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Explain the experimental method to determine the wavelength of spectral line using diffraction grating.

What is the highest order spectrum which can be seen with monochromatic light of wavelength 6000 A by means of a diffraction grating with 5000 lines/cm?

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• The diffraction grating is often used in the laboratories for the determination of wavelength of light.
• The grating spectrum of a given source of monochromatic light is obtained by using spectrometer. The arrangement is as shown above.
• The spectrometer is first adjusted for parallel rays. The grating is then placed on the prism table and adjusted for normal incidence.
• In the some destruction as that of the incident light the direct image of the slit or the zero order spectrum can be seen in the telescope.
• On either side of the direct image a symmetric diffraction pattern consisting of different orders can be seen.
• The angle of diffraction θ for a particular order 'n' 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

a + b = 1/(number of lines per cm) = 2.54/no of lines per cm

• Thus using the equation

(a+b)sinθ = nλ

• The unknown wave/length λ can be calculated by putting the value of the grating element (a+b) the order 'n' and the angle of diffraction θ