If the focal length of a spherical mirror is $20 \ cm$,what is its radius of curvature (in $cm$)?

What is the effect on the wavelength of light when it travels from a rarer to a denser medium?

$(a)$ Calculate the distance at which an object should be placed in front of a convex lens of focal length $10\, cm$ to obtain a virtual image of double its size.
$(b)$ In the above given case,find the magnification,if the image formed is real. Express it in terms of the relation between $v$ and $u$.

The image formed by a convex mirror is only one-third of the size of the object. If the focal length of the mirror is $12 \, cm$,the image formed will be

An object $1 \, cm$ tall is placed at a distance of $15 \, cm$ from a concave mirror of focal length $10 \, cm$. Find the position,size,and nature of the image formed.

$A$ ray of light is incident on a plane mirror normally. What are the angle of incidence and the angle of reflection?