How many images are formed by two parallel mirrors?

What type of lens behaviour will an air bubble inside water show?

The figure below shows an object $AB$ placed on the principal axis of a lens $L$. The two foci of the lens are $F_1$ and $F_2$. The image formed by the lens is erect,virtual and diminished. Copy the diagram and answer the following questions: $(i)$ Draw the outline of the lens $(L)$ used. $(ii)$ Draw a ray from $B$ passing through $O$. Show the ray after refraction by the lens. $(iii)$ Draw a ray of light starting from $B$,which after passing parallel to the principal axis,is incident on the lens and emerges after refraction from it. $(iv)$ Locate the final image formed.

If the magnification of an image formed by a mirror is $(-\frac{1}{3})$,what type of mirror is it?

The power of a lens is $5$ dioptre. What is the focal length of the lens (in $m$)?

$A$ ray is incident along $ACB$ on a spherical mirror,where $C$ is the centre of curvature. In which direction will the ray be reflected?