Match the corresponding entries of Column $1$ with Column $2$. [Where m is the magnification produced by the mirror]

S.No.	Column 1	S.No.	Column 2
A	$m=-2$	a	Convex mirror
B	$m=-1/2$	b	Concave mirror
C	$m=+2$	c	Real image
D	$m=+1/2$	d	Virtual image

Two identical glass $(n_g= \raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.)$ equiconvex lenses of focal length $f$ each are kept in contact. The space between the two lenses is filled with water ($n_w=\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$). The focal length of the combination is:

A beam of light from a source $L$ is incident normally on a plane mirror fixed at a certain distance $x$ from the source. The beam is reflected back as a spot on a scale placed just above the source $L$. When the mirror is rotated through a small angle $\theta$, the spot of the light is found to move through a distance $y$ on the scale. The angle $\theta$ is given by:

An astronomical refracting telescope will have large angular magnification and high angular resolution, when it has an objective lens of