Transcript Purpose
Purpose of this Minilab
Physics 2225 : The Spectrometer • 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 spectroscopy etc.
Each of these spectroscopic techniques uses a different type of apparatus.
Physics 2225 : The Spectrometer
Optical Spectroscopy in Astronomy
Light from a star Optical telescope Spectrograph Spectrum Analysis of spectrum Temperature 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 Light from lamp d Diffraction grating
The Diffraction Grating
d sin
Q n
(= difference in path length)
Q n Whenever d sin Q n = n l : All waves are in phase (constructive interference); n = 0,1,2….
Otherwise they cancel each other (destructive interference).
Different l means constructive interference for different Q n !
Physics 2225 : The Spectrometer collimator
The Spectrometer
Spectrometer table telescope
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 2) wood plate to achieve that.
Use level to make sure spectrometer table surface is level, too (there are
three
thumb screws for that).
Spectrometer table surface level Wood surface Thumb screw
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.
telescope collimator Slit Slit width adjust screw collimator 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 piece after adjustment.
graticule Slit image
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 slit is fixed as seen through the telescope).
Align the fixed side of the slit image with the vertical line of the graticule by turning the
telescope rotation fine adjust knob
.
telescope Telescope rotation fine adjust knob Telescope rotation lock-screw Fixed edge of slit image aligned with graticule.
Reading the Spectrometer Scale
Physics 2225 : The Spectrometer 1.
2.
3.
4.
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
.
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: 155.0
+ 15’ = 155.25
(Note: 60’=1
).
Physics 2225 : The Spectrometer
Using/Aligning Diffraction grating
Make sure grating is perpendicular to collimator axis 90 Diffraction grating: 600 lines/mm collimator telescope
Na light
Side view Wood blocks to adjust height of collimator to Na light source 90
Na light
collimator telescope Top view
Physics 2225 : The Spectrometer
Using/Aligning Diffraction grating
Na light
collimator telescope Top view 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
Na light
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.
Na light Physics 2225 : The Spectrometer
Using/Aligning Diffraction grating
Q Yellow sodium line : l= 5890Å = 5890 x 10 -10 m For n = 1: Q = 20 39’ 1.
2.
3.
4.
5.
Loosen table rotation lock screw.
Rotate the rotating table base (the one with the vernier scale) but Tighten table rotation lock screw.
not
the spectrometer table (the one with the grating on it). Rotate until vernier scale reads exactly 20 39’.
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 its lock screw as well.
The spectrometer vernier scale is now calibrated for use.
Physics 2225 : The Spectrometer
Measuring
Q
for an Unknown Spectral Line
Q r light light Q l 1.
2.
3.
4.
5.
Find the spectral line by moving telescope to the right.
Determine Q r .
Find the spectral line by moving the telescope to the left.
Determine Q l Calculate Q .
by taking the average of Q r and Q l .
Physics 2225 : The Spectrometer
Activity 2: Unknown Light Source
???
collimator telescope
Look at the spectrum.
Measure lines if necessary.
Determine the element.
Dimmer
Using the Desk Lamp
Physics 2225 : The Spectrometer Lamp Plug (black) must be plugged into dimmer plug.
Dimmer plug (white) must be plugged into power outlet.