$A$ diminished virtual image can be obtained only in

$(a)$ State Snell's law of refraction.
$(b)$ When a ray of light travelling in air enters obliquely into a glass slab, it is observed that the light ray emerges parallel to the incident ray but it is shifted sideways slightly. Draw a ray diagram to illustrate it.

An object is placed at the following distances from a concave mirror of focal length $15 \, cm$.
$(a)$ $10 \, cm$ $(b)$ $20 \, cm$ $(c)$ $30 \, cm$ $(d)$ $40 \, cm$
Which position of the object will produce:
$(1)$ Virtual image
$(2)$ $A$ diminished real image
$(3)$ An enlarged real image
$(4)$ An image of same size

State Snell's law of refraction.

Find the position,nature,and size of the image of an object $3 \, cm$ high placed at a distance of $9 \, cm$ from a concave mirror of focal length $18 \, cm$.

$A$ concave mirror and a convex lens are held in water. What changes, if any, do you expect in their focal length?