If a spherical mirror is dipped in water, does its focal length change?

An air bubble is formed inside water. Does it act as a converging lens or a diverging lens?

Two converging lenses of unequal focal lengths can be used to reduce the aperture of a parallel beam of light without losing the energy of the light. This increase the intensity. Describe how the converging lenses should be placed to do this.

If a thin lens is dipped in water, does its focal length change?

Can mirrors give rise to chromatic aberration?