How to simulate convex lens and concave lens?

i dont know about the shader but a tried some method the generate convex or concave effect. 1) Create a render texture. 2) Add a camera and use the above created render texture as renderer in camera. 3) create a new material (Unlit->Texture). Enter the above render texture as its texture. 4) Apply this material on the object on which you want the effect. Hope this might work for you !!!