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. 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.

4. 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.

5. The Diffraction Grating
d sin Qn (= difference in path length)
Light from lamp Qn d
Diffraction
grating
Whenever d sin Qn = nl : 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 Qn !

6. The Spectrometer
Spectrometer
table
telescope
collimator

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 of
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

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 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

9. Spectrometer Alignment Procedure
Have slit partially open. Align telescope and collimator.
telescope
collimator
Slit
Slit width
adjust screw
- 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).
collimator
View through eye piece
after adjustment.
Focus knob
graticule
Slit image

10. 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.

11. Reading the Spectrometer Scale
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: .
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 axis
Diffraction grating:
600 lines/mm
90 Na
light
collimator
telescope
Side view
Wood blocks to adjust height of collimator to Na light source
90 Na
light
collimator
telescope
Top view

13. Using/Aligning Diffraction gratingNa
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.

14. Using/Aligning Diffraction gratingNa
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.

15. Using/Aligning Diffraction gratingNa
light Q
Yellow sodium line : l= 5890Å = 5890 x 10-10m
For n = 1: Q = 20 39’
Loosen table rotation lock screw.
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 scale
reads 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 its
lock screw as well.
 The spectrometer vernier scale is now calibrated for use.

16. Measuring Q for an Unknown Spectral Line
light Qr
light Ql
Find 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
???
collimator
telescope
Look 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 plugged
into dimmer plug.
Dimmer plug (white) must be plugged
into power outlet.
Dimmer
On/Off
switch
of lamp