The Becke Line Immersion Method
The human eye can detect transparent objects in air such as glass fragments, because they refract light. If two objects have the same refractive index, then our eyes would not see any difference in the light that passed through them. The refraction of the light causes light beams that pass only through the air and beams that pass through the air and the object to reach our eyes at slightly different times and angles, thus enabling the eye to distinguish them. If a piece of glass is immersed in a liquid whose refractive index matches that of the glass, then the glass should be invisible because light that passes through the glass would have the same refractive index as the liquid and our eyes would not be able to detect any difference. In reality, if a piece of glass is immersed in a liquid with the same refractive index, the glass usually does not completely disappear. This is because there are other effects caused by light passing through glass. These are more pronounced when the glass is thick. The result is that we can still see faint borders of the glass even if they have the same refractive index. When a piece of glass is immersed in a liquid of different refractive index and observed under a microscope with transmit ted light, a bright halo in the shape of the glass, will appear to surround the glass. This halo is called the Becke line and it is caused by the difference in refraction by the glass and the liquid. If the glass and liquid have the same refractive index, the Becke line will disappear even if the glass does not. The Becke line can be clearly seen in Figure 1.
FIGURE 1 A piece of glass immersed in a liquid of different refractive index. The Becke line can be seen as a bright halo around the glass. If the liquid and the glass had the same refractive index the halo would disappear. In theory the glass would also disappear; however, if it is relatively thick, then there are other factors such as diffraction that prevent the glass from disappearing completely.
In practice, the Becke line method takes advantage of several properties of refractive index. As an example, consider the case described at the beginning of this section of the chapter, of the hit and run where pieces of glass from the head light of a car are found in the clothing of the victim. Samples would be collected from the headlight (knowns) and from the victim’s clothes (unknowns). The glass chemist would need to have a set of liquids that are made to have an accurately determined refractive index. Each sample would be tested to determine its approxi mate refractive index using the standard liquids. This can be done by mounting a piece of glass that is immersed in a liquid under a microscope. The Becke line is found and then the focus of the microscope is changed so that the objective lens and the glass are moved away from each other. The Becke line will move in or out toward the medium (glass or liquid) that has the higher refractive index. Using this information, a liquid is chosen that has a slightly higher refractive index than the glass being examined. A microscope with a hot stage can be used to perform this analysis. This is shown in Figure 2. A commercial instrument known as a Glass Refractive Index Measuring Instrument (GRIM) is used in most forensic science laboratories to determine the refractive index of small glass particles. A particle of glass is immersed in a small amount of a liquid whose refractive index is slightly higher than that of the glass. The GRIM has a sensor that can determine when the Becke line disappears. It also has a thermocouple that measures changes in temperature of the refractive index liquid. Under computer control, the GRIM raises the temperature of the liquid until the Becke line disappears and the refractive index of the glass can be determined.
FIGURE 2 A compound microscope with micro hot stage that is used to determine the refractive index of a microscopic particle such as glass.
The Becke line method is very accurate and precise, but it is always advisable to get as much data as possible before rendering an opinion about the association of evidence. The Becke line experiment is usually performed using light at 589 nm (the sodium D line). If a different wavelength of light is used, the refractive index of the liquid and the glass will be different, so the experiment can be repeated at several different wavelengths. Then a plot of wavelength versus refractive index can be constructed for each glass particle. This is called a dispersion plot. See Figure 3. It is not uncommon for two pieces of glass to have the same refractive index at 589 nm (or another single wavelength) and yet have different refractive indices at other wavelengths. If two pieces of glass have the same dispersion curve, this is good evidence that they could have come from a common source.
FIGURE 3 A dispersion curve. This curve measures the variation of refractive index of glass with the wavelength of light used to measure it. Shorter wavelengths produce higher refractive indices. These lines are constructed from three data points and the best fit line is constructed by regression analysis.
