Diffraction of light, when light waves encounter obstacles equal to or less than its wavelength, can bypass obstacles. When encountering a single slit, bright lines appear on the screen after diffraction, which darken from the middle to both sides in turn. Grating diffraction can obtain a row of bright stripes with alternating light and dark and uniform brightness.
The number of slits in the grating is very large, generally tens to thousands per millimeter. The diffraction of monochromatic parallel light through the slits of the grating and the interference between the slits form patterns of wide dark stripes and thin bright stripes, which are called spectral lines. The position of the spectral line varies with the wavelength. When polychromatic light passes through the grating, spectral lines with different wavelengths appear in different positions, forming spectra.
The spectrum formed by light passing through grating is the same result of single-slit diffraction and multi-slit interference. It can be considered as a group of infinitely long and narrow slits with equal spacing, and the spacing between the slits is d, which is called grating constant. When the plane wave with incident wavelength is vertically incident on the grating, the points on each slit act as secondary wave sources, and the light emitted by these secondary wave sources propagates in all directions.
Because the slit is infinitely long, we can only consider the situation on the plane perpendicular to the slit, that is, simplify the slit into a row of points on the plane. Then the light emitted by each slit is coherently superimposed to form a light field in a certain direction on the plane.
Principle of diffraction grating:
Usually, diffraction gratings work based on Fraunhofer multi-slit diffraction effect. The formula describing the relationship between grating structure and incident angle and diffraction angle of light is called grating equation. When the wave propagates, every point on the wave front can be regarded as an independent secondary wave source; These secondary wave sources emit spherical secondary waves again, and then the wavefront at a certain moment in the future is the envelope surface of these spherical secondary waves at that moment.
When interference occurs, because the phase of the light emitted by each slit is different at the interference point, they will partially or completely cancel each other out. However, when the optical path difference between two adjacent slits and the interference point is an integer multiple of the wavelength of light, the phase of the two beams of light is the same, and interference enhancement will occur.