Two identical narrow slits are illuminated separately, first by an ordinary filament bulb and then by a single-mode laser of the same average wavelength. In a separate trial, the slits are fed by two different lasers that happen to emit the same wavelength but are not phase-locked to each other. Answer the following questions.
(a) Explain why a stable interference pattern is not observed with the ordinary filament bulb even though it covers the required wavelength range.
(b) In the case of the single-mode laser, identify the property that allows sustained interference and explain its role.
(c) Give one qualitative reason why two independent, non-phase-locked lasers of identical wavelength still fail to produce permanent fringes on the screen.