Images Videos In the reflection of light, the angle of incidence is equal to the angle of reflection, measured from the normal (the line perpendicular to the point of impact). Figure 1: The law of refraction. Plane light wave at position AA′ in medium of index n and BB′ in medium of index n′ (see text). Figure 2: Relationships between refractive indices and dispersive powers of several representative optical glasses and plastics. The hair-thin fibres used in fibre optics. Figure 3: Graphic refraction procedures (see text). Figure 4: Trigonometrical ray tracing (see text). Figure 5: The Gauss theory (see text). Figure 6: Operating principle of the telescopic rifle sight (see text). Figure 7: Porro prism. Light ray passing through an optical fibre. Figure 8: Lens aberrations. Image formation in a microscope, according to the Abbe theory. Specimens are illuminated by light from a condenser. This light is diffracted by the details in the object plane: the smaller the detailed structure of the object, the wider the angle of diffraction. The structure of the object can be represented as a sum of sinusoidal components. The rapidity of variation in space of the components is defined by the period of each component, or the distance between adjacent peaks in the sinusoidal function. The spatial frequency is the reciprocal of the period. The finer the details, the higher the required spatial frequency of the components that represent the object detail. Each spatial frequency component in the object produces diffraction at a specific angle dependent upon the wavelength of light. Here, for example, a specimen with structure that has a spatial frequency of 1,000 lines per millimetre produces diffraction with an angle of 33.6°. The microscope objective collects these diffracted waves and directs them to the focal plane, where interference between the diffracted waves produces an image of the object. Figure 9: Two-lens coherent optical processing system, showing how the raster periodicity is removed but the scene information is retained (see text). Figure 10: Holography. Optics of the pupil Explanation of concave and convex lenses.