Presentation on theme: "Purpose of this Minilab"— Presentation transcript:
1 Purpose of this Minilab Gain experience with optical spectroscopic techniques.- Understand the basic origin of spectroscopic lines (homework).- Understand how diffraction gratings work (homework).- Determine the wavelength of particular spectroscopic lines.- Identify a material based on it’s spectrum.
2 The Electromagnetic Spectrum Light = The part of the electromagnetic spectrum which is visible to the human eye.
3 SpectroscopySpectroscopy = Analysis of the composition of some region of the electromagnetic spectrum.(Determination of the amplitudes and frequencies/wavelengths contained within the examined region of the spectrum.)Different regions of spectroscopy exist:Optical spectroscopy (examines visible light)Radio frequency (rf) spectroscopyX-ray spectroscopyGamma ray spectroscopyetc.Each of these spectroscopictechniques uses a differenttype of apparatus.
4 Optical Spectroscopy in Astronomy Light from a starOptical telescopeSpectrographSpectrumAnalysis of spectrumTemperature of star, velocity of star relative to earth…..Absorption lines (missing wavelengths) show thecomposition of the gas surrounding the star.
5 The Diffraction Grating d sin Qn (= difference in path length)Light from lampQndDiffractiongratingWhenever d sin Qn = nl : All waves are in phase (constructive interference); n = 0,1,2….Otherwise they cancel each other (destructive interference).Different l meansconstructive interferencefor different Qn !
6 The SpectrometerSpectrometertabletelescopecollimator
7 Activity 1: Spectrometer Alignment Procedure Spectrometer must be “level”:This really means that spectrometer table surface must be parallel to wood surface.The easiest way to achieve that:Use a level and make sure wood surface is level (put papers underneath legs ofwood plate to achieve that.Use level to make sure spectrometer table surface is level, too (there are threethumb screws for that).Spectrometertable surfacelevelWood surfaceThumb screw
8 Spectrometer Alignment Procedure Eyepiece adjustment:Look through telescope.Rotate the graticule (crosshair) alignment ring until one line of graticules is vertical.Slide eyepiece in/out until graticule is in focus.Focus telescope to infinityRotate spectrometer so that you can look through telescope at a far away object(e.g., across the room).- Turn the focus knob until that far away object is in focus.telescopeEye pieceFocus knobGraticule alignment ring
9 Spectrometer Alignment Procedure Have slit partially open. Align telescope and collimator.telescopecollimatorSlitSlit widthadjust screw- Look through telescopeRotate telescope a bit if not totally alignedAdjust focus knob (on the collimator) untilslit comes into sharp focus.- Turn slit assembly if necessary to make it vertical(aligned with graticule).collimatorView through eye pieceafter adjustment.Focus knobgraticuleSlit image
10 Spectrometer Alignment Procedure Tighten the telescope rotation lock-screw.Look through telescope while turning backand forth the slit width adjust screw.(this will show you which side of the slitis fixed as seen through the telescope).Align the fixed side of the slit image withthe vertical line of the graticule by turningthe telescope rotation fine adjust knob.telescopeTelescoperotationfine adjustknobTelescoperotationlock-screwFixed edge of slit image alignedwith graticule.
11 Reading the Spectrometer Scale Find the number of degrees (with 0.5 degree accuracy) on bottom scale aligned with the 0from top scale (if between two marks, pick the lower number):Shown example: .Use magnifying glass: Find which mark on top scale is aligned with a mark on bottom scale.Read off on the top scale the number corresponding to the aligned mark.Shown example: 15 on top scale is aligned with a mark from bottom scale.Add that number (in units of minutes of arc) to the number from step 1.Shown example: + 15’ = (Note: 60’=1 ).
12 Using/Aligning Diffraction grating Make sure grating is perpendicular to collimator axisDiffraction grating:600 lines/mm90NalightcollimatortelescopeSide viewWood blocks to adjust height of collimator to Na light source90NalightcollimatortelescopeTop view
13 Using/Aligning Diffraction grating NalightcollimatortelescopeTop viewIn this position (collimator and telescope aligned) you should see an image of the slit when looking through the telescope. This is the n=0 maximum. All colors (lines) in the spectrum are on top of each other.
14 Using/Aligning Diffraction grating Nalightcollimator1. Move telescope to the right side.you should first see successively the n=1 maxima of different colors.2. Carefully tighten the telescope rotation lock screw such that the n=1 maximum of the bright yellow line is aligned with the vertical graticule.3. Use the telescope rotation fine adjust knob to exactly align the fixed edge of theyellow line with the vertical graticule.
15 Using/Aligning Diffraction grating NalightQYellow sodium line : l= 5890Å = 5890 x 10-10mFor n = 1: Q = 20 39’Loosen table rotation lock screw.Rotate the rotating table base (the one with the vernier scale) but not thespectrometer table (the one with the grating on it). Rotate until vernier scalereads exactly 20 39’.Tighten table rotation lock screw.You can use the table rotation fine adjust knob to get the angle aligned perfectly.Make sure the spectrometer table with the grating is still aligned and tighten itslock screw as well. The spectrometer vernier scale is now calibrated for use.
16 Measuring Q for an Unknown Spectral Line lightQrlightQlFind the spectral line by moving telescope to the right.Determine Qr .Find the spectral line by moving the telescope to the left.Determine Ql .Calculate Q by taking the average of Qr and Ql .
17 Activity 2: Unknown Light Source ???collimatortelescopeLook at the spectrum.Measure lines if necessary.Determine the element.
18 Using the Desk Lamp On/Off switch of lamp Lamp Plug (black) must be pluggedinto dimmer plug.Dimmer plug (white) must be pluggedinto power outlet.DimmerOn/Offswitchof lamp
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