Why diffraction grating have closely spaced rulings?
A diffraction grating is a glass plate having a large number of close parallel equidistant slits mechanically ruled on it.A diffraction grating is used to study the diffraction effects of light.
Since the phenomenon of diffraction is found to be prominent when the wavelength of light is large as compared with the size of the opening or aperture of the slit.Since the wavelength of visible light is very small ranging from 400 nm to 700 nm,so the diffraction grating have closely spaced rulings in order to compare the size of the incident light.A typical diffraction grating has about 400 to 5000 rulings per centimeter.
Difference between interference and diffraction
Diffraction of light