We need to be careful to distinguish between the kinds of "units" that we produce in writing, especially formal academic writing, and the kinds of "units" that we produce in conversational speech. Sentences in academic writing are usually syntactically "complete" units with full subjects and predicates. On the other hand, conversational speech proceeds with little bits and pieces of language--few people speak in full sentences. Some grammarians and linguists use the term utterance to refer to the units of spoken conversation.They've invented this term as a way to clarify the unit being studied and as a recognition that while both speech and writing use the same basic grammar they are not completely alike in how they use that grammar.
For a rough surface, reflected light rays scatter in all directions. This is called diffuse reflection. Diffuse reflection is when light hits an object and reflects in lots of different directions. This happens when the surface is rough.
