An air bubble in water will act like a

An air bubble in water acts as a diverging (concave) lens because the refractive index of air is lower than that of water, so rays bend away from the normal on entering and then refract back on exit, producing overall divergence and a virtual image. Treating the spherical bubble as two refracting surfaces shows the system behaves like a lens with negative focal length: the curved interfaces are convex toward water but effectively act as concave surfaces for rays inside the denser medium, causing spreading of rays and virtual image formation. Experimental and theoretical treatments describe bubbles as inverted spherical lenses (diverging) and note the bubble produces a virtual, reduced image, consistent with concave (diverging) lens behaviour [1].

Sources

1. [1] https://www.sciencedirect.com/science/article/abs/pii/S092777571500117X