Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?

Where should the light source (bulb) be placed in the reflector of a searchlight or torch so that the light rays travel over a long distance?

The figure below shows a point source of light $S$,a convex lens $L$,and a plane mirror $M$. The three are placed such that rays of light from $S$ return to it after reflection from $M$. $(i)$ What is the distance $OS$ called? $(ii)$ To which point (left of $S$,on $S$,or right of $S$) will the rays return if $M$ is moved to the left and brought in contact with $L$?

An object is brought towards a concave mirror. How does the position and size of the image change?

Which type of image cannot be obtained by a concave mirror?

$(i)$ Describe an activity to find the approximate value of the focal length of a concave mirror.
$(ii)$ What happens to the size of the image of an object when it is moved gradually away from a convex mirror?
$(iii)$ In an experiment to study refraction through a glass slab,it is observed that a ray of light undergoing refraction emerges parallel to the direction of the incident ray. Why does it happen so? Explain with the help of a diagram.