The cutoff between visible and infrared light is usually said to be somewhere between 700 and 800nm. why is silicon transparent to most infrared light but opaque to visible light?

The  infrared  photon  does  not  have  greater  energy as  compare  to   visible  rays  so  infrared  rays  pass  through  but  due  to  greater  energy  than  the  gap  visible  rays  could  not  hence  silicon  is  transparent      to infrared  rays  but  silicon  is   opaque  to   visible  rays .

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