wave motion (urgent)

1. Yes, the lowest frequency is 50 Hz. Statement 3 is wrong.

2. The peak of P appears earlier in time than the peak of Q (notice that on a displacement-time graph, the peak nearer to the origin ( t = 0) occurs earlier than one that occurs farther away from the origin). Hence, P leads Q.

3. The graphs shown on the "model answer" and "my answer" are wrong. At the instant shown, particle A has already passed the "peak" and is returning to the equilibrium position. Hence, the curve should start (at t = 0) with a +ve displacement after the peak.

The following animation gives a good show on how the particles move:

http://faraday.physics.utoronto.ca/IYearLab/Intros/StandingWaves/Flash/long_wave.html

4. When one end is free to move, there is only half of a "loop" at the lowest frequency. But one "loop" equals to half a wavelength, hence "half loop" equals to one-quarter of a wavelength.

Following from above, fundamental frequency corresponds to one "half loop", hence "1.5 loop" consists of 3 "half loops", i.e. 3x fundamental frequency.

5. As far as the light incident onto the grating is well collimated (this is to ensure normal incidence), the distance of the light source from the grating is not crucial.

6. Although the two fromulae looks similar, the physics concept behind are different. The "no. of antinodal lines" case is light interference from two coherent sources, which is not a diffraction effect that occurs in grating.

7. No. It is reflection, as it follows the Law of reflection.


2011-09-16 14:02:33 補充：
3. Yes.
4. The free end is an antinode. From an antinode to a node (the fixed end), there is 1/4 wavelength.