An idealised grating is made up of a set of slits of spacing "d", that must be wider than the wavelength of interest to cause diffraction. Assuming a plane wave of monochromatic light of wavelength "λ" with normal incidence (perpendicular to the grating), each slit in the grating acts as a quasi point-source from which light propagates in all directions (although this is typically limited to a hemisphere). After light interacts with the grating, the diffracted light is composed of the sum of interfering wave components emanating from each slit in the grating. At any given point in space through which diffracted light may pass, the path length to each slit in the grating varies. Since path length varies, generally, so do the phases of the waves at that point from each of the slits. Thus, they add or subtract from each other to create peaks and valleys through additive and destructive interference. When the path difference between the light from adjacent slits is equal to half the wavelength, , the waves are out of phase, and thus cancel each other to create points of minimum intensity. Similarly, when the path difference is "λ", the phases add together and maxima occur. For a beam incident normally on a grating, the maxima occur at angles "θ"m, which satisfy the relationship = | "m" |, where "θ"m is the angle between the diffracted ray and the grating's normal vector, and "d" is the distance from the center of one slit to the center of the adjacent slit, and "m" is an integer representing the propagation-mode of interest.