phy wave

1. The correct answer is option B.
Compressions and rarefactions are alternate in positions in a wave and both of them travel together (to the right in this case) producing a wave motion.

2. The answer is option C.
For a wave crest to move more than one wavelength, the time required is longer than before. Hence, the stroboscope has to decreases its rotation speed.
Simialrly, for a crest to move less than one wavelength, the time needed is shorter than before, the stroboscope thus has to rotate faster.

3. The answer is option C.
Option 1 is correct. In a concave lens, a real object produces a virtual image. Hence by the Principle of Reversibility of Light, a virtual object will produce a real image.

Option 2 is wrong.
Light rays intersect at the focus will be refracted parallel to the principal axis after passing through the lens.

Option 3 is correct.
A virtual object could produce a virtual image.

4. The rays are incident at 45 degrees to each of the plane reflectors. After reflection two times at each of the plane reflectors, the rays will be reflected 180 degrees and thus travelling in the opposite direction to the original rays.