Polymerizable composition containing reactive silsesquioxane compound containing phenanthrene ring

A polymerizable composition comprising:

• 100 parts by mass of a reactive silsesquioxane compound (a), which is a polycondensate of an alkoxy silicon compound A of formula [1] and an alkoxy silicon compound B of formula [2], and 10 to 500 parts by mass of a polymerizable compound having at least one polymerizable double bond (b):
• X—Si(OR1)3 [1]
• (wherein X is a phenyl group having at least one substituent having a polymerizable double bond, naphthyl group having at least one substituent having a polymerizable double bond, biphenyl group having at least one substituent having a polymerizable double bond, or a C1-10 alkyl group having at least one (meth)acryloyl group, and R1 is a methyl group or ethyl group);
• Ar1—Si (OR2)3 [2]
• (wherein Ar1 is a phenanthryl group optionally substituted with a C1-6 alkyl group, and R2 is a methyl group or ethyl group).