EXPERIMENT NO. 3 Apparatus used: Spectrometer, diffraction grating, mercury ly magnifying glass, sprit level ele. Formula used; The wavelength of light a is given by the following grating formula: (a+b)sinO =n, (a+b)sin@ n ll) where (a +b) = grating element n= order of spectrum angle of diffraction ‘number of rulings per inch on the grating, then or a was) iP: F----4 0 ef nid W2 Grating | Convex lens |4¢—Screen . m Procedure: ‘eS fA) Adjustment Of spectrometer: (1) Adjust the sp ectometer ae shen ase yar tn gegen, i it Sk the following adjustments:optics 405 (i) (ii) ii) (B) 2) (3) 4) 6) (6) a” «) (8) ‘The axis of the telescope and of collimator must intersect the principal axis of rotation of telescope. Prism table should be made horizontal using a sprit level. Telescope and collimator are adjusted for parallel rays. Adjustment of grating for normal incidence: Mount the grating on the prism table such that it faces the incident light, clamp the prism table. Collimator and telescope are arranged in a line and the image of the slit is focussed on the cross wire. Take the reading () of one of the vernier scale (V,). Rotate the telescope through 90°. it means now reading becomes (6 +90°) (Fig. 3.2]. Prism table Grating Source § = Collimator C Telescope (T) Fig, 3.2 Adjustment of grating for normal incidence Release prism table and turn it from this position through 45° or 135°. In this position the grating is normal to the incident beam of light. Clamp the prism table. Rotate the telescope to the left and right side of direct image turn by turn to get the first order diffraction, Determination of diffraction angle (8): Rotate the telescope to the lett side of direct imag: spectral line of violet colour on the vertical cross wire for I“ order. Note down the readi of both the vgrniers V; and imilarly, adjust the different spectral lines turn by turn. oa the cross wire and take the reading of V, and V3. and adjust thePRACTICAL Phiyg, Ry 106 : a further to obtain the second orders Spectrun, te the telescope on the vertical cross wire ang note thy 6) esate tne epee eS { again ne sa sam of the direct image and 2 to the right 0! imag ey tO Ne anit ie fist ‘order as well as for second order 7 te [} above procedun | j came spectral lines. arate teal s ifference of the same kind of 3 feadings for g | | _ = ore on first order and then in the second order, this na | reese of 20 for that particular colour (Fig. 3.3). Half of it wilt i the je ot 2 | , ance of ifiastion 0, fr tat order. ae , { x | ec gST iy . if é X Prism table | ve Telescope (T) Central on f Sede, maximum y Be, My t cd Fig. 33 Determination of diffraction angle q 03) Finally ‘0 determine the grating element (a+b) , note the number of rulings per inch on grating (N), 13 — (13) Calculate the wavelength of each spectral line for first and second order of diffraction Separately using i n swavelength ofeach colour 'Y using equation (2) and then meat 1 Observations: ose CollimstorC —— Gratlag ()) Determination of angle of diffraction : te pectrum on fet | Spectrum on right Side of dir prec iect image | side ofuirect image | 29 er MSKivig] Total) | Tea fe al 7 Mselvsal Tesal (a2) cee) (d) SERRSa Rec pene rere siiaiitiaermnemttol PTICS 107 vi 1 V2 vi Ve | vi v: Green| Viole Second. Red @ e 4 6 (6) Reading after rotating the prism table through 45° or 135°=. ") Number of rulings per inch on the grating. N=... ) Least count of spectrometer ) Reading of telescope for direct image ) Reading of telescope after rotating it through ($+90°)=..... Calculations: a GB 4) ) IFN = number of rulings per inch on gr: 54 Grating element (a + b) = N ) The wavelength of various spectral lines in the first order spectrum can be calculated by equation (2) with n =1 ) Similarly, the wavelength of various spectral lines in the second order spectrum can be calculated by equation (2) on substituting the values of 8 with n=2 2.54. = sin® Nx2 }) Calculate the values of wavelength for each spectral line using above relations in A.U. ri A ‘Mean Mercury spectral Wavelength (4) A I wae Hine From n=1 (a) From (b) | KE=S-A | 2 Violet | Violet Ut Blue ; GreenPRACTICAL pyy Sy 08 a 1 ee | | ~ el i | Yedow 8 i { i | L Rei , J a y light for various spectrat fing welengths of mercury lig 1S ay Reentt: The wavelens ven matable - Viet Blse i ron i Yalow Yellow fl Red Precantions and sources of error: (1) Ths grating surface should not be touched by fingers, but hold it from ie edge ited (2) Before performing the experiment, the spectrometer should be adjusted. 18) The adjustment of grating for normal incidence must be pad czzefully_ An error in this causes appreciable deviations in corte ‘sult Therefore, use of grating in minimum deviation position i recommended 1) While taking observations, clamped (5) Reading of both the verniers is essential. This eliminates the error due 40 the exis of rotation not passing through the centre of the gradulat circle telescope and prism table should be Vive Voce What do you mean by wavelengaiy grating? i 2 3. What is « plane diffrectio 4 iffraction of Tigh? What do you mean ty di Bei dcceeotirnOPTICS 5s 6. 403 What is the difference between interference and diffraction? What is the difference hetween prism and grating spectrum? Which spectrum is more intense and why? What is grating element? What do you mean by the dispersive power of a grating ard on what factors does it depend?