State Snell's law of refraction.

$A$ convex mirror always forms a virtual and erect image.

$A$ $5 \, cm$ tall object is placed perpendicular to the principal axis of a convex lens of focal length $20 \, cm$. The distance of the object from the lens is $30 \, cm$. Find the $(i)$ position,$(ii)$ nature,and $(iii)$ size of the image formed.

Two lenses of power $+3.5 D$ and $-2.5 D$ are placed in contact. Find the power and focal length of the lens combination.

Which of the following can make a parallel beam of light when light from a point source is incident on it?

$(a)$ Write the relation between $u, v, f$ for lenses and for mirrors,where $u, v, f$ are object distance,image distance,and focal length respectively.
$(b)$ The magnification produced by a concave mirror is $m = +4$. Write the information about the image given by this statement.
$(c)$ Draw a ray diagram for the following and show the formation of the images in the case of a concave mirror when the object is placed:
$(i)$ Between the pole and the focus point.
$(ii)$ At the centre of curvature.