Question

Why it is impossible too observe an interference pattern with two independent bulbs

Answer 1

Interference is possible when:
1. The both light should have same wavelength.
2. They should have a constant path(which in turn causes phase) difference, same frequency.
3. They should be coherent. 

So for better result we consider monochromatic waves that's two waves from a single wave by Young's double slit experiment. It satisfies above conditions.