$A$ mirror of focal length $f$ is immersed in water $(\mu = 4/3)$. The focal length of the mirror in water will be ......

$A$ spherical mirror forms an erect image of an object,three times the size of the object. If the distance between the object and the image is $80 \ cm$,then the focal length of the mirror is ......... $cm$.

An object ' $O$ ' is placed at a distance of $100\,cm$ in front of a concave mirror of radius of curvature $200\,cm$ as shown in the figure. The object starts moving towards the mirror at a speed of $2\,cm/s$. The position of the image from the mirror after $10\,s$ will be at ...... $cm$.

$A$ thin rod of length $f/3$ lies along the axis of a concave mirror of focal length $f$. One end of its magnified image touches an end of the rod. The length of the image is

$A$ concave mirror of focal length $f$ (in air) is immersed in water $(\mu = 4/3)$. The focal length of the mirror in water will be

An object $1 \, cm$ tall is placed $4 \, cm$ in front of a mirror. In order to produce an upright image of $3 \, cm$ height,one needs a: