diffraction of light explained
When light passes a narrow opening or a sharp edge it does not go straight. It bends and spreads. The intensity pattern of one opening is drawn as sinc², and that is the classical intensity of light, not a probability density. A diffraction grating of many openings in a row, and Huygens's picture of little wavelets, is a story of its own. The opening is what bends the light aside.
When light passes one opening of width a, a bright central peak stands on the screen and dark minima sit to either side. The minima appear at a·sinθ = mλ, and the intensity curve has the shape sinc². The narrower the opening, the wider the diffraction spreads. The first minimum sits at sinθ = λ/a, so the spread is fixed by the ratio of wavelength to width, and when the opening narrows to about a wavelength the light spreads almost in a half circle. Narrower means a wider spread.
Huygens said that every point on a wavefront is the source of a new wavelet. Add the wavelets from the many sources inside the opening, direction by direction, and they reinforce and cancel, and that builds the diffraction pattern. Pack the sources tighter and their sum gathers into a smooth sinc² envelope. Straight ahead, every wavelet arrives in the same phase and they add without remainder, but off to the side the phases slip a little and they cancel. That is why the center is the brightest. The forward direction is where the phases agree.
A diffraction grating is many openings lined up at a regular spacing. As the number of openings grows, the main peaks stay in the same places and grow sharper and brighter. The place of a peak is fixed by the spacing of neighboring openings, d·sinθ = mλ. The more openings, the more completely the light between the peaks cancels, so the line width narrows in proportion to 1/N, and a spectrometer engraves a great many lines to split wavelengths more finely. More openings make a sharper line.
Even on the same grating, a different wavelength diffracts at a different angle. In d·sinθ = mλ a longer wavelength means a larger angle, so the colors spread like a fan. Splitting light by wavelength like this is the heart of spectroscopy. When the wavelength exceeds the spacing, that order cannot appear. A disc glints with rainbow colors because its fine grooves act as a grating and split white light by wavelength. The grooves of a DVD are that grating. The grooves split the colors.
Now move a probe directly over the pattern. Read the angle at each place, and with the integer in a·sinθ = mλ you can check which minimum that place is. At the center the intensity is greatest, and a dark minimum sits at every integer, regularly. Between the dark minima sit side peaks that grow fainter and fainter. Those are the side ripples of the sinc² curve, and their intensity is far weaker than the central peak. The side peaks are paler than the center.