A magician during a show makes a glass lens `mu = 1.5` disappear in a through of liquid. What is the refractive index of the liquid ? Is the liquid water ?

Explanation

The refractive index of the liquid must be equal to 1.47 in order to make the lens disappear. This means, `n_(1)=n_(2)`. This gives `1//f`=0 or `fto infty`. The lens in the liquid will act like a plane sheet of glass. No, the liquid is not water. It could be glycerine.