O6: The Diffraction Grating Spectrometer

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1 2B30: PRACTICAL ASTROPHYSICS FORMAL REPORT: O6: The Diffraction Grating Spectrometer Adam Hill Lab partner: G. Evans Tutor: Dr. Peter Storey 1

2 Abstract The calibration of a diffraction grating spectrometer using a source of known wavelength was made. The calibrated spectrometer was then used to determine the wavelengths of lines in the spectra of atomic hydrogen and other atoms. This was achieved by calibrating a transmission diffraction grating using a sodium spectral source, whose line wavelengths were well known. The spectral lines in atomic hydrogen and helium were then measured and their corresponding wavelengths calculated using the data obtained through the calibration. The experimental results yielded an approximation of the Rydberg constant, R, was found; R 1.09 x 10-7 m -1. 2

3 Contents Abstract 2 Contents 3 Introduction 4 Experimental Method 5 Experimental Data Diffraction Grating Calibration 7 Measuring the Sodium doublet 8 Measuring the Hydrogen Balmer series 9 Measuring the Helium spectral lines 10 Error Analysis 11 Summary of Results 11 Conclusion 13 Appendix A 14 References 15 3

4 Introduction When atoms or molecules are excited, for example through heating, then electrons gain energy and are promoted to higher energy levels within the atom or molecule. When the electrons drop down again a photon of light is given off with a specific wavelength corresponding to the energy lost by the electron. Every element or compound has a specific line spectrum associated with it. This leads to spectroscopy being a fundamental tool used by astronomers to determine what astronomical bodies are made up of. In this experiment a transmission diffraction grating is used. This consists of a mask with a large number of evenly spaced slits. A light from the source passes through a collimator so that parallel rays of light fall onto the diffraction grating. The rays of light are diffracted through the slits and the diffracted rays recombined to form an image of the collimator slit at the telescope focus. For the diffracted image to have a maximum intensity the path difference between adjacent slits must contain an integral number of wavelengths, see Fig. 1. Figure 1: Diffraction at the grating θ i θ p 1/N This results in the equation, 1/N (sinθ p - sinθ i ) = pλ (1) Where: - θ p = diffracted angle θ i = incident angle N = number of lines per unit length of grating p = order of interference λ = wavelength of light 4

5 The angular deviation of the beam, D, is given by the formula below, D = θ p - θ i (2) The undeviated beam, when D=0, corresponds to the zeroth order, i.e. p=0. When the grating is normal to the beam, θ i =0, θ p =D. From equation (1) we thus get, SinD = pnλ (3) Experimental Method The experimental apparatus was set up as shown below, Fig. 2. Figure 2: Experimental apparatus Diffraction grating Turntable slit Spectra source Telescope vernier Collimator Turntable vernier Angular scale Telescope 5

6 Procedure Before any measurements were taken the spectrometer had to be adjusted and calibrated. The telescope eyepiece was focussed on the crosswire and adjusted until the crosswire was in sharp focus. The axes of the telescope and collimator were set to be perpendicular. The vertical angle of the grating plane was made perpendicular to the incident beam and the grating is set to be normal to the beam in the horizontal plane. To make these adjustments the following procedure was carried out. 1. The telescope was set on the undeviated image of the slit and its angular position recorded. 2. The telescope was rotated through ninety degrees to make the telescope and collimator axes perpendicular. 3. The grating was rotated so that the reflected image from its front was centred in the telescope. This set the grating at a 45 angle to the horizontal. 4. To adjust the vertical angle of the grating, the vertical centre of the collimator slit was reduced in length to a small element about the centre. 5. The reflected image was observed in the telescope and the grating levelling screws adjusted until the slit centre was aligned with the horizontal crosswire. 6. The grating angle was recorded and the grating rotated 45 to make it perpendicular to the collimator axis. To ensure that the grating rulings were parallel to the spectrometer axis, the levelling screws were adjusted such that on rotation of the telescope the image of the slit centre stays in the same place. The collimator slit was made parallel to the grating rulings. 6

