SPECTROSCOPY

PRE-LAB 4: SPECTROSCOPY

OBJECTIVES for the overall lab:

  1. Review the basics of spectroscopy, including how to identify different materials using spectra.
  2. Develop an understanding of general spectroscopic features of rocks and minerals
  3. Learn how to identify specific rocks and minerals in the visible and near-infrared wavelength range

BACKGROUND

The above figures illustrate the electromagnetic radiation spectrum, including visible light.

Spectroscopy is the study of the interaction between radiation and an object, as a function of wavelength. A spectrum is a 2-d plot of the intensity of radiation from an object vs. its wavelength. [Note the word ‘spectrum’ is singular, and ‘spectra’ is plural]. The radiation that is emitted from an object is modified by its compositional properties. The emitted radiation of a rock is controlled by its chemical makeup (the atoms and bonds within its minerals). Since the spectrum of a rock will change based on variations in rock chemistry, this is one way we can identify different rock compositions using spectra. In the visible wavelength region, the chemistry of an object is reflected in its color. This color strongly controls the shape of its spectra, as we will see later in lab.

INTRODUCTION

A trained geologist can gather diagnostic information just by looking at the color, texture, density, and other physical properties. A typical human eye will respond to wavelengths from about 380 to 750 nm (the ‘visible’ portion of the wavelength spectrum). To generate the data for the first part of this lab, we used a portable reflectance spectrometer to extend our “vision” to a slightly longer wavelength range. The ALTA II spectrometer wavelength range is 470-940 nm (or 0.47-0.94 m — see table below). In the second part, we will look at the reflectance spectra of minerals out to even longer wavelengths into the near-infrared.

Useful Wavelength Units

Converting nanometers to meters: 1 nm = 1×10-9 m

Converting microns to meters: 1 m = 1×10-6 m

Converting microns to nanometers: 1 m = 1000 nm

Watch the lab video where your instructor demonstrates the use of the ALTA II spectrometer, and uses it to collect the data you’ll need for the 1st part of the lab exercise.

  1. Below is the data table generated using the ALTA II Reflectance Spectrometer on the orange and the basketball. It is your job to plot this data: either sketch and label their spectral shape on the plot below, or graph it (with labels) in Excel if you are comfortable with using that software for graphing. (Note that relative reflectance value is only needed to draw the spectral shape. While the overall shape of a spectrum should remain constant, it may shift up and down along the y-axis due to changing lighting conditions. Since these are RELATIVE values only, this is one of the ONLY times in this class you are not required to specify the units).

Wavelength (nm) Basketball Orange
470 blue 80 180
525 cyan 113 375
560 green 133 545
585 yellow 236 1030
600 orange 344 1150
645 red 404 1380
700 deep red 302 1462
735 IR1 426 1233
810 IR2 376 1413
880 IR3 383 1130
940 IR4 360 1005

Note that you’ll have to establish your own scale for the y-axis (I recommend 0 to 1500).

Reflectance

1000

900

800

700

600

500

Wavelength (nm)

Examining your reflectance data, and comparing it to the wavelengths associated with different colors of light on page 1, what can you conclude about the relationship between the color you see and reflectance patterns? Does the position of your peak intensity (highest reflectance) make sense, given the color of the objects we analyzed?

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