LYCOS RETRIEVER 
Quantum Theory: Photoelectric Effect
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Quantum theory was developed in the early 20th century to account for unexpected atomic behavior that could not be explained by classical mechanics. But at larger scales, objects' heat and motion blur out quantum effects, and interactions are ruled by classical mechanics, including gravitational forces and electromagnetism.
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Quantum mechanics, a theory of modern physics formulated in the first half of the twentieth century, successfully describes the behavior of matter on small scales. It explains and quantifies three effects that classical physics cannot account for:
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The introduction of external sources in the quantum theory will ... be discussed in detail. These sources are interpreted as representations of classical objects within the theory, and will lead to the concept of the functional generators of the correlation functions, and ultimately to the important concept of the effective action. Unlike the usual treatment, all these objects will be defined directly on the Euclidean lattice. The physical interpretation of the effective action, as well as its relation to the classical limit of the theory, will be discussed in detail.
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Photoelectric Effect