t | wormat inet SPECTROMETER-GRATING AIM: To determine wavelength of various colors of light radiation using diffraction grating with normal incidence method. APPARATUS: plane transmission grating, spectrometer, mercury light, reading lens. FORMULA: Wavelength of prominent line in the mercury source A = 22 Where 6 = angle of diffraction n= order of spectrum N= number of lines per centimeter in grating procedure Moasuromont of A:- PROCEDURE: A) ADJUSTMENT OF THE GRATING FOR NORMAL INCIDENCE: ) Preliminary adjustments of spectrometer are made, focusing and adjusting the eye piece of tne telescope to a distant object. 2) The grating table is to be leveled with spirit level. 3) The slit of the collimator is illuminated with mercury vapor lamp. The direct reading is taken when the telescope is in front of the collimator. The telescope is turned from this position by 90° and fixed. 4) The grating is vertically mounted on the grating platform, the rulings on it being parallel to the slit in the collimator. 5) The platform is now rotated until the image of the slit as reflected by the glass surface is seen in the telescope. The vertical crosswire is made to coincide with the fixed edge of image. The platform is fixed in this position. The vernier table is now rotated in the appropriate direction through 45°, so that the rays of light from the collimator fall normally on the grating.8) WAVELENGTH OF THE SPECTRAL LINES OF THE MERCURY SPECTRUM: 1) The telescope is released and rotated to catch the first order diffraction image on one side, Say right or left as shown in fig.( ). 2) In the first order, readings are taken by coinciding the prominent lines namely violet, indigo, blue green, yellow, orange and red respectively with the coincidence of vertical cross wire. The Teadings of vernier-I and vernier-II are noted for each line. 3) Now the telescope is focused to the direct ray passing through the grating and vertical cross wire is set on direct ray. The readings in vernier-I and vernier —II are noted. 4) The difference of the readings correspond to vernier-| of direct ray reading and that of prominent line gives angle of diffraction(6,).similarly difference of vernier-ll of direct ray and that Of prominent line gives(@2). The average of (81) and (82) gives (8). 5) Similarly (8) correspond to all prominent lines are calculated. Sin@ values for all prominent lines are calculated, 6) The wavelengths of various colors are determined by using given formula. sind aN OBSERVATIONS: Least count of the spectrometer (L.C) = mes =30730= 1°. Number of lines per inch of grating (N) = 15000 LPI = 15000 / 2.54 =5905.5 cm Order of the diffraction(n) = 1. TABLE: ‘Reading Tor the affacied mage ‘iference Right Sige between ie mC] Spectral tenes a readings | Mean on tine varaiar® Vernier Veraier Vernier 2] a9 | 28 | slitaction | (colours) ca 0) wa Wa x cy | do |e | on eg WERT VSO] TR] WERT VSC] TR) MSR [WSC] TR] WSR [VSS] TR] Ys (cess | (ain | caoos | caea) | cai | car | cea) | cai | ceo | ter | cai | caeon | ceo | $2 PRECAUTIONS: 1) The grating should be held from the edges and ruled surface should not be touched. 2) The ruled surface should be face away from the collimator. 3) The slit and grating should be at same height. 1RESULT: The wavelength of various colors of given source is determined in normal incident method by using diffraction grating. . Violet (A) = . Indigo ( A) = . Blue (A) = . Green (A) = . yellow (A) Orange (A ) . Red (A)