A lens inside a liquid is not visible. Why? Are lenses inside liquid water invisible?
 A: The refraction index of water is about 1.33, of air is 1. How about the lens material?  Probably about 1.5.  So it’s somewhat less of a difference, and in the other direction.
Also, did you try it?  Seems easy enough to answer that way.
A: For a full analysis have a look at Fresnel Equations
For normal incidence the amount of light reflected at an interface depends on the refractive index of the medium through which the light is travelling $n_1$ and the refractive index of the medium it is trying to enter $n_2$.
The fraction of the light which is reflected is 
$$\left | \dfrac {n_1-n_2}{n_1+n_2}\right |^2$$ 
So there is going to be more reflected light between an air, $n=1.00$, and glass $n=1.50$ interface than a water, $n=1.33$, and glass $n=1.50$ interface.  
A: A lens suspended in a refractive medium would not be visible if it has the same, or at least very similar, optical properties. This means that the refractive index, opacity, and absorption spectra of both the lens and the liquid should have values very close to each other. Additionally, the lens in question should transmit much more light than it reflects, as reflected light would make the lens visible in the liquid.
A: This is simply because the Refractive index of the material of the lens is equal or nearly equal to the Refractive index of the liquid medium. The geometrical bending of light rays do not take place as the velocity of light in both the medium is same, which is clear from the idea of equal Refractive index.So the lens is not visible.
So lenses inside liquid water may not be invisible.It depends on the difference of refractive indices of the material of the media. 
