A double convex air bubble in water would behave as a

A double convex air bubble in water behaves as a divergent lens. According to the Lens Maker's formula, the focal length of a lens depends on the refractive index of the lens material relative to the surrounding medium [t1, t9]. For a standard convex lens, the material (glass) has a higher refractive index than the surrounding air, causing convergence. However, in the case of an air bubble in water, the refractive index of the lens material (air, n ≈ 1.00) is lower than that of the surrounding medium (water, n ≈ 1.33) [t5]. When light enters a medium of lower refractive index from a higher one, it bends away from the normal [t5]. Consequently, the convex surfaces of the air bubble cause parallel light rays to diverge rather than converge [t1, t2]. Thus, despite its physical convex shape, it acts as a concave or diverging lens [t3, t6].