Introduction

If you have not done so please make an optical spectrometer or obtain one similar to that used in the Patterns in Nature class or from "Project STAR".

Procedure

• Gather these items from your Optics Kit:
  • the spectrometer built earlier
  • an incandescent (household) light bulb

• Observe the light spectrum from an incandescent light bulb. Note that it covers the visible spectrum from red to blue without noticeable lines (see Reading on Sources of Light).

• Observe the spectrum from fluorescent lights where there is a green line (the mercury line at 546 nanometers). If you can view "neon" lights you will also find lines at different colors and wavelengths. You are retracing the footsteps of the early (turn of the century) spectroscopists who found that each element in a gas discharge tube produced its own set of characteristic lines.

Background

Figure 1. X-ray spectrum of molybdenum showing the K_alpha and K_beta lines superimposed on a continuous spectrum. The wavelengths of the lines are characteristic of the target element.

spectrum from molybdenum showing two peaks at wavelengths between 0.6 and 0.8 Ångstroms (Å). The K_alpha and K_beta identify the electron transitions where the electrons move into the innermost electron level (the K-shell). The characteristic peaks (termed line spectra) have specific energies that can be identified with each element. Measurement of the energies of the line spectra energies allows one to identify the elements emitting the X-rays.

Most laboratories with a scanning electron microscope (SEM) have an X-ray spectrometer attached for the measurement and detection of X-rays. These instruments that measure X-ray energies contain liquid-nitrogen (LN₂) -cooled detectors and can be easily noticed by the cylindrical dewar (similar to a thermos bottle) about 8 inches in diameter and 12 inches tall. If you are in a laboratory with an SEM, see if it has an instrument for measuring the energy of X-rays.

Go to the Reading on The Electron