We are not talking about the nature of God here or transubstantiation, we are talking of opposites, good and evil. Scripture and Tradition tell us that the Trinity exists, that Christ has 2 natures. We have Our Lord's word that His Body and Blood become actually present at Mass. These are mysteries which have been revealed to us which we accept through the gift of Faith not initially through reason.
What we don't have is any revelation whatsoever that something can be both good and evil simultaneously. 
As far as the nature of light is concerned I believe that scientific observation has established this and being no scientist I accept this.