I. Introduction

Spectroscopy is the measurement of the intensity of light at many different wavelengths, and the interpretation of those measurements using theories of physics. Spectroscopy is absolutely crucial to astronomy. With few exceptions, such as the study of rocks returned from the Moon or data from landers on Mars, almost everything we know about the universe comes from analysis of light from astronomical objects. It is from spectroscopy that we have learned of the temperatures, luminosities, and chemical compositions of the stars.

Spectroscopy is also of importance in other fields of science and technology. It can be used to measure the chemical and physical state of ocean water, glucose levels in human blood, and in industrial procedures. Spectroscopy is one of the better examples of a field of physics that has significantly impacted society.

The purpose of this laboratory exercise is to illustrate some of the capabilities of spectroscopy, using a sophisticated spectroscopic instrument. You will first study spectra from different types of objects in the lab and measure wavelengths of spectral lines. You will then study and measure the spectrum of an extremely important astronomical object: the Sun.

This project will not require a formal lab writeup. Concentrate on understanding the measurement and what it is telling you. There is space on this form for responding to questions, noting observations, and making sketches. Hand it in to your teaching assistant at the conclusion of the lab period.

II. Explanation of the Equipment

Most spectrometers are fundamentally simple in design. A thin beam or ray of light passes through, or is reflected from an object which spreads out, or disperses the light according to wavelength. An easy way of visualizing this is to think of a prism which spreads out light into all the colors of the rainbow. The dispersing element ( a prism or diffraction grating) sends the violet light in one direction, the yellow light in a slightly different direction, the red light in still a different direction, and so on. This dispersed, polychromatic light is then focused onto a surface which acts as a detector. In lecture demonstrations, this is just the overhead projector screen, and your eye is the detector that sees that the different colors have different intensities. For much of twentieth century astronomy, the detector was a photograph plate. Photographic plates are still used in some spectroscopic applications. Modern instruments use a CCD (charge-coupled device) in which an electronic wafer builds up an electrical charge when light shines on it. This charge is later read out and measured by a computer.

The spectrometer which is used in this exercise is a USB2000 device manufactured by Ocean Optics company. It is an amazingly compact device which has one input (a fiber optics cable which shines the light into the spectrometer) and a USB port to send data to the analysis computer. Software provided with the spectrometer permits display and analysis of the spectra.

A diagram of the USB2000 is shown in the figure below.

Light comes in from the fiber optics cable, reflects from a mirror on the far wall of the spectrometer, then strikes the diffraction grating, where it is dispersed, or spread out according to wavelength. The focusing mirror then focuses the dispersed light on the CCD array detector. There is a relation between position on the detector and the wavelength of light. The intensity of light as a function of position on the detector therefore corresponds directly to intensity as a function of wavelength, which is the spectrum. Not shown are the electronics which read out the charge on the detector, digitize the signal, and format it for the USB port. All of this is crammed in a box the size of a deck of cards!

When the spectrometer is connected to the computer, and the control program is running, there are a number of simple controls the user has over the display and analysis of the spectrum.

· The vertical cursor measures the wavelength of observation and gives the intensity of light at that wavelength. It is controlled by the mouse. The wavelength and intensity reading are shown in the lower left corner of the screen.

· Right above the spectrum are a number of data boxes that can be set by the user. The one at the far left gives the integration time, or the length of time the device averages the signal before readout. The units are milliseconds. The longer the integration time, the larger is the signal recorded. Next to it is the number of spectra that are averaged before display. The larger the number of spectra averaged, the clearer and less noisy the spectrum will appear. You will find it helpful to manipulate these control parameters when studying the spectra of the gas discharge tubes and the spectrum of the Sun.

· Finally, at the top of the screen will be a set of standard Windows menu bars. The one labeled “View” can be used to set the scale of the spectrum. If you bring up the dialog box, you can set the range of the abscissa (x coordinate) and ordinate (y coordinate). This is a very useful feature for making precision measurements of spectral lines, or examining the shape of spectral lines.

