wave motion

1. The answer is option B.
Because a real image is formed at Q, and hence it can be produced on a screen.

2. The focus of L2 overlaps with that of L1 at a point between the two lenses. Parallel light rays thus meet at the common focus after passing through L2. These rays will be refracted parallel at L1. Because the distance of L2 from the common focus is shorter than that from L1, the rays are then widely separated.

3. The answer is option C.
Constructive interference is produced along the path of the receiver.

4. Diffraction is prominent when the wavelength is longer than the dimension of the barrier.
Reflection of wave depends on the reflectivity of the reflecting material.

2013-12-30 10:40:27 補充：
Your supl questions:
1. The image is formed at Q. One cannot see the image if his eyes are at Q, as the distance of distinct vision is about 25 cm. That means your eyes must be at least 25 cm behind Q.
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2013-12-30 10:40:55 補充：
You also cannot see the image at distance in front of Q, as the image is not yet formed at that position.

2013-12-30 10:46:36 補充：
2. You need to consider every given situation one by one.
Case 1: this could only be possible if the two foci overlap to the left of L4. But it is not always guaranteed unless the focal length of L1 is long enough.
Case 2: Ray diagram shows that the emergent ray would not be parallel.

2013-12-30 10:48:40 補充：
Case 4: The emergent rays are parallel, but their separations are smaller than the incident rays. This can be shown on drawing a ray diagram.