different liquids acting like lens on watch glass
Answers
You need to keep in mind that lens are devices that work on the principle of refractive index of the medium. The ray of light when travels from one medium to another, it simply experiences a change in its direction.
This change in direction can actually magnify or diminish the size of the image formed at the other end. The images in a lens are formed as the ray of light is allowed to penetrate through the mediums used.
A plain watch glass is already a curved surface that has slightly different refractive index as compared to air. Apart from this, if you use a liquid that is also transparent but thicker in density it will offer with different refractive index as the ray of light passes from air to the liquid medium. This is what will force the medium to behave as a lens.
Water is one of the examples of such liquids that behaves as a lens as it magnifies the image to certain extent. If you are using another liquid medium like a clear Gel, then it will simply offer with greater refractive index and thus behave as a magnifying lens.