Spectroscopy

Spectroscopy lab

Objective: To measure the wavelength of spectral lines from a discharge tube with a thorough error analysis (the error is not the difference between your measurement and the most recent published value). Other groups who make this measurement should get the same results as you within the error you specify.

Model: The spectrometer instrument utilizes diffraction from a grating to separate the spectral lines for a gas discharge. The equation is n lambda = d (sin[thetaIN] + sin[thetaDIFF]) where lambda is the wavelength of the incident EM wave while thetaIN and thetaDIFF are the incident and diffracted angles of the EM radiation from the grating normal. See http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/instruments/phy217_inst_grating.html https://en.wikipedia.org/wiki/Diffraction_grating

Method and Procedure: The optical spectrometer is described in these links https://en.wikipedia.org/wiki/Optical_spectrometer https://www.pasco.com/file_downloads/Downloads_Manuals/Student-Spectrometer-Manual-SP-9268A.pdf

Please include an appendix addressing the following points, which are associated with thinking critically about the lab :

1.) Applying principles: what fundamental principles are associated with the model?

2.) What assumptions did you make in the model?

3.) What assumptions did you make relating to how the apparatus appropriately tests the model?

4.) How do you know that the sources (citations) you used are reliable?

5.) Emphasis on supporting your conclusion with evidence: what type of evidence are you using to draw the conclusion that the data do or do not support the model?

6.) Assessing the impact of additional evidence: what additional ways could you have tested the model?

Questions to think about and ask the TA: -how does a grating work? Start with a double slit pattern. Then move to a triple, then N slit diffraction pattern. The maximum obeys n lambda = d (sin[thetaIN] + sin[thetaDIFF]). Sketch a diagram to help understand what this eqn means.

-What are you going to measure? You could check to see if this equation is a valid model of the way the spectrometer works, if you know the wavelength and grating spacing but measure the angles. Or, you could assume the equation models the apparatus and use it to measure the wavelength.

-How do you use a vernier scale to measure the angles?

-How do you deal with thetaIN? You can’t just assume it is zero.

-What happens when two lines are close together?

Second week team projects:

-construct a spectrometer using the prism and light source provided (not the discharge tube used to collect data). You don’t need to make measurements only demonstrate that it spreads the spectrum.

-use the small plastic spectrometer to check your wavelength results from the spectrometer. Sketch a diagram which shows how this device works.