Below is the picture of a newton rigns experiment but u can see the rings at the lens surface outside the apparatus , what is the reason for these fringes to occur ? The pic was taken with a mobile camera (2MP) 1st explanation :I think this is like two virtual sources of fresnel biprism , here the lens is ciruclar so the light from sodium lamp strikes it & there must be something inside the lens that produces these fringes . 2nd explanation : Light travels thru glass plate from fringes formed in apparatus & intereferes at the lens of camera with the direct light come from sodium lamp & both r in same phase to give interference pattern . I am more in favour of my 1st thought , what do u guys think ??