Objectives
1. Measure the refractive indices of solids and liquids by different methods:
- Direct ray tracing
- Traveling microscope (Duc de Chaulnes’ image displacement method)
- Methods of total internal reflection or critical angle measurement (also by use of refractometers – Abbe, Pfund, Pulfrich)
- Methods of minimum angle deviation
- Method of liquid lenses (by using both cylindrical and spherical lenses)
- Laser beam displacer
- Method of hollow cells (both rectangular and prismatic)
- Method of liquid immersion (including refractive index-matching liquids)
2. Compare these techniques for experimental errors, time consumption, affordability, reliability, amount of materials needed to complete measurements, and universality.
Background
Direct Ray Tracing
The simplest way to measure the refractive index of materials is direct ray tracing. This process includes tracing an input/output light beam, measuring angles of incidence α
i and of refraction α
r. Refractive index n can be found by applying Snell's law (sinα
0/ sin α
r = n This method was described in Chapter 5. The drawbacks of this method include low accuracy and special material requirements, such as an exact geometrical shape.
Recently, an interesting modification of direct ray tracing has been proposed in several papers (e.g. S. Lombardi, G. Monroy, I. Testa, E. Sassi, Measuring variable refractive indices using digital photos, Physics Education, 45(1), 83–92 (2010)). The authors proposed taking a digital picture of the laser rays propagating through a medium and measuring all the required angles from this photo.
Traveling Microscope (Duc de Chaulnes’ Image Displacement Method)
If an object S is observed through a transparent slab in the shape of a parallel plate, it appears displaced along the direction of observation by a distance SS′ = δ, as shown in Fig. 15.1. This fact can be used to measure the refractive index of the slab.