(i) A lens is an optically transparent medium bounded by two spherical refracting surfaces or one plane and one spherical surface.
(ii) Lens is basically classified into two types. They are: (i) Convex Lens (ii) Concave Lens.

(i) The rays coming from the objects which are at a distance less than 25 cm are not properly a focused on retina therefore ciliary muscles cannot be contracted beyond this limit.
(ii) The focal length of the eye cannot be reduced below 25 cm.

(i) At sunrise and sunset, the light rays from the sun have to travel a larger distance in the atmosphere than at noon.
(ii) Hence, most of the blue lights are scattered away and only the red light which gets least scattered reaches us.
(iii) Therefore, the colour of the sun is red at sunrise and sunset.

(i) Due to ageing, ciliary muscles becoming weak and the eye-lens becoming rigid (inflexible), the eye loses its power of accommodation.
(ii) Because of this, an aged person cannot see the nearby objects clearly. So, it is also called as 'old age hypermetropia'.

(i) Near point : The minimum distance required to see the objects distinctly without strain is called least distance of distinct vision. It is 25 cm for normal human eye.
(ii) Far point: The maximum distance up to which the eye can see objects clearly is called as far point of the eye. It is infinity for normal eye.