., an emergent ray thus always exists.

2. Let D be the distance between the source and screen, and x be the distance of the microphone from the source when it is placed at A.

Thus, path difference between direct and reflected waves
= (2d-x)-x
= 2d - 2x
Hence, for minimum sginal, 2d - 2x = m入 + 入/2
where m is an integer
When the microphone is placed at B,
path difference = [2d - (x+2)] - (x+2)]
= 2d -2x -4
Hence, 2d - 2x -4 = (m-30)入 + 入/2 for mimimum signal at B
i.e. (m入+入/2) - 4 = m入 - 30入 + 入/2
4 = 30入
入 = 4/30 m
Speed of sound = 4000 x 4/30 m/s = 533 m/s
The answer should be option C.

3. The same equation can be used if the situation meets the assumptions in that equation, i.e. the minima are close to the centre of line LM.

4. Stationary wave is formed by two identical travelling waves moving in opposite directions. The "speed" here refers to the speed of that travelling wave.




2013-12-18 12:20:39 補充：
Your suppl question:
There is only ONE source.
The wave directly trave from source to microphone is of distance = x
The indirect wave, travels a distance of d from source to the reflector, and then reflected and travels to the microphone a distance (d-x).

2013-12-18 12:21:29 補充：
(cont'd)...Hence, total distance travelled by the indirect wave = d + (d-x) = 2d - x
Path difference = (2d - x) - x