In Young's double slit experiment with sodium vapour lamp of wavelength 589 nm and slit 0.589 mm apart, the half angular width of the central maxima is

Two identical light waves, propagating in the same direction, have a phase difference $$\delta$$. After they superpose the intensity of the resulting wave will be proportional to

A plastic sheet (refractive index = 1 6. ) covers one slit of a double slit arrangement for the Young’s experiment. When the double slit is illuminated by monochromatic light (wavelength = 5867 $$\mathop A\limits^o $$), the centre of the screen appears dark rather than bright. The minimum thickness of the plastic sheet to be used for this to happen is