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

A plane mirror is placed at the bottom of a tank containing a liquid of refractive index ' $\mu$ ', ' $p$ ' is a small object at a height ' $h$ ' above the mirror. An observer ' $O$ ' vertically above ' $p$ ' outside the liquid sees ' $p$ ' and the image in a mirror. The apparent distance between these two will be

Optical path of a particular ray of light has travelled a distance of 3 cm in flint glass is same as that on travelling a distance ' $x$ ' cm through another medium. The value of ' $x$ ' is [refractive index of flint glass $=1 \cdot 6$, refractive index of another medium $=1.25]$