So I stand up and hold it in front of me ... how does it know how far back the light should be bent? It could be bent in front of me ... the same goes for lying

So I stand up and hold it in front of me ... how does it know how far back the light should be bent? It could be bent in front of me ... the same goes for lying on the ground. Also, if it is dark under the blanket then it must be one way ... if it is one way it would be dark underneath so it would show darkness even if it could detect the level it should be showing between the person lying down and the earth beneath them.