Cardboard life hacks

In a class, teacher gave two cardboard pieces having equal area which are in the shape of a parallelogram to two groups. Find h.

Letters A, B, C and D are written on a cardboard as shown in the figure below. The cardboard is kept at a suitable distance behind a transparent empty glass of cylindrical shape. If the glass is now filled with water, one sees an inverted image of the pattern on the cardboard when looking through the glass. Ignoring magnification effects, the image would appear as

A source S and a detector D are placed at a distance d apart. A big cardboard is placed at a distance sqrt(2) d from the source and the detector as shown in figure. The source emits a wave of wavelength = d//2 which is received by the detector after reflection from the cardboard. It is found to be in phase with the direct wave received from the source. By what minimum distance should the cardboard be shifted away so that the reflected wave becomes out of phase with the direct wave?

Raman has a square cardboard of area 4sq.ft, from which he clips away a small square of area 1 sq.ft ,from one of the corners.Now,if he wants to cut up the remaining piece of cardboard into exactly four identical smaller pieces,the perimeter of each smaller piece is:

If we put a cardboard (say 20 cm xx 20 cm) between a light source and our eyes, we can't see the light. But when we put the same cardboard between a sound source and our ear, we hear the sound almost clearly, Explain.

A tiny mirror M is fixed on a piece of cardboard placed on a table. The cardboard is illuminated by light from a bulb. The position of eye with respect to position of bulb is shown in Fig.16.2 as A, B, C and D. In which position mirror will be visible?