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## QuestionPhysicsClass 12

A magician during a show makes a glass lens with $$n_g=1{\cdot}47$$  disappear in a trough of liquid. What is the refractive index of the liquid? Could the liquid be water?

Let $$n_g=$$  ref. index of glass, $$n_l=$$  ref. index of liquid, $$f_l=$$  focal length of liquid
$$\frac 1{f_l}=\left(n_g-1\right)\left[\left(\frac 1{R_1}\right)-\left(\frac 1{R_2}\right)\right]$$
For $$f_l={\infty}$$  (to make glass disappear), $$n_g=n_l$$ ,
$$n_l=1{\cdot}47$$ , thus it will behave as a plane glass.
No, the liquid can not be water because it’s ref. index is not $$1.33.$$