LIGHT USING DIFFRACTION GRATING AND SPECTROMETER AIM To find the wavelength of the constituent colours of a composite light using diffraction grating and spectrometer. APPARATUS REQUIRED Spectrometer, mercury vapour lamp, diffraction grating, grating table, and spirit level. FORMULA λ θ = sin nN å where, λ → Wavelength of the constituent colours of a composite light (å) N → Number of lines per metre length of the given grating (No unit) (the value of N for the grating is given) n → Order of the diffraction (No unit) θ → Angle of diffraction (degree) DIAGRAMS T T T G ° T G Figure (a) Normal incidence Figure (b) Angle of diffraction T G G G Y Y PROCEDURE ) Initial adjustments of the spectrometer • Eye-piece: The eye-piece of the telescope is adjusted so that the cross-wires are seen clearly. • Slit: The slit of the collimator is adjusted such that it is very thin and vertical.
• Base of the spectrometer: The base of the spectrometer is adjusted to be horizontal using leveling screws. ( ) ( ) ( ) ( ) XII- - ) XII- - ) - - - - PRACTICAL • Telescope: The telescope is turned towards a distant object and is adjusted till the clear image of the distant object is seen. Now the telescope is adjusted to receive parallel rays. • Collimator: The telescope is brought in line with the collimator.
Collimator is adjusted until a clear image of the slit is seen in the telescope. Now the collimator gives parallel rays. • Grating table: Using a spirit level, the grating table is adjusted to be horizontal with the three leveling screws provided in the grating table. ) Adjustment of the grating for normal incidence • The slit is illuminated with a composite light (white light) from mercury vapour lamp.
• The telescope is brought in line with the collimator. The vertical cross-wire is made to coin- cide with the image of the slit (Figure (a) ). • The vernier disc alone is rotated till the vernier scale