Have you attempted to do such a simple experience: immerse biconvex ("magnifying"glass in the water and look through it shipped items? Try it - you will be impressed by the surprise: the magnifying glass in the water almost does not increase! Immerse in water "diminutive" (biconcave) glass, and it turns out that it is almost lose the property to decrease. If you do experience not with water, and vegetable oil (e.g., cedar), having a refractive index larger than that of the glass, biconvex glass will reduce the items, a biconcave to increase them!
Recall, however, the law of refraction of light rays, and these miracles will not surprise you with its originality. Biconvex lentils increases in the air because the glass refracts light stronger than the surrounding air. But between the refractive power of the glass and water, the difference is small; therefore, if you put a glass lentils in water, the rays of light passing from water into the glass, will not experience large deviations. That is the reason that underwater magnifying glass is much weaker increase than in air, and the diminutive - weaker decreases.
Vegetable oil refracts rays stronger than glass, and therefore, in this liquid "magnifying" glass is reduced, and the "diminutive" increase. There are under water and empty (or air) lenses: concave increasing, convex reduce it. Goggles for diving represent such hollow lens.
Goggles for diving consist of hollow PLANO-concave lenses.
Ray MN, refracted, follows the path MNOP, leaving the inside of the lens from the perpendicular fall and coming closer to him (i.e. to or SIG) outside of the lens.
Therefore, the lens acts as a collective glass.