We have analyzed what is called a transmission grating, with many parallel slits. In practice, most diffraction gratings are manufactured as reflection gratings. The simplest reflection grating, shown in FIGURE 33.10a, is a mirror with hundreds or thousands of narrow, parallel grooves cut into the surface. The grooves divide the surface into many parallel reflective stripes, each of which, when illuminated, becomes the source of a spreading wave. Thus an incident light wave is divided into $N$ overlapped waves. The interference pattern is exactly the same as the interference pattern of light transmitted through $N$ parallel slits, so Equation 33.15 still applies.

FIGURE 33.10 Reflection gratings.

FIGURE 33.10 Full Alternative Text

The rainbow of colors reflected from the DVD surface in Figure 33.10b is an everyday display of this phenomenon. The surface of a DVD is smooth plastic with a mirror-like reflective coating in which millions of microscopic holes and lines, each less than $\text{5.0 }\mu \text{m}$ in width, encode digital information. From an optical perspective, the array of holes in a shiny surface is a two-dimensional version of the reflection grating shown in Figure 33.10a. Reflection gratings can be manufactured at very low cost simply by stamping holes or grooves into a reflective surface, and these are widely sold as toys and novelty items. Rainbows of color are seen as each wavelength of white light is diffracted at a unique angle.

Naturally occurring reflection gratings are responsible for some forms of color in nature. The brilliant colors of a peacock feather, seen in the photo at the beginning of this chapter, are due not to pigments but to the structure of the feather. A microscope would reveal that each feather consists of millions of regularly spaced microscopic structures called barbules. The barbules act as a reflection grating and create the ever-changing, multicolored hues of iridescence as the angle between the grating and your eye changes. The iridescence of some insects is due to diffraction from parallel microscopic ridges on the shell.