1 A diffraction grating is an arrangement consisting of a large number of parallel slits of same width and separated by equal opaque spaces.

2 It is obtained by ruling equidistant parallel lines on a glass with the help of a diamond.

3 The lines act as opaque spaces and and the incident light cannot pass through them. The space between the two lines is transported to light and acts as a slit.

4 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 inches.

Advantages of increasing the number of lines in the grating are:

a The number of principle maxima that can be seen on a screen increase.

b The distance between two adjacent principle maxima increases.