Figure 1-8.–The law of refraction. Figure  1-9.–Diffraction. the index of refraction for that substance; for example, light travels about 1 1/2 times as fast in air as it does in glass, so the index of refraction for glass is about 1.5. Refraction, or change of direction, always follows a simple rule. “In passing from one transparent substance into another of greater density, refraction is toward the normal. In passing from one transparent substance into another of lesser density, refraction is away from the normal.” In this rule the normal is defined as a line perpendicular (90°) to the surface between the mediums. Refraction is shown in figure 1-8. The ray of light (AB) strikes the glass at an oblique angle. Since the glass is denser than air, the ray of light is bent toward the normal (RS) and emerges from the glass at (C). Upon entering the air again, the ray is bent away from normal (RS) and travels along the path (CD). All rays striking the glass at an angle other than perpendicular   are   refracted.   In   the   case   of   the perpendicular ray (ME) that enters the glass normal to the  surface,  no  refraction  takes  place  and  the  ray continues through the glass and into the air in a straight line. DISPERSION The speed of light in a medium depends on the wavelength of the light. As light enters a more dense medium, the short waves, such as blue, are slowed more than the long waves, such as red. Thus the index of refraction  of  a  medium  varies  with  the  wavelength,  and the different colors of light are bent different amounts. This changing index of refraction or the breaking up of white  light  into  its  component  colors  is  called dispersion. This then ties in with the previous discussion of the colors of light where we saw the way a prism creates a spectrum from white light. The prism is able to create this spectrum because of dispersion. DIFFRACTION We have said that light travels in a straight line. Well, that is not always true. An exception to this rule occurs when light travels close to an opaque edge. Because of the wave nature of their travel, light rays passing near an opaque edge are bent ever so slightly (fig. 1-9). This bending is called diffraction and is evidenced by the formation of a shadow with a fuzzy edge when light passes an opaque object. In this case, the outside edge of the shadow is light and indistinct, but it gradually darkens into the true black of the shadow that indicates that some of the light is scattered into the shadow area. Unlike  refraction,  in  diffraction  the  long  wave- lengths of light are bent the most. Diffraction is important to the photographer when the light passes the edges of a lens diaphragm. When the lens diaphragm is opened fully, the amount (actually the percentage) of diffracted light is quite small. But when the  diaphragm  is  closed  to  a  small  opening,  the percentage  of  diffracted  light  is  quite  large  and  reduces the sharpness of the image formed by the lens. In other words,  a  small  aperture  opening  interferes  with  the image-forming  light  more  than  a  large  aperture  does. POLARIZATION Energy in the form of wave motion radiates from its source and travels through a medium. For example, 1-6