The focal length of a plane mirror is:

An object $O$ is placed between two parallel plane mirrors $M_1$ and $M_2$. The distance of the object from $M_1$ is $6\,cm$ and from $M_2$ is $15\,cm$. The position of the $4^{th}$ image formed by mirror $M_1$ is:

$A$ small object is placed $10\, cm$ in front of a plane mirror. If you stand behind the object $30\, cm$ from the mirror and look at its image,the distance focused for your eye will be........$cm$.

It is desired to photograph the image of an object placed at a distance of $3\;m$ from a plane mirror. The camera,which is at a distance of $4.5\;m$ from the mirror,should be focused for a distance of......$m$.

An object is at a distance of $0.5 \ m$ in front of a plane mirror. The distance between the object and its image is ..... $m$.

Assertion : If the angle between the two plane mirrors is $72^o$ and the object is asymmetrically placed between the two mirrors,then $5$ images of the object will be formed.
Reason : For a given system of mirrors,the total number of images formed due to successive reflection is equal to either $\frac{360^o}{\theta}$ or $\frac{360^o}{\theta} - 1$ accordingly as $\frac{360^o}{\theta}$ is odd or even respectively.