Physics 2225 : The Spectrometer
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.
Physics 2225 : The Spectrometer
The Electromagnetic Spectrum
Light = The part of the electromagnetic spectrum which is visible to the human eye.
Physics 2225 : The Spectrometer
Spectroscopy
Spectroscopy = 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) spectroscopy- X-ray spectroscopy- Gamma ray spectroscopyetc.
Each of these spectroscopictechniques uses a differenttype of apparatus.
Physics 2225 : The Spectrometer
Optical Spectroscopy in Astronomy
Light from a star
Optical telescope
Spectrograph
Spectrum
Analysis of spectrumTemperature of star, velocity of star relative to earth…..Absorption lines (missing wavelengths) show the composition of the gas surrounding the star.
Physics 2225 : The Spectrometer
Whenever d sin Qn = nl : All waves are in phase (constructive interference); n = 0,1,2….Otherwise they cancel each other (destructive interference).
The Diffraction Grating
Light from lamp
Diffractiongrating
d sin Qn (= difference in path length)
Qn
Different l meansconstructive interference
for different Qn !
d
Physics 2225 : The Spectrometer
The Spectrometer
telescopecollimatorSpectrometer table
Physics 2225 : The Spectrometer
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:1) Use a level and make sure wood surface is level (put papers underneath legs of
wood plate to achieve that.2) Use level to make sure spectrometer table surface is level, too (there are three
thumb screws for that).
Spectrometertable surface
Wood surface Thumb screw
level
Physics 2225 : The Spectrometer
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 infinity- Rotate 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.
telescope
Eye piece
Focus knob
Graticule alignment ring
Physics 2225 : The Spectrometer
Spectrometer Alignment Procedure
Have slit partially open. Align telescope and collimator.
collimator
Slit Slit widthadjust screw
collimatortelescope
Focus knob
- Look through telescope- Rotate telescope a bit if not totally aligned- Adjust focus knob (on the collimator) until
slit comes into sharp focus.- Turn slit assembly if necessary to make it vertical
(aligned with graticule).
View through eye pieceafter adjustment.
Slit imagegraticule
Physics 2225 : The Spectrometer
Spectrometer Alignment Procedure
- Tighten the telescope rotation lock-screw.- Look through telescope while turning back
and 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 turning the telescope rotation fine adjust knob.
Telescoperotationlock-screw
telescope
Telescoperotationfine adjustknob
Fixed edge of slit image alignedwith graticule.
Physics 2225 : The Spectrometer
Reading the Spectrometer Scale
Figure 1Figure 1
1. Find the number of degrees (with 0.5 degree accuracy) on bottom scale aligned with the 0 from top scale (if between two marks, pick the lower number):Shown example: 155.0 .
2. Use magnifying glass: Find which mark on top scale is aligned with a mark on bottom scale.3. 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.4. Add that number (in units of minutes of arc) to the number from step 1.
Shown example: 155.0 + 15’ = 155.25 (Note: 60’=1 ).
Physics 2225 : The Spectrometer
Using/Aligning Diffraction grating
Nalight
collimator telescope
90
Nalight
collimator
90
Diffraction grating:600 lines/mm
Side view
Top view
telescope
Make sure grating is perpendicular to collimator axis
Wood blocks to adjust height of collimator to Na light source
Physics 2225 : The Spectrometer
Using/Aligning Diffraction grating
Nalight
collimator
Top view
telescope
In 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.
Physics 2225 : The Spectrometer
Using/Aligning Diffraction grating
Nalight
collimator
1. 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 the yellow line with the vertical graticule.
Physics 2225 : The Spectrometer
Using/Aligning Diffraction grating
Yellow sodium line : l= 5890Å = 5890 x 10-10m For n = 1: Q = 20 39’
1. Loosen table rotation lock screw.2. Rotate the rotating table base (the one with the vernier scale) but not the
spectrometer table (the one with the grating on it). Rotate until vernier scalereads exactly 20 39’.
3. Tighten table rotation lock screw.4. You can use the table rotation fine adjust knob to get the angle aligned perfectly.5. Make sure the spectrometer table with the grating is still aligned and tighten its
lock screw as well. The spectrometer vernier scale is now calibrated for use.
Nalight
Q
Physics 2225 : The Spectrometer
Measuring Q for an Unknown Spectral Line
light
Qr
light
Ql
1. Find the spectral line by moving telescope to the right.2. Determine Qr .3. Find the spectral line by moving the telescope to the left.4. Determine Ql .5. Calculate Q by taking the average of Qr and Ql .
Physics 2225 : The Spectrometer
Activity 2: Unknown Light Source
??? collimator telescope
Look at the spectrum.Measure lines if necessary.Determine the element.
Physics 2225 : The Spectrometer
Using the Desk Lamp
Dimmer
Lamp Plug (black) must be pluggedinto dimmer plug.Dimmer plug (white) must be pluggedinto power outlet.
On/Offswitchof lamp