Describe how the wavelength of monochromatic light can be measured using a diffraction grating and a spectrometer.

Describe how the wavelength of monochromatic light can be measured using a diffraction grating and a spectrometer.

Solution

Make the following adjustment

• The collimator C is adjusted such that parallel rays emerge from it.
• The telescope is adjusted so that parallel rays entering it are brought to a focus at the cross wire near its eyepiece.
• The table is levelled so that the plane of P is parallel to the axis of rotation of the telescope.

• The grating is placed on the table so that its plane is normal to incident light
• The readings of the first diffraction image are observed on both sides of the normal at T₁ and T₂.
• The angular difference is 2θ, and the wavelength is calculated from λ = d sin θ, where d is the spacing of the slits, obtained from the number of lines per centimeter of the grating.
• If a second order image is obtained for a diffraction angle θ, then λ = d sin θ/2

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