The photographs are built up from the interaction of photons of light with either small silver halide crystals, or the pixelated grid of an electronic light sensor. The image is formed by discrete interactions, not continuous waves, so the photos are grainy.
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This is discussed on page 588. The graininess can also be attributed to the printing process - for example, the printing process may also use discrete dots of ink to create a photograph. Search for "halftone" to read much more about this answer.