Spectrometry Lab, Masterson

Spectrometry Lab

Brooke Masterson

AST 115 H, Plavchan

4/9/2016

Introduction

            (The below information was taken from the lab itself.)

          

      Spectrum = an array of colors or other electromagnetic radiation produced by a prism or similar device. The study of spectra is one fo the most important parts of observational astronomy.  Analysis of the lines appearing in an object’s spectrum permits the astronomer to understand the conditions present on the object emitting the light.  Careful determination of the positions, strengths and shapes of these lines can help to determine an object’s surface chemical composition, motion, surface temperature, rotation, speed, rough size, and other properties.  Much of all that we have discovered about objects in the Universe has been learned from studying their spectra. 

Purpose of the Lab

            Knowing how to determine what a gas is from its spectrum is important for many reasons.  The purpose of this lab is to provide the lab participant to understand how to use a spectrometer, observe different pure gases, and use the information provided by the instructor and from other resources to make their best guesses on what those unlabeled gases are.  It is important to learn this technique because correctly identifying an object’s/gas’ spectrum by the lines they emit or absorb is very important in Astronomy. 

Discussion and Results

            In this section I will present to the audience both what was supplied to me in my lab, the questions and information, along with my results.

Part I: Observing Spectra using a Diffraction Grating

   A.     Your instruction will provide you with a spectrometer with which to observe spectra.  The most important part of the spectrometer is called a diffraction grating.  The grating in your spectrometer is used to disperse the light into a spectrum for viewing on the spectrometer scale.  Before using the spectrometer, study the 2”x2” diffraction grating found in your lab packet.  This sample grating is similar to the one in the spectrometer.  You should notice bright colors seen through the grating and also reflected off its surface.  A grating uses the wave phenomena of diffraction and interference to produce spectra.  A prism, on the other hand, uses the fact that different colors of light are refracted at different angles as they pass from air through glass.

   B.     Hold your grating close to one eye and look through it toward an incandescent light bulb.  You should see a continuous spectrum of color off to either side of the bulb.  You may have to shift your head around somewhat to see these.  Attempt to capture a picture of this with your cell phone or describe what you see.  Pay careful attention to the order of the colors and locations compared to the light bulb. 

  C.     Using the spectrometer:  The remainder of the exercise will be done using the spectrometer.  One looks through the diffraction grating at the narrow end of the device.  Notice that there is a narrow opening, or “slit”, opposite the diffraction grating.  Hold the end with the diffraction grating near your eye.  As you look through the grating, find the slit.  Holding the spectrometer steady near your eye, twist your upper body slowly horizontally, left and right, until you see light from a source.  Then, without moving your body, glance to the side fo the interior of the spectrometer and see the spectrum displayed on the scale.  Use the wavelength scale that is labeled 700 through 400 running from left to right.  These are wavelength values in units of nanometers (where 1nm = 10^-9 m).  Note that the corresponding range in units of Angstroms would be from 7000 through 4000.  You may need to calibrate the wavelength scale of your spectrometer.   Your instructor will explain how.  If you have any difficulties using the spectrometer, be sue to ask your instructor for help.  (Don’t be shy – our whole class asked for help!)

Part II: Observing the Continuous Spectrum of an incandescent light bulb

  A.     You now need to look at the spectrum of a light bulb through the spectrometer.  You should see all the colors of the spectrum aligned with the wavelength scale.  When you use your calibrated spectrometer to observe the spectrum, notice that each color falls on a different portion of the wavelength scale.  Attempt to capture a picture of it with your cell phone or describe what you see.  Include what colors correspond to what wavelength. 

a.     I was not able to observe a light bulb’s spectrum with the spectrometer. 

   B.     Look carefully at the left edge of the spectrum and note the wavelength scale reading at which the light disappears.  Do the same on the right edge of the spectrum.  These are the wavelength “limits” of the sensitivity of your eye.  Record these wavelength numbers.

a.     I found these limits to be 650nm for the left edge and 430nm to be the right edge for my eye.