7 Experimental Data Calibration of the grating Through the calibration of the grating we found the number of lines per unit length of the grating. This was done using the sodium lamp as the light source. The diffraction angle, θ p (deviation from straight through image), was measured for one of the sodium lines in as many orders as possible. Table 1 Order (p) Telescope Angle θ p Sin (θ p ) (º) (º) (4 s.f.) The data in Table 1 was plotted resulting in Graph 1. To find the equation of the line the least squares method was used. This yielded a measurement for the gradient of the line, m, of m = / x10-1. Equation (3) gave, Sin(D) = Np Therefore the gradient of the line should be m = N. N = / x10-1 = 589 x 10-9 m N = 297 +/- 2 x 10 3 m -1 This gives the number of lines per mm for the diffraction grating at, N 300 lines mm -1 (2 s.f.) This is an appropriate result for a diffraction grating. 7

8 The Sodium Doublet The separation of the sodium doublet was also measured to as many orders as possible. The data collected is displayed below in Table 2. Table 2 Order Telescope Telescope Mod D (p) Angle 1 1 D Angle 2 2 Cos(D) (D 2 - D 1 ) λ (nm) ( (º) (º) (º) (º) (4 s.f.) (4 s.f.) The data shown in Table 2 was used to calculate the angular separation of the sodium doublet. Equation (3) gives, Differentiating gives, Sin(D) = Np Cos(D) D = pn λ This gives, λ = Cos(D) D/ pn (4) Using the data in table 2 and the statistical equations from Appendix A, the mean, standard deviation, standard deviation of the sample and standard error on the mean can be found. < λ > = x m (4 s.f.) s = x m (4 s.f.) σ = x m (4 s.f.) σ m = x m (4 s.f.) This gives an average value for λ as, 0.8 +/- 0.2 nm (4 s.f.) The sodium doublet is found at 589nm and nm. This gives a true value of λ of 0.6 nm. This is within the uncertainty of the experimental measurement. 8

9 Measurement of the Balmer Series With the diffraction grating calibrated it became possible to measure the Hydrogen Balmer series. The angle of the violet, green and red lines produced by the hydrogen source were measured to as high an order as possible, the results are displayed in Table 3 below. Table 3 Order Telescope Angle (º) Violet line Green/blue line Red line The telescope angles found in Table 3 together with equation (3) can be used to calculate the wavelengths of the lines. Table 4 Order Violet line Green/blue line Red line D V Sin (D V ) D G Sin (D G ) D R Sin (D R ) The data in Table 4 is plotted in Graph 2. To find the equations of the lines of best fit that would correspond to the plots in Graph 2, the least squares method was applied. This gives us the gradient of each line. Using equation (3) we find that the gradient of each line should be Nλ. Violet line, m = / x10-1 λ = nm (4 s.f.) Green line, m = / x10-1 λ = nm (4 s.f.) Red line, m = / x10-1 λ = nm (4 s.f.) 9

10 Measurement of the atomic spectrum of Helium The previous part of the experiment was repeated using a Helium source instead of the Hydrogen lamp. The readings obtained are n Table 5 below. Table 5 0rder Telescope Angle ( ) Violet Green Orange Red The telescope angles found in Table 5 together with equation (3) can be used to calculate the wavelengths of the lines. Table 6 Order Violet line Green/blue line Orange line Red line D V Sin (D V ) D G Sin (D G ) D O Sin (D O ) D R Sin (D R ) Using the same method as for the Hydrogen Balmer series the wavelengths of the lines can be calculated. Violet line, m = / x10-1 λ = nm (4 s.f.) Green line, m = / x10-1 λ = nm (4 s.f.) Orange line, m = / x10-1 λ = nm (4 s.f.) Red line, m = / x10-1 λ = nm (4 s.f.) 10

