A person at the beach looking vertically down onto the sand can just distinguish individual grains from one and other. Given that her eyes are 2 m above the sand, the pupils of her eyes are 3 mm in diameter and assuming a wavelength of 555 nm for the light (ie the middle of the visible spectrum) determine the size of the grains. Assume also that her eyes are perfect lenses and any atmospheric effects are negligible.

Q7) In Young’s twin slit interference experiment, the formulae for constructive and destructive interference both contain the term m which is an integer. In the case of constructive interference, substituting m = 3 allows you to find the position of the third bright fringe above (or below) the central maximum but for destructive interference m = 3 finds the fourth dark fringe. Explain this difference.