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From "Meso compound" in Wikipedia:

A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereogenic centers, the molecule is not chiral.

What is the meaning of "optical activity" in this context? In other words, what kind of photo activity requires chirality?

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  • $\begingroup$ Chirality can be observed by the way a beam of light is refracted after passing it through a solution of the subject compound $\endgroup$
    – Technetium
    Commented Oct 28, 2015 at 11:00
  • $\begingroup$ Rotation of plane of polarised light, as always. $\endgroup$
    – Mithoron
    Commented Oct 28, 2015 at 11:38

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The meaning of optical activity in this context is the same as usual: the ability to rotate the plane of polarized light. To have it, a molecule must be chiral.

Chirality is the absence of certain type of symmetry. Since most small molecules have that symmetry, many students get the wrong perception that chirality is a property which must be obtained by having some functional group (asymmetric center, etc.), much like having a $\ce{-COOH}$ group makes a compound an acid. This is not quite true. Not only a compound may be chiral without any chiral centers, but it can also be achiral despite having some chiral centers, as are meso compounds.

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