ELECTROMAGNETIC RADIATIO
Electromagnetic radiation can be visualized in a number of ways. One way is to think of it as existing in sine waves irradiating out from a source in all directions. There are many sizes of waves that can be described by their wavelength (λ), which is the distance between corresponding points on two adjacent waves. This is shown in Figure 5.1. Some types of radiation can have extremely short wavelengths such as X-rays. Others can be very long. Radio carrier waves have wavelengths that exceed 2 m. Sometimes it is more convenient to measure light in terms of its frequency (ν), which is the number of waves that passes a given point in 1 s. Frequency is measured in cycles per second (Hertz). Since light travels very fast and the wavelengths are generally very short, the number of waves that passes a given point in 1 s is generally very high and most light has high numbers of cycles per second. There is a simple inverse relationship between wavelength and frequency, as shown in Equation (5.1). c=λν In Equation (5.1), “c” is the speed of light (3 × 108 m/s or about 186,000 miles/s). This equation shows that, as frequency increases, wavelength decreases and vice versa. Given the speed of light and either the frequency or wavelength, the other variable can be calculated. For example, your favorite FM radio station might be located at 90.5 on the dial. This is shorthand for a broadcast frequency of 90.5 MHz (millions of Hertz) or 9.05 × 107 Hz. Using Equation (5.1), the wavelength of this light would be about 3.1 m, which is about 10 ft. Radio waves are very long waves compared to other types. Frequency can also be expressed in other units. One of the more common measurement units is wavenumbers. A wavenumber is the inverse of the wavelength measured in centimeters. Thus, one wavenumber is 1 cm−1. Another way of expressing the broadcast frequency of the above-mentioned radio station (0.5 MHz) would be in wavenumbers. To convert Hertz to wavenumbers, change the wavelength from 3.1 m to 310 cm and then take the reciprocal; the result is 3.2 × 10−2 cm−1. Likewise wavelength can be expressed in any unit of length. As you study various regions of the electromagnetic spectrum, you will see that sometimes the radiation is described
FIGURE 5.1 Electromagnetic radiation can be viewed as a sine wave. The wavelength is the distance between two corresponding peaks or valleys and is denoted by the Greek letter λ. The number of waves that passes a given point in 1 s is referred to as the frequency of the light and is denoted by the Greek letter υ.
in wavelength units and sometimes as frequency. Also, you will see that different units of wavelength or frequency are used depending upon the type of radiation. These conventions have arisen over time purely as a convenience. Spectroscopists (scientists who study light and matter) like to work in small, whole numbers if possible. They choose units of wavelength or frequency for a particular region so that they can work with small numbers. For example, in the ultraviolet and visible region of electromagnetic radiation, scientists generally use wavelengths as a measuring unit. In particular, they measure wavelength in billionths of meters (10−9 m or nanometers (nm)). Using this unit, ultraviolet light comprises 200 to about 450 nm and visible light runs from 450 to about 750 nm. Electromagnetic radiation can be thought of as consisting of tiny packets of energy (E) called photons. The energy of a photon can be described in terms of the wavelength or frequency of the radiation as shown in Equations (5.2) and (5.3).
E=hν (Equation 5.2)
E=hc/λ (Equation 5.3)
In these equations, “h” is a constant of proportionality called Planck’s constant. It ensures that the units are the same on both sides of the equation. These equations show that, as the frequency of light increases, so does its energy and as the wavelength increases, the energy decreases. Electromagnetic radiation exists as a continuum of wavelengths from the very short to very long. It is probable that there exists radiation of wavelengths that are too short for modern measuring instruments to even detect. The continuum of electromagnetic radiation that we are aware of and can measure is depicted in a chart in Figure 5.2.
At the far left of this electromagnetic spectrum are gamma rays. These are very energetic and can pass through matter. They can be dangerous to life in that they can damage or destroy cells. Next lower in energy are X-rays. These rays can also pass through most matter but are deflected by dense matter such as bones. This is the principle behind the cameras that are used to take X-ray pictures of peoples’ insides. The X-rays reflect off the bone and other dense tissue and are detected while the others pass through soft tissue. The next major region of the electromagnetic spectrum is called the ultraviolet. This region contains ultraviolet radiation and vis ible light. These two areas are lumped together because both UV and visible light have the same effects on matter. Light in this region is not energetic enough to pass through matter. Instead, when a molecule absorbs this light, electrons are shifted from one orbital to another. An orbital is an energy level where an electron resides. The ultraviolet region is so-called because it borders on the violet area of the visible region, which is the light that human eyes can detect and see as color. As frequencies of visible light decrease, the light changes from violet down to red at the lowest frequencies. Figure 5.3 shows the color spectrum produced by visible light. Only certain types of molecules will absorb ultraviolet light. Most substances do not. The UV/visible region has many applications in the analysis of forensic evidence.
FIGURE 5.2 The electromagnetic spectrum. At the top of the chart are the frequencies of electromagnetic radiation or light in decreasing order. Scientists divide the spectrum into regions. Within each region, electromagnetic radiation has different effects on matter that it comes in to contact with. For forensic science purposes, the most important regions are the ultraviolet/visible (UV/visible) and the infrared (IR).
FIGURE 5.3 The color spectrum. When white light is refracted by a prism, it breaks up into various colors. Light of these wavelengths is called the visible spectrum because when photons reach our eyes, our optic nerves send images to our brain that registers as a color. The highest frequency (shortest wavelength) light is violet and the lowest is red. Frequencies higher than violet are in the ultraviolet region. We do not see this light as colored. Frequencies below the red are in the infrared region. We do not see this light as being colored either.
Lower in energy than the red region of visible light is the IR (infrared) region (infra means “below”). When absorbed by matter, this type of light causes bonds between atoms in a molecule to vibrate like two weights on either end of a spring. Every substance absorbs light in the IR region. Taken as a whole, the wavelengths of IR radiation absorbed are different for every substance. This makes it useful as a tool for the identification of a pure substance. The IR region is also very important in the analysis of chemical evidence in forensic science. At still lower frequencies than IR light is the microwave region. These light waves cause molecules to rotate or spin.
At the lowest end of the light spectrum are radio waves. These have very long wavelengths and thus very low frequencies and relatively little energy. Some of these waves are meters long! They carry radio and TV signals. Remember the example of the radio station at 90.5 MHz. Its wavelength is more than 10-feet long! Radio waves are transported through the air to the radio receiver by a carrier wave. This process is called “modulation”. Modulation can be accomplished using either amplitude (AM) or frequency (FM). Once the waves reach the radio receiver, the radio wave and the carrier wave are separated using a process called demodulation. Radio waves are not commonly used in forensic science as analytical tools.
