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