The official Google Books page for the third edition offers a substantial preview (often 20-30% of the book). This is excellent for checking specific derivations (e.g., the quantization box volume) without paying.
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Searching for a "loudon quantum theory of light pdf link" will likely lead you to a mix of academic repositories, illegal file-sharing sites (Library Genesis, Sci-Hub, etc.), and fake "free PDF" spam pages. The official Google Books page for the third
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