A concave mirror of focal length ' $f$ ' produces an image ' $n$ ' time the size of the object. If the image is real, then the distance of the object from the mirror is

A drum of radius ' $R$ ' full of liquid of density ' $d$ ' is rotated at angular velocity ' $\omega$ ' $\mathrm{rad} / \mathrm{s}$. The increase in pressure at the centre of the drum will be

In a vibrating string with fixed ends the waves are of type

The focal length of combination of lenses formed with lenses having power of +2.50 D and $-3.75$ D will be