Composition for optical materials and use thereof

A composition for optical materials is herein disclosed which comprises the following two components (a) and (b): (a) a component containing at least one of poly isocyanates represented by the formula (1) A+545 S-B-NCO)n (1) wherein A is an alkanediyl group or an alkanetetrayl group which may contain a sulfur atom, or a dithianetetrayl group, and the alkanediyl group or the alkanetetrayl group may be substituted by a phenyl group; B is a methylene chain represented by +545 CH2+546 m (m is an integer of 1 to 4) which may contain a sulfur atom, and B's may be the same or different; and n is an integer of 2 or 4, and (b) a component containing at least one of polythiols having two or more mercapto groups. The composition is useful to provide optical products such as plastic lenses, filters, substrates for recording media and optical fibers which have a very high refractive index and which are excellent in heat resistance.