3.1 Overview: Electromagnetic Energy

Introduction

You may be well aware of the importance of visible light for making observations in the geosciences.  For example, the color of minerals and rocks can help us identify them (although it is not always the best identifier!).  Color is determined by the relative absorbance or reflection of different wavelengths of visible light.

For example, a gem-quality emerald is a deep green color.  This is because it is reflecting the green wavelengths of light to our eyes, and is absorbing the other visible wavelengths of light.  Emerald is the mineral beryl, but it is trace Cr3+ in the mineral structure that causes the green color.

Emerald from Muzo Mine, Columbia

Visible light represents a small range of the electromagnetic spectrum.  Even though no human can see beyond the visible range, we can still use technology to “see” the electromagnetic spectrum beyond anyone’s reach.

The electromagnetic energy we cannot see (but can detect with instrumentation) can be used to study the chemical composition of materials, can detect the vibrations of molecules and minerals, and can see the structure of solid materials, among other things.

This chapter provides a practical basis for applying knowledge of electromagnetic energy to analytical methods.

Learning Objectives

At the end of this module, students should be able to:

• Students should be able to:
• Describe the categories of electromagnetic energy within the electromagnetic spectrum, and be able to place these categories relative to each other in wavelength and energy.
• Describe the wavelengths of visible light and name a color for a given wavelength range.
• Describe the ways in which electromagnetic energy can behave when it interacts with an object.
• Define characteristics of light, including the speed of light, wave front, and light ray.
• Define refractive index.
• Differentiate between the electric and magnetic components of light.
• Use the electric (E) vector of light to describe the behavior of plane polarized light.
• Explain how the double slit experiment demonstrates wave-particle duality at the quantum scale.
• Describe the relationships between energy, wavelength, frequency, and wavenumber with the help of Planck’s equation.
• Convert wavelengths of electromagnetic energy between commonly used units.
• Convert the wavelength of electromagnetic energy to wavenumbers (cm-1), a commonly used unit in infrared and Raman spectroscopy.