the glass has a higher index of refraction (n) than the air in the bubble, when light goes from a material with a larger n to one with a smaller n, it refracts away from the normal, when the light exits the bubble it refracts back towards the normal, such that the light is exiting the bubble at the same angle as it entered the bubble, this means that the light rays aren't convergent anywhere on the other side of the bubble as the light source, this is the same as a concave (diverging) lens