NB 'a' is the aperture size - intensity maxima for the single slit pattern envelope occur at the straight through position (principal maximum) and thereafter half way between the minima. The minima occur at multiples of  /a.

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Here are links to interactive programs for you to explore the effect of single and double slit diffraction on particles as well as photons!.

See also diffraction gratings - basically multiple slits!

Question

A beam of green light is diffracted by a slit of width 0.550 mm. The diffraction pattern forms on a wall 2.06 m beyond the slit. The distance between the positions of zero intensity on both sides of the central bright fringe is 4.10 mm. Calculate the wavelength of the laser light in nm.