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

Figure 1-10. Controlling polarized light.

when a section of line is secured at one end and the free end is held in your hand and given a shake, a wave travels down the length of the line from the end that was shaken to the secured end just like an oscillator. A light source acts as an oscillator. The wave motion in the line, however, does not represent the true wave motion of light because light waves move in all possible directions at right angles to their direction of travel. A much clearer picture of light wave motion can be seen by having a number of parallel lines with each one being shaken in a different direction-one up and down, one sideways, and the others at various angles in between.

Ordinarily, light waves vibrate in all directions at right angles to their direction of travel. However, when light waves strike a series of parallel microscopic slots, all the light that passes through vibrates in one direction. This is polarized light. Filters that polarize light, termed polarizing filters, have a practical use in photography (fig. 1-10).

Specular reflected light, from a nonmetallic surface at any angle between 32 and 37, is polarized in such a manner that the light rays vibrate in a direction parallel to the reflecting surface. Light reflected in this manner is said to be plane polarized and is seen as glare (fig. 1-11). There is no polarization whatsoever produced by reflections from metallic surfaces.