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Online text: www.rp-photonics.com/group_index.html

Cited passages: Rüdiger Paschotta, RP Photonics Encyclopedia, Group Index: What is a Group Index? and Group Index and Time Delays.

Online text: www.rp-photonics.com/optical_path_length.html

Cited passages: Rüdiger Paschotta, Optical Path Length: waveguide caveat; Rüdiger Paschotta, RP Photonics Encyclopedia, Optical Path Length: definition and applications.

Online text: www.rp-photonics.com/wavefronts.html

Cited passages: Rüdiger Paschotta, RP Photonics Encyclopedia, Wavefronts: What are Wavefronts? and Various Details; Rüdiger Paschotta, Wavefronts: Various Details, anisotropic propagation.