COMPOUND MAGNIFYING SYSTEMS: REFRACTIVE INDEX
The refraction of visible light is an important characteristic of lenses that allows them to focus a beam of light onto a single point. Refraction (or bending of the light) occurs as light passes from one medium to another when there is a difference in the index of refraction between the two materials, and it is responsible for a variety of familiar phenomena such as the apparent distortion of objects partially submerged in water. Refractive index (RI) is defined as the relative speed at which light moves through a material with respect to its speed in a vacuum. By convention, the RI of a vacuum is defined as having a value of 1.0. The index of refraction, N (or n), of other transparent materials is defined through the equation N=C/v where C is the speed of light and v is the velocity of light in that material. Because the RI of a vacuum is defined as 1.0 and a vacuum is devoid of any material, the refractive indices of all transparent materials are therefore greater than 1.0. For most practical purposes, the RI of light through air (1.0008) can be used to calculate refractive indices of unknown materials. Refractive indices of some common materials are presented in Table 4.2.
When light passes from a less dense medium (such as air) to a more dense medium (such as water), the speed of the wave decreases. Alternatively, when light passes from a more dense medium (water) to a less dense medium (air), the speed of the wave increases. The angle of refracted light is dependent on both the angle of incidence and the composition of the material into which it is entering. The normal is defined as a line perpendicular to the boundary between two substances. Light will pass into the boundary at an angle to the surface and will be refracted according to Snell’s Law: N1 ×sin(q1) =N2×sin(q2) In this equation, N represents the refractive indices of material 1 and material 2 and q are the angles of light traveling through these materials with respect to the nor mal. There are several important points that can be drawn from this equation. When N(1) is greater than N(2), the angle of refraction is always larger than the angle of incidence. Alternatively, when N(2) is greater than N(1), the angle of refraction is always smaller than the angle of incidence. When the two refractive indices are equal (N(1) = N(2)), then the light is passed through without refraction. The concept of RI is illustrated in Figure 4.9 for the case of light passing from air through both glass and water. Notice that while both beams enter the more dense material through the same angle of incidence with respect to the normal (60°), the refraction for glass is almost 6° more than that for water due to the higher RI of glass. Samples to be viewed in transmitted light must be in a material with a RI that is close to their own. Numerous materials are commercially available to use as mounting media or mountants. The RI of water is about 1.33 and therefore makes a poor mounting medium because it refracts the light so much less than a hair, which has a RI of about 1.5.
FIGURE 4.9 60 Samples to be viewed microscopically must be mounted in a material that has a refractive index (RI) near their own. If the RI of the sample and the mountant are too different, like this straw in water (RI = 1.33), then optical distortion results. The RI of glass is about 6° more than that for water, meaning that a light ray gets bent more passing through glass than water.
