1. Diffraction Grating And Emission Spectra To determine the wavelengths emitted by the atomic element in a discharge tube and to identify the element

2. Excitation of atoms (most often gaseous matter) light emission Introduction First quantitative observation of emitted light with a prism Study of atomic spectra Quantum theory of matter Find: sharp lines of different colours emission spectrum The spectrometer used for precise wavelength measurements This experiment: calibrate grating spectrometer measure the atomic spectrum identify the element Experimental improvements diffraction grating grating spectrometer

4. Atomic Theory: The energy, or wavelength of each photon determined by the energy difference between states of the atom Quantum theory: prediction of the energy levels Hydrogen atom

5. Characteristic for each atom well defined spectrum of wavelengths from the energy states The spectrum identification of the element Identification procedure: measurement of the wavelengths of as many lines in its spectrum as possible and look up in a table of prominent lines

6. Theory of the Diffraction Grating Collimated (parallel) monochromatic light past a narrow slit a pattern of maxima and minima in intensity on a screen The pattern fringes of diffraction pattern Large number of evenly spaced slits narrow bright fringes Wide dark spaces (minima) between the fringes A diffraction grating a flat plate with a large number of uniform slits scribed at regular intervals

7. Practical arrangement: f  S P Wavefronts f 1 f -- focal length collimator telescope

8. Each slit becomes a radiator of waves in all directions. Many rays spread out from the grating b a P x z R 1 l  y Various rays arriving at a point P on a screen cover different distances from a slit The difference between any two lengths to the same screen path difference R n

9. The path difference = n  the sine waves in phase and the intensities add up to produce a bright fringe The difference is ½  , 3/2  , 5/2  , … the waves 180 0 out of phase give a dark spot or line Rays diffracted by  path difference = d sin  Thus for a maximum aka. the grating equation n = 1, 2, … are the 1 st , 2 nd , etc orders  = wavelength of the line d = grating space i.e., distance between successive slits  = off axis angle measured from the straight through Set 1/d= N = number of slits per cm the grating eq n

10. Apparatus The spectrometer is made up of three parts: the collimator – an optical device which produces a parallel beam of light from your monochromatic light source the telescope – an optical device which focuses a beam of parallel light in the plane of the crosshairs the turntable – supports the diffraction grating and enables measurement of the angular position of the telescope DO NOT TOUCH THE RULED SURFACE OF THE GRATING

14. All wavelengths in nm Xe 462.4 Zn 518.2 Ne 609.6 Kr 450.2 Fe 516.9 Ne 607.4 Zn 463.0 Rb 526.0 Ne 614.3 Hg 404.7 Fe 466.8 Fe 527.0 Rb 616.0 He 406.2 Xe 467.1 Rb 536.3 Ne 616.4 Hg 407.8 Cd 467.8 Rb 543.1 Rb 620.6 H 410.1 Zn 468.0 Hg 546.1 Ne 621.4 He 412.2 He 471.3 Kr 556.2 Ne 626.6 Rb 420.2 Zn 472.2 Kr 557.0 Rb 629.8 Rb 421.6 Xe 473.4 Rb 564.8 Ne 630.5 Kr 427.4 Cd 480.0 Rb 572.4 Ne 633.4 Fe 430.8 Zn 481.0 Hg 577.0 Zn 636.2 Kr 432.0 Xe 482.9 Hg 579.0 Ne 638.3 H 434.0 H 486.1 Ne 586.9 Ne 641.0 Hg 435.8 Hg 491.6 Kr 587.1 Cd 643.8 Kr 436.3 He 492.2 He 587.6 Kr 645.6 Fe 438.4 Xe 492.3 Ne 588.2 Ne 650.6 He 438.8 Fe 495.8 Zn 589.4 Ne 653.2 Cd 441.4 He 501.6 Ne 594.5 H 656.3 Kr 446.4 He 504.8 Ne 598.2 Ne 659.9 He 447.2 Cd 508.6 Ne 603.0 He, Ne 667.8 Xe 450.1 Cd 515.6 Rb 607.1 Ne 671.7 Element Wavelength Element Wavelength Element Wavelength Element Wavelength

18. To adjust the turntable The turntable must be level in order to be sure that your images will appear in the middle of the screen, not off at an angle where the telescope may not be able to pick them up; x, y, and z are the three leveling screws under the turntable; BC is the diffraction grating (a) Place the diffraction grating on the turntable so the lines on the grating face the telescope