Grating diffraction

1. Grating diffraction

The diffraction fringes of grating refer to the comprehensive result of single slit diffraction and multi slit interference

1.1 Superposition of single slit diffraction

Give a conclusion , In Fraunhofer diffraction experiment , If the position of the lens does not change , Just a small upward movement of the single seam , Then the diffraction pattern on the light screen will not change . That is, the result of single slit diffraction is independent of the position of single slit , It depends on the position of the lens .

We know , After the parallel light irradiates the single seam , Due to different diffraction angles , A single slit diffraction fringe is formed on the screen by focusing through a lens

Those who have N A slit grating , Under the irradiation of parallel light , Each slit produces its own diffraction pattern , Although the position of each slit is different , But the position of the lens is fixed , therefore N The single slit diffraction fringes of the group will be superimposed through the lens , Like the diffraction fringes formed by a single slit .

1.2 Multi slit diffraction

Because all the slits are on the same wave front , The wavelet emitted from all the corresponding points of the two adjacent slots arrives on the screen P The optical path difference of points is equal , So the light passing through all the slits is coherent , On screen P Coherent superposition also occurs at the point , Form interference fringes , This is multi slit interference .

Let the total number of slots of the transmission grating be N, The seam width is a, The light tight width between the seams is b,（a+b）=d,d It is called a raster constant .
Among them a It is given that the seam width is a Light intensity diagram of single slit diffraction pattern
chart b The light intensity distribution of multi slit interference pattern with infinite slit width is given .
The total light intensity of grating diffraction determined by multi slit interference and single slit diffraction is shown in the figure c Shown .
The intensity of interference fringes is modulated by single slit diffraction .
If the number of grating slots is large , The width of each seam is very small , The central bright zone of single slit diffraction will become very wide , We have observed the grating diffraction pattern , It is the diffraction beam of each slit, the interference fringes in the central bright grain region of single slit diffraction .

1.3 Grating equation

1.4 Lack of rank