Experiment 2 Radiation in the Visible Spectrum In the following experiments we will examine the radiation given off by sources radiating in the visible region We will be using a spectrometer produced by Ocean Optics Light enters the spectrometer via a fiber optic cable Inside the spectrometer a diffraction grating diffracts the different frequencies onto a CCD The CCD basically counts the photons according to wavelength The data is transfered via a usb port to a PC The Ocean Optics software displays the spectrum as counts versus wavelength The wavelength bin size is 0 5 nanometers After the spectrum is collected we will save it in text format We will import the text files into Excel for data analysis After instruction on how to use the software we will examine the spectra given off by the following sources incandescent light filament hydrogen mercury argon various light emitting diodes a laser pointer and florescent lights These experiments will be carried out over three lab periods Data Collection 1 For each measurement you must first take a dark spectrum scan After you take a dark spectrum scan you can take as many measurement scans as needed However if you change any sampling variable integration time averaging smoothing angle etc you must store a new dark spectrum scan 2 All data obtained should be saved in excel form and submitted as part of the lab report Operating the Spectrometer The spectrometer is operated using the PC 1 Boot up in Windows2000 2 Double click on the Ocean Optics icon on the desktop 3 When the softwear boots up the spectometer should automatically start taking data 1 Part A Incandescent Light Source Our incandescent light source will be a tungsten filament 1 Data Collection Record and save the spectra for the light bulb when it is operating at its normal brightness 2 Qualitative Assessment Describe in words the nature of the spectrum e g is it continuous or discrete Do you notice any unusual features Do you think it is representative of perfect black body radiation Why or why not 3 From the spectrum estimate the temperature of the filament This can be done by modeling the filament as a black body radiator In this case the Wien displacement law is kT 0 2014hc max Is your result chose to what is listed in the literature as the temperature of a light bulb 4 Reduce the voltage across the filament and repeat Part 3 above You might have to increase the collection time since the intensity will be less with a dimmer light bulb Is max the same less or greater as in part 3 Is your observation consistent with the change in temperature Part B Line Spectra of Gases Mercury Argon spectrum calibration 1 Data collection Record and save the spectra for the mercury argon lamp Using the Ocean Optics spectrometer observe the spectra of your lamp To operate the Ocean Optics software double click on the icon Ocean Optics Spectrometer on the desktop In the menu displayed click Go to start data acquisition You may start with the initial parameters of integrating time 100 ms and Average 1 Save the data for analysis with excel 2 Qualitative assessment Describe in words the nature of the spectrum e g is it continuous or discrete Did you observe any radiation below around 300 nm from the discharge tubes If not why not 2 3 Compare the spectrum you have observed to the known spectrum from a physics reference Make a chart for each has showing your recorded wavelength the accepted wavelength for this line and the difference between these values 4 You may have noticed in the known spectrum from a physics reference that mercury has an emission lin at 253 7 nm did you observe this line If not why not 5 Describe the purpose of this measurement hint look at the title of this section Hydrogen Spectrum In this experiment we will examine the radiation given off by a hydrogen discharge tube 1 Data Collection Record and save the spectra for the hydrogen tube 2 Qualitative Assessment Describe in words the nature of the spectrum e g is it continuous or discrete 3 Print out a graph of the spectrum from Excel and on the graph write down the atomic transition that is producing each peak For example 3p 2s You can use the file next to this writeup to help you identify the transitions 4 Identify as many peaks from the Balmer series as you can For these Balmer transitions use the text file of the data to estimate the wavelength of the peak center Make a data table of these wavelengths and the corresponding photon energies in eV Make a graph of 1 versus 1 n2i where ni is the quantum number of the initial state The Balmer equation is 1 1 1 RH 2 2 nf ni 1 where nf is the quantum number of the final state 5 From the slope of your graph determine the Rydberg constant RH 6 From the intercept of the vertical axis and your value for RH determine nf which is the quantum number of the final state 3 7 Do the values you obtained for RH and nf agree with those stated in the literature What are some of the reasons that they might be a little bit different Part C LED Spectrum In this experiment we will examine the radiation given off by different Light Emitting Diodes LED 1 Data collection For each LED you will slowly increase the voltage across it For each step in voltage you will also record the current through the LED Keep recording voltages and currents until the LED lights up Do not put too much voltage as you might damage the LED The goal of this exercise is to determine at which voltage the diode starts conducting The charge of an electron e times this starting voltage should be equal to the energy of the photon that is emitted 2 When the LED is producing light record and save a spectrum with the spectrometer 3 Qualitative Assessment Describe in words the nature of the spectrum e g is it continuous or descrete 4 For each LED determine the wavelength that has the most counts This is the characteristic wavelength of the LED Estimate the Full Width at Half Maximum FWHM of the peak in nanometers This is the range of wavelength for which the counts drop to 1 2 the maximum value to the left and right of the peak 5 If you examine the graph of current versus voltage for each LED you will be able to estimate the voltage at which the current starts This starting voltage times the charge of the electron should equal the energy of the photons emitted That is eV hc 2 6 Make a graph of voltage V versus 1 for each LED Do your data points lie in a straight line If so find the slope of the line Does it agree with the literature