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**Diffraction grating:**

A diffraction grating is an arrangement consisting of a large number of parallel slits of same width and separated by equal opaque spacing. It is obtained by ruling equidistant parallel lines on a glass plate with the help of a fine diamond point.

**Grating element:**

Let N be the number of parallel slits, each of width 'a' and separated by opaque space 'b'. Then, the distance between the centers of the adjacent slits is d=a+b and is known as grating element.

**Determination of wavelength of the spectral line using diffraction grating:**

The diffraction grating is often used in the Lab for the determination of wavelength of light. The grating spectrum of the given source of monochromatic light is obtained by using a spectrometer as shown -

Symmetrical diffraction pattern consisting of different orders can be seen. The angle of diffraction θ for a particular order 'm' of the spectrum is measured.

The number of lines per inch of grating are written over it by the manufacturer. Hence the grating element is

Thus, using equation **(a+b)sinθ=mλ**

The unknown wavelength ′λ′ of the spectral line can be calculated by putting the value of the grating element (a+b), the order 'm' and the angle of diffraction ′θ′