Make That Invisible! Refractive Index Matching
Students determine the refractive index of a liquid with a simple technique using a semi-circular hollow block. Then they predict the refractive index of a material (a Pyrex glass tube) by matching it with the known refractive index of a liquid using the percent light transmission measurement. The homemade light intensity detector uses an LED and multimeter, which are relatively inexpensive (and readily available) compared to commercially available measurement instruments.
Figure 1. Look carefully: Is one of the glass stirring rods invisible?Copyright 2013 Marjorie Hernandez, RET Program, College of Engineering, University of Houston
Refractive index is a fundamental optical property of materials, and knowing the accurate value of a material's refractive index enables us to predict the angle that light is bent as it passes through the material, which is important in many real-world applications. Chemical, environmental and biomedical engineers take advantage of refractive index matching to minimize (if not remove) multiple scattering when capturing images to study the properties and behavior of micro- and nano-sized particles (such as bacterial and colloidal systems). Optical engineers use accurate measurements of refractive index to design optical instrument components such as lenses, microscopes, telescopes as well as other equipment that utilize the properties of light. Mechanical engineers must know the refractive index of fluids and other materials to build efficient and affordable machines. These examples illustrate the importance of knowing and understanding the concept of refractive index. Numerous methods and modern instruments are available to accurately measure the refractive index of various materials.
After this activity, students should be able to:
- Determine the relationship of the angle of incidence and the angle of refraction between two different media.
- Measure the refractive index of a given liquid using Snell's law.
- Determine the refractive index of an unknown material using percent light transmission.