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 spectroscopy etc. Each of these spectroscopic techniques uses a different type of apparatus.

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.

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 !

The Spectrometer Spectrometer table telescope collimator

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

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

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

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 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 ).

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

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.

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.

Using/Aligning Diffraction grating Na 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.

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 .

Activity 2: Unknown Light Source ??? collimator telescope Look at the spectrum. Measure lines if necessary. Determine the element.

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