$A$ magician during a show makes a glass lens with $n = 1.47$ disappear in a trough of liquid. What is the refractive index of the liquid? Could the liquid be water?

(N/A) For a lens to disappear in a liquid,the refractive index of the lens $(n_{l})$ must be equal to the refractive index of the liquid $(n_{m})$.
According to the Lens Maker's Formula: $\frac{1}{f} = (\frac{n_{l}}{n_{m}} - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$.
If $n_{l} = n_{m}$,then $\frac{n_{l}}{n_{m}} = 1$,which implies $\frac{1}{f} = 0$,or $f \rightarrow \infty$.
Thus,the lens behaves like a plane glass plate and becomes invisible.
Therefore,the refractive index of the liquid must be $1.47$.
Since the refractive index of water is approximately $1.33$,the liquid cannot be water. It could be a liquid like glycerine,which has a refractive index of approximately $1.47$.