If coherent waves of equal amplitude have travelled different paths and the difference between their paths is half a wavelength, or an odd number of half wavelengths, then they will be half a wavelength out of phase and the result will be complete destructive interference
Light from ordinary sources is not polarised because the light vibrations occur in all directions perpendicular to the direction in which the light is travelling
A piece of transparent material on which a very large number of parallel lines are engraved. The spaces between the lines behave as slits and allow the light to pass through.