Conceptual Questions on Interference of Waves

1. To demonstrate the interference of waves, Thomas Young allowed light from a single illuminated slit to fall on a pair of narrowly spaced slits. Explain
a. Why light from two separate lamp bulbs can never produce an interference pattern
b. Why double slits used as above give an interference pattern
How would the interference pattern differ if
c. The double slits had been closer together

2. Why must two loudspeakers be producing sounds of exactly the same frequency in order for us to hear the effects of interference?
3. Two loudspeakers are placed 4 m apart for an open-air concert. They are playing back a flute sounding a note of 680 Hz. Members of the audience sit in a row, 20 m from the loudspeakers, parallel to the line between the loudspeakers.
Take the speed of sound as 340 m s–1.
Describe, as precisely as possible, what different people in the row will hear.
4. When a spherical surface is put in contact with an optically flat plate, circular interference patterns, called Newton's rings, can be observed in reflected light. Why there is a dark spot at the center of the pattern of Newton's rings?
5.  When sunlight reflects from a thin film of soapy water (held against the force of gravity), the film appears multicolored at the bottom and dark at the top. Why?
OR
The top portion of a soapy water film on a vertical loop appears black when viewed by reflected light. Why?
6. In Young’s double slit experiment, what changes would occur to the fringe width if the wavelength of the incident laser light is changed from green to red?
7. Explain what happens to Newton’s rings when (i) monochromatic light source is replaced by white light source (ii) the plane glass plate is replaced by silver glass plate (iii) the centre of plano convex lens is not in contact with the plane glass plate and (iv) some oil is placed between the glass plate and lens.  
8. What is the role of compensating glass plate in Michelson interferometer?
9. A camera lens is covered with a nonreflective coating that eliminates the reflection of perpendicularly incident green light. If the light were incident on the nonreflective coating at 45°, rather than perpendicularly, it would not be eliminated by the coating and an observer would see it.
10. Why it is not possible to see interference fringes when the wavelength of the light is greater than the distance between the slits.
11. Can two independent light sources (say sodium lamps or candles) produce observable interference pattern on screen? Why should the distance between the two coherent sources be small and the distance between sources and screen be large?
12. Two coherent light waves arrive at a particular point on a screen. The optical path difference between the waves is 3m. Determine the nature of interference at the point if the wavelength of the wave is 3900 Å.
13. If we are to observe interference in a thin film, why must the film not be very thick (with thickness only on the order of a few wavelengths)?