Explain how a bright line is formed by the diffraction grating at the first order diffraction angle

A diffraction grating is basically a series of very small, point-like light sources, where the adjacent ones are always a given distance away. To have a bright line, the light waves from all of the point-like sources must interfere constructively. This happens when the path difference between the adjacent light rays is an integer multiple of the wavelength. Therefore, there will a bright line for all the angles for which the path difference is the 0, the wavelength, twice the wavelength, and so on. Another name for the integer multiple is called the order of diffraction. Hence, at the first order diffraction angle, the path difference between the adjacent rays is exactly the wavelength of the light we use, so the light from each source on the grating will interfere constructively. This results in a bright line.