What type of Len is an air bubble inside water
Air bubble inside water is behaving like a Diverging Lens as shown in figure.
when light ray enters from water to air, it is refracted so that it deviates away from normal because light ray entering from denser medium to rarer medium. when light ray emerging from air bubble again refraction is taking place at air-water boundary. But the overall effect is it is giving divergence to light ray as can be seen from figure