Part III: Observing the Emission Spectra of Several Elements

  C.    In this section, you will look at several spectrum tubes, each filled with the gas of a specific chemical element.  When a high voltage is applied to the ends of the spectrum tube, current flows through the gas and heats it.  A hot gas emits only certain colors of light.  Each type of gas produces a unique pattern of bright spectral lines.  As you view each spectrum tube with your spectrometer, note the wavelengths of dominant bright lines of each element.  Attempt to capture a picture of it with your cell phone, or describe what you see.  Also note the general color of the tube of gas as seen with the unaided eye in your blog post.  Hot gases do not produce a continuous band of colors, and the unaided eye will see only one combined color, which will be a mix of all the different colors in the spectrum of a particular element.  Use this information to identify the unlabeled gases.  You should study the emission spectra of hydrogen, helium, mercury, neon, and sodium. 

In my class’ lab, there were seven unlabeled gases.  I will identify each gas by the number they are under, such as Gas 1, Gas 2, Gas 3, etc. 

                      1.  Gas 1.  I have no good answer to what that gas was.  It emitted all colors in the spectrum evenly, and I ... I'm lost for an answer besides this one: It has multiple gases in that tube.  

Part IV: Observing Other Sources of Light

   A.     Look at a fluorescent light bulb with your spectrometer.  Note that it appears to have a continuous spectrum with emission lines on top of it.  Record what wavelengths you observe the emission lines to be at. 

These were my findings

            Red was at 600nm

            Green was at 540nm

            Light blue was at 470nm

            Navy blue was at 440nm

These emission lines were pretty bright, too

   B.     Now observe two other light sources of your own choosing (either in the building or outside).  Note: During the day, NEVER point the spectrometer directly at the Sun.  You could damage your eyesight.  It is safe, however, to observe the solar spectrum reflected off of white clouds, concrete surfaces, or the moon. 

I observed a purple notebook and the grey desk.  The purple notebook’s emission lines in its spectrum were not very bright, and all of the colors (red, green, and blue) blended together with no lines that seemed brighter than the others.  The grey desk was even more faded in color, and it seemed like all of the colors on the spectrum were present.  My best guess for why the purple notebook reflected the fluorescent light bulb’s red, green and blue colors was because the purple notebook reflects the colors that it is – I’m going to try to explain this complicated process of colors as best as I can.  Every object on earth absorbs all matters of light, visible and not, but the color they reflect is the color that humans see and associate that object with.  We learned that in our class eons ago.  So, my best guess for why the purple notebook emitted mostly red, green and blue lines is because those are the colors purple is made up of! (although I don’t really understand why I saw a lot of green, but hey, that’s what I saw).  As for the grey desk, I’m guessing it was so devoid of showing one particular color over the others that all of the colors in the visible scope were faded when I saw them in the spectrometer. 

   C.     Compare these spectra with your previous results.  Try to determine which gases are in these lights by the kind of spectra they have and by the patterns in the spectral lines that they produce. 

These were inanimate objects.  So I’m not sure if they have gases, and I definitely didn’t see any from the website that resemble the purple notebook and the grey desk.  However, I can predict a gas for the fluorescent bulb. 

There wasn’t much color between these bright lines emitted from the color spectrum.

I have a good guess! I think it is Strontium.  Maybe.  Not sure.  But that’s what I think.

Conclusion

            Unfortunately, with all sciences, scientists are trained to observe the natural universe and through these observations, they hope to come to conclusions that are correct.  But they never really know.  All scientists, even the most amateur ones, hope that with meticulous notes and by double-checking their work a million times that they reach a great explanation to whatever answer they were seeking.  That is their deepest hope.  Their motivation is to be as right as we possibly can. 

            I’m not certain I was right in identifying all of these gases, but I put my absolute best foot forward, and if that wasn’t enough, that’s okay.  I did learn a lot about spectrums and emission lines and different gas’ spectrums, so even if I didn’t label any gas correctly, I walk away from this lab with a better understanding of spectrometry.  For that, I am grateful.