What is grating and grating element?
A grating is an arrangement consisting of a large number of parallel slits of same width and separated by equal opaque spaces.It is obtained by ruling equidistant parallel lines on a glass plate with the help of a diamond.
The lines act as opaque spaces and the incident light cannot pass through them.The space between the two lines is transparent to light and acts as a slit.The spacing between the lines is of the order of wavelength of visible light.The number of a lines in a plane transmission grating is of the order of 15000 to 20000 per inch.
The width of each slit is a and opaque spacing between two consecutive slits is b.
(a+b) is called grating element or grating constant.
It can be seen that distance between two consecutive slits is grating element.