III. Procedure

The computer will probably be in the Windows desktop when you arrive. Double click on the OOI Base 32 icon to start the program. Look around on the lab table and identify the USB2000 unit, the fiber optics cable connected to it, the stand for holding the fiber optics cable, and the USB cable connected to the computer. You’re ready to start.

There are a number of steps or parts to this lab, intended to give you a clear idea of what the spectrometer is doing, and the information we have in the light from an object.

Spectra of light sources. There is a gray metal box on the lab table with several small light bulbs on the top. When you turn the switch on, the light bulb lights up. The first light bulb is not “dead”; it emits in the infrared at a wavelength to which your eye is not sensitive (you can check us out on this!). For each of the lightbulbs, measure the central wavelength (wavelength at which the light bulb is brightest), and the range of wavelengths over which the light emits significant amount of light. Record your data in the table below.

Bulb Number	Color	Central Wavelength	Range of Wavelengths	Comments

Additional Comments:

Spectrum of Continuous Source. Shine the light from an overhead projector into the fiber optics cable. The overhead projector bulb is a regular incandescent light. Make a reasonably accurate sketch of the spectrum you see on the plot below. Be sure and put numbers on the axes!

Now calculate the temperature of the filament, using Wien’s Law. Here is a chance to apply an equation you have learned about in class to a real physical situation. Remember that Wien’s Law is a relationship between the temperature of an object and the wavelength at which it is brightest. The relationship is

T=2.9e-03/L_max

Where T is in degrees Kelvin, and L_max is the wavelength at which the object emitting the radiation is brightest. This wavelength must be input in meters. Carry out the calculation on the space to the right of the spectrum above.

Question: with the data you have, and have shown above, what is an uncertainty that limits the degree of precision to which you can measure the temperature T?

Spectra of Gas Discharge Tubes. Kirchoff’s second law of spectroscopy says that a hot, tenuous gas emits a spectrum which consists of isolated, bright emission lines. The wavelengths at which these lines occur are a unique fingerprint of the gas that is being excited. Each lab table will have two discharge tubes, one of hydrogen and the other of helium. Turn on the hydrogen lamp and bring the optical fiber up to the light. Draw the spectrum of hydrogen, taking care to place the right numbers on the axes, and generally making the spectrum as accurate as possible. Measure the wavelengths of the six most prominent lines (if you can find six), and record the data in the table below. Remember to increase the sensitivity of the spectrometer by increasing the integration time, or the number of cycles to average. Also, when measuring the wavelength of a line, be sure the cursor is exactly in the middle of the line. Otherwise, a significant and avoidable error will result.

When you have completed your observation of hydrogen, do the same with helium.

Element	Wavelength	Line Strength	Element	Wavelength	Line Strength

Solar Spectrum. Use the ring stand to hold the entrance to the fiber optics cable at the window of the lab room. Direct sunlight is not necessary, and even on a relatively cloudy day, the light will be bright enough. The spectrum of this light will be the spectrum of sunlight. Adjust parameters such as integration time, number of cycles to average, and scale of the spectrum to give you a good display that is convenient for making measurements. First of all, draw an accurate sketch of the spectrum on the form below.

How would you characterize the solar spectrum in terms of the types of spectra described in Kirchoff’s laws, i.e. a continuous spectrum, an emission line spectrum, or an absorption line spectrum?

Using data from your plot (and with the help of the cursor measurer), and applying Wien’s law, measure the surface temperature of the Sun. Put your calculations in the space to the right of the plot.

Measure the wavelengths of some of the strongest spectral lines in the spectrum of the Sun. Record them in the table below. Which of them can you identify from Part C. above? If you can identify it, indicate what element is responsible for it. You have thereby demonstrated that this element is present in the Sun.

Wavelength	Strength	Identification

When you have completed your table, check with your teaching assistant for the accepted table of lines in the solar spectrum.