The diffraction effect of such a large slit is not so strong. Of course, there will be diffraction effect. What finally hits the grating is indeed zero-order diffracted light, but diffraction is not an effect, but an inevitable process produced by using slits.

If an extended light source such as mercury lamp is used (that is, it is not a point light source), the spatial coherence of the light source will be poor if it is directly incident. If such light enters the grating, it will be the superposition of countless groups of grating diffraction fringes (if the mercury lamp is regarded as many point light sources, the grating diffraction fringes produced by each point light source are related to the position of the point light source, and the superposition effect is messy), so it can not be used well. Grating +6544.

The purpose of adding slits is to make light sources such as mercury lamps be regarded as point light sources (as a whole, as slit light sources) in the restricted direction of slits (also the diffraction direction of gratings), so that diffraction fringes of gratings can be seen at the exit end, and light with a specific wavelength can be extracted and measured by adding slits at the exit end.

The angle between the entrance and exit slits and the grating, * * *, forms a strict diffraction optical path, so that we can accurately know the entrance and exit angles from the exit end when the light diffracts on the grating, and thus we can accurately calculate the wavelength or frequency.

If a laser is used as the light source, in principle, the parallel light generated by the beam expanding system can be used for experiments. However, in this experiment, the original slit-grating-slit system is still simpler and more effective, so the instrument structure can be used as an extended light source without deliberate change. (Of course, laser is usually incident on a narrow-band high-resolution monochromator because of its narrow linewidth, and its spectral distribution is measured [very narrow].)

As for preventing the incidence of stray light, it is also the edge of the slit, and here we should pay attention to the edge of the slit, its plane part should face the direction of light, and the wedge part should face the direction of light, which can also reduce stray light to a certain extent.