The bob of a pendulum of length ' $l$ ' is pulled aside from its equilibrium position through an angle ' $\theta$ ' and then released. The bob will then pass through its equilibrium position with speed ' $v$ ', where ' $v$ ' equal to ( $g=$ acceleration due to gravity)

On the surface of the liquid in equilibrium, molecules of the liquid possess

An optician makes spectacles having a combination of a convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm . The power of this combination of lenses in dioptre is

A ray of light travelling through a rarer medium is incident at very small angle ' $i$ ' on a glass slab and after refraction its velocity is reduced by $25 \%$. The angle of deviation is