Question

what is the opaque substance ?

Answer 1

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Opacity (optics) ... An opaque object is neither transparent (allowing all light to pass through) nor translucent (allowing some light to pass through). When light strikes an interface between two substances, in general some may be reflected, some absorbed, some scattered, and the rest transmitted (also see refraction).

Answer 2

the substancethrough which light cannot pass through is called an opaque substance