When a plane mirror is rotated through a certain angle,the reflected ray turns through twice as much and the size of the image

An object $4 \ cm$ high is placed at a distance of $27 \ cm$ in front of a convex lens of focal length $18 \ cm$. Find the position,nature,and size of the image formed.

$(a)$ For the same angle of incidence $45^{\circ}$,the angle of refraction in two transparent media $I$ and $II$ is $20^{\circ}$ and $30^{\circ}$ respectively. Out of $I$ and $II$,which medium is optically denser and why?
$(b)$ Light enters from air to diamond,which has a refractive index of $2.42$. Calculate the speed of light in diamond,if the speed of light in air is $3.00 \times 10^{8} \text{ m s}^{-1}$.

Name two spherical mirrors. Define them. Write any three points of difference between them.

What is the $SI$ unit of the power of a lens?

$A$ convex mirror is used to form the image of a real object. Then tick the wrong statement.