Purpose of this Minilab

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.


The Electromagnetic Spectrum

Light = The part of the electromagnetic spectrum which is visible to the human eye.


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.


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.


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 interferencefor different Qn !

d


The Spectrometer

telescopecollimatorSpectrometer table


Activity 1: Spectrometer Alignment Procedure1.1 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


Spectrometer Alignment Procedure1.3 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.1.4 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


Spectrometer Alignment Procedure1.5 Have slit partially open. Align telescope and collimator.

collimator

Slit Slit widthadjust screw

collimatortelescope

Focus knob

1.6 - 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


Spectrometer Alignment Procedure1.7 - 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.


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 ).


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



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.



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.



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


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 .


Activity 2: Mercury Light Source

Hglight collimator telescope

You may need to use books or other objects tomake the height of the light opening match thecollimator height.

Hglight

Hg light is extremely bright.Put cardboard with a small hole in front of the opening to reduce brightness.

Cardboardwith smallhole.

We will not do

Activity 2 – ignore it!


Activity 3: Unknown Light Source

??? collimator telescope

Look at the spectrum.Measure lines if necessary.Determine the element.


Using the Desk Lamp

Dimmer

Lamp Plug (black) must be pluggedinto dimmer plug.Dimmer plug (white) must be pluggedinto power outlet.

On/Offswitchof lamp

Documents

Purpose of this Minilab