Since you put so many questions in one post, I could only give brief answers within the given limit of words.
1. Use Right-Hand Srew Rule to find direction. The resultant field points to the upper left corner of the square.
Magnitude can be found using Ampere's Law.
2. The aperture is the minimum diameter of the region on the lens such that you could see the whole image.
3. The "diffraction" graph indeed shows a resultant light intensity distribution from diffraction and interference. The intensity of the central fringe is max, with decrease in intensities on two sides.
4. The reason is already given in your answer. It is not due to diffraction effect. The metal mesh attenuates the electric field components of the microwave because of electromagentic induction. The so-called "Faraday Cage" effect.
5. The region shown in option A is reached by waves from the two slits, hence interference occurs with the formation of the fringes.
6. It can be shown that the speed of sound v is given by
v = square-root[RT/M]
where R is the Universal Gas Constant (=8.31 J/mol.K)
T is the absolute temperature
M is the molar mass of the medium
Air has a molar mass of about 29 kg/mol, whereas water is 18 kg/mol.
7. The highest sound intensity occurs at the point that is directly opposite to S1, i.e. at the foot of the perpendicular from S1 to the line OQ.
Point O is at a longer distance than the perpendicular distance.
2014-09-20 17:00:16 補充：
Some correction to Q6
The speed of sound v = square-root[kRT/M] where k is the adiabatic index
The molar masses for air and water are respectively 0.029 kg/mol and 0.018 kg/mol.
2014-09-20 17:03:07 補充：
Therefore, if other conditions are equal, speed of sound in a high molar mass medium (e.g. air) is lower than that in a small molar mass medium (e.g. water).
2014-09-24 16:17:40 補充：
Your auppl questions
1. Magnetic field at X = 4.(4.pi x 10^-7).(2.5^2).cos(45)/(2.pi x 0.1768) T = 2 x10^-5 T
where 0.1768 m is the distance from X to a corner of the swuare.
The direction is vertically upward.
(sorry there is a mistake in my original answer)
2014-09-24 16:19:17 補充：
See 意見column for further answers
2014-09-24 16:22:58 補充：
5. The fringes occur on the ENTIRE light overlapping region where interference takes place.