ok....this is a very nice Q. provided that ur solving this one for the first time.....

there's a glass cube of edge length 'a'.......and made of material of refractive index  .......there's a dust particle placed at the geometrical centre of the cube........wats the amount of area that ill have to cover on the surface of the cube so that i am not able to see the dust particle from outside..........

im telling u.....ull feel good after solving this one.......