Zhe, Zha, or Zhu, sometimes transliterated as Že (Ж ж; italics: Ж ж or Ж ж; italics: Ж ж) is a letter of the Cyrillic script. It commonly represents the voiced retroflex sibilant /ʐ/ (listen) or voiced postalveolar fricative /ʒ/, like the pronunciation of the ⟨s⟩ in "measure". It is also often used with D (Д) to approximate the sound in English of the Latin letter J with a ДЖ combination. Zhe is romanized as ⟨zh⟩, ⟨j⟩ or ⟨ž⟩.
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In optics the Lagrange invariant is a measure of the light propagating through an optical system. It is defined by
where y and u are the marginal ray height and angle respectively, and ȳ and ū are the chief ray height and angle. n is the ambient refractive index. In order to reduce confusion with other quantities, the symbol Ж may be used in place of H. Ж is proportional to the throughput of the optical system (related to étendue). For a given optical system, the Lagrange invariant is a constant throughout all space, that is, it is invariant upon refraction and transfer.
Zayin (also spelled zain or zayn or simply zay) is the seventh letter of the Semitic abjads, including Phoenician zayn 𐤆, Hebrew zayīn ז, Aramaic zain 𐡆, Syriac zayn ܙ, and Arabic zāy ز. It represents the sound [z]. It is also related to the Ancient North Arabian 𐪘, South Arabian 𐩸, and Ge'ez ዘ. The Phoenician letter gave rise to the Greek zeta (Ζ), Etruscan z 
, Latin Z, and Cyrillic Ze З, as well as Ж.