Most objects are seen by virtue of their reflected light. For a “rough” surface, the law of reflection ${\theta }_{\text{r}}={\theta }_{\text{i}}$ is obeyed at each point but the irregularities of the surface cause the reflected rays to leave in many random directions. This situation, shown in FIGURE 34.9, is called diffuse reflection. It is how you see this page, the wall, your hand, your friend, and so on.

FIGURE 34.9 Diffuse reflection from an irregular surface.

By a “rough” surface, we mean a surface that is rough or irregular in comparison to the wavelength of light. Because visible-light wavelengths are $\approx 0.5\mu \text{m}$, any surface with texture, scratches, or other irregularities larger than $1\mu \text{m}$ will cause diffuse reflection rather than specular reflection. A piece of paper may feel quite smooth to your hand, but a microscope would show that the surface consists of distinct fibers much larger than $1\mu \text{m}$. By contrast, the irregularities on a mirror or a piece of polished metal are much smaller than $1\mu \text{m}$.