Name(s): Date: Course/Section: Spectroscopy

Transcription

1 Name(s): Date: Course/Section: Grade: Spectroscopy Part 1: Visible Light 1. Fill in the table below that summarizes the colors of the lights on the LED array. The table should include the bulb s color, the manufacture's peak wavelength (λexpected ), the range in wavelength (λ range i.e., where the peak begins and ends), and the peak wavelength you measure, λpeak. Color λexpected λ range λpeak

2 2. Qualitatively compare your measurements of the peak wavelength of each bulb to the manufacture s specifications. Speculate about the differences. Part 2: Analyzing Emission Spectra 1. Observe the spectrum of the hydrogen and helium samples in the spectrum tube carousel. Record the peak wavelengths for each line. Line λpeak Strength Hα Hβ Hydrogen Balmer Lines Hγ Hδ Hε Strongest He I Lines Weakest

3 2. Sketch the spectra of nitrogen, carbon dioxide, and air and label the strongest lines. There are more lines than you need to write down, but it will be useful later if you have some quantitative data. Use the same scale for the x-axis for easier comparison. Nitrogen Carbon Dioxide Air 3. From your spectra, prove or disprove that there is carbon dioxide and nitrogen in air. 4. Why do different elements produce different emission lines? Part 3: The Solar Spectrum 1. In the optics lab, turn on the light source at the first optics bench and observe the spectrum. What type of light source are you seeing in the spectrum (color, emission type)?

4 2. Examine the spectrum for the instrumental noise by looking at the tail ends. What is the average amplitude of the noise? Is it different for the blue and red portions of the spectrum? 3. Qualify what you consider strong lines by with your answer from Question 2. (Hint, you may look up the term signal to noise to guide you.) 4. Hypothesis what kinds of lines you would expect to see in the solar spectrum (emission, absorption, hydrogen? Iron?)

5 At your lab computer, navigate to /scratch/current/gen_astro_fall_2017/ and open solar_spectrum.cmbl This spectrum of the Sun was taken with the same spectrometer you just looked at in the optics room. There are many spectral lines (emission and absorption) present in this spectrum. 5. In Logger Pro, use the Examine (crtl+e) tool to find the peak wavelengths of the strongest lines. Record your measurements on your printed copy of the solar spectrum. Then using the Solar Spectrum poster or by Googling, identify the elements responsible for as many lines or bands as possible on your printed sheet. You most likely identified the Ca doublet lines (K and H) in the solar spectrum without realizing they are a doublet. Doublet lines are from an element in the same ionization state but the electron is allowed to have spin +1/2 or -1/2, which produces photons of slightly different wavelengths (energies) depending on the spin of the electron when the photon was created. 6. What is the Δλ between the Ca K and H lines? 7. There are two more doublets in this spectrum. One is produced by Mg and the other by Na. Can you resolve them if the spectrometer has a resolution of 1.7 nm? Write out your thought process and include λpeak and Δλ for each line. Then, identify the doublets on your plot. If you cannot identify both peaks, can you identify the unresolved peak where the doublet should be?

6 8. Compare your hypothesis to the lines you observed in the solar spectrum. What surprised you? Think of 2 questions the solar spectrum raises in your mind about the Sun. Part 4: Gratings Read the background section on the lab website about gratings. In this section of the lab, you will determine the lines per mm of 4 different gratings. The Grating Equation In the pre lab question, you found the wavelength of light of a laser given the lines per mm (1/d), the spectral order (m), the angle of incidence (θi), and the diffracted angle (θr). In this part of the lab, we will be finding the lines per mm. Rearrange the grating equation to find d, the mm per lines. Assume that the incident angle is 0 degrees. In the optics room, you will find a nm green laser, a screen, a ruler, four gratings, and a slide holder. Mount a grating onto the holder and then plug in the laser. Adjust the holder such that the laser is going through the grating. WARNING: Do NOT look directly at the laser as it can severely damage your eyes. Analyzing the Configuration and the Problem at Hand 1. Turn on the laser and examine the dots on the screen. Sketch a diagram of the configuration.

7 2. Move the holder closer to the laser and further away. What happens to the separations between the dots on the screen? 3. Try twisting the grating in the holder such that the laser is entering the grating at an angle. What happens to the separations between the dots on the screen? Adjust the holder such that separation between the dots is minimized i.e., the angle of incidence is as close to 0 degrees as possible. Adjust the distance of the holder to be midway between the screen and the laser. 4. Using the geometry of the configuration, determine about how you could measure the diffracted angle, θr. Write out your process and relevant equations.

8 5. Determine the lines per mm for each of the 4 gratings. Three of the are labeled, but you will determine your own measurements. The fourth slide, labeled Team C is unknown. Make a table and clearly represent your measurements and calculated values. Be sure to include units as well. Suggestion: When measuring the separation between orders, measure from order -1 to 1, instead of 0 to 1, and then divide the separation by 2.

9 Analysis Questions 1. Why was it recommended that you measure between the order +/- m and divide by two instead of measuring from 0th order to m? (Hint: Think about the assumption in the grating equation.) 2. Compare your answers to the manufacture s lines per mm. Do you find a systematic offset in your measurements? How do they compare? (Hint: Consider how you made your measurements or what assumptions went into the experiment.) 3. What is the lines per mm of the the Team C grating? How accurate is your measurement?

10 4. Find the angle of incidence, θi, for the three labeled gratings using the measured diffraction angle, θr, the manufacture's lines per mm, and the wavelength of the laser. 5. Are the angles very large? How sensitive is your measurement of the lines per mm to the incident angle? Bonus Credit Question 1. Read the Error Analysis, Propagation of Error section of the lab website. In Part 3, if the spectrometer has a precision of 1.7 nm, what error should be attached the answer to Question 6?