11 Error Analysis The primary random error associated with this experiment is the human reading of the rotation angle of the telescope. As an initial reading and then a final reading are taken this error is compounded. The error in the telescope scale is +/- 5. However due to human error in reading the Vernier scale a more realistic angle error would be +/- 30. The overall error in D is, ( D) 2 = ( D 1 ) 2 + ( D 2 ) 2 D = +/- 42 This would affect the wavelength calculations as, λ = SinD /pn λ = cosd x D/Np λ/λ = +/- cot D x D This formula gives the error in each separate measurement of the wavelength of the spectral lines. However the measurements obtained used least squares to find the overall average wavelength making this formula difficult to apply to the collected data. Summary of Results The calibration of the diffraction grating yielded a value for N, the number of lines per unit length of; N 298 +/- 2 lines mm -1 The separation of the sodium doublet yielded a value of, λ = 0.8 +/- 0.2 nm The true value falls within the experimental value as, λ =0.6 nm. The wavelengths of the Hydrogen Balmer lines were found to be, Colour Measured value (nm) Uncertainty (nm) True Value (nm) 1 Violet Green/blue Red It can be seen that the true and measured values are similar but fall outside of the error. 11

12 The Rydberg constant, R is given by, 1/λ ij = RZ 2 [1/j 2 1/i 2 ] where, Z = atomic number j and i = energy levels within the atom. For the violet line i=5; j=2 For the green line i=4; j=2 For the red line i=3; j=2 The corresponding approximations for R are: Violet, R = x 10-7 m -1 Green, R = x 10-7 m -1 Red, R = x 10-7 m -1 The mean value of R, <R> = 1.09 x 10-7 m -1 The actual value of R is, R = x 10-7 m -1 ( 2 ) The wavelengths of the Helium spectrum were found to be, Colour Measured value (nm) Uncertainty (nm) True Value (nm) 3 Violet Green/blue Orange Red It can be seen that the true values of the Helium spectral lines fall within the errors of the experimental measurements. 12

13 Conclusion The results obtained from the experiment gave approximate values of the separation of the sodium doublet as, 0.8 +/- 0.2 nm (1 s.f.) The known separation of the doublet is 0.6 nm, which just falls within the experimental uncertainty giving us an accurate result. The values of the wavelengths of the Hydrogen Balmer lines were found to be, Balmer line Measured value (nm) Uncertainty (nm) True Value (nm) 1 χ β α These values fall relatively close to the true values although they fall outside of the errors. The values for the Hydrogen Balmer series also gave rise to an approximation of the Rydberg constant, R R = 1.09 x 10 7 m -1 (3 s.f.) The actual value of R is given as, R = x 10 7 m -1 so the experimental measurement was an accurate approximation. The values of the Helium spectral lines were found to be, Colour Measured value (nm) Uncertainty (nm) True Value (nm) 3 Violet Green/blue Orange Red The true values agree very closely with those measured and fall within the experimental uncertainty. The errors quoted in the results are statistical errors calculated from the sample data. More precise analysis would include a method of calculating the known actual errors. These could have been found for individual measurements but could not be incorporated into the least squares method. As the angular separation between lines is smaller at low orders any error in measurement would have a larger effect on the calculations than the higher order measurements. This may have been a large factor in the error on the sodium doublet. Low intensity lines at higher orders for the Hydrogen and Helium spectra also made measurement difficult. The precision of the experiment could be improved by using higher intensity sources, more precise equipment i.e. more accurate turntable mechanism, more measurements. 13

14 Appendix A Statistical formulae ( 4 ) The mean, <x> = Σx i /N The standard deviation, The standard deviation, of the sample. The standard error on the mean, s = {[Σ( x i - <x>) 2 ]/N} σ = s [N/N-1] σ m = s [1/N] 14

15 References 1 The American Institute of Physics Handbook (12 th edition) 2 Introduction to the Structure of Matter, JJ Brehm & WJ Mullin, Wiley (inside cover) 3 The American Institute of Physics Handbook (12 th edition) 4 Laboratory II: Student Handbook, session 1999/2000 (Data Analysis notes, Dr.MJ Esten) 15

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