Synopsis

The human eye functions on the principle of the sensation of vision. We see objects because of the light scattered from them falls on the eye. The eye has a lens in its structure. This eye lens forms a real and inverted image of an object on the retina. The distance between the lens and retina is about 2.5 cm i.e., for any position of the object in front of the eye the image distance is fixed and about 2.5cm. The maximum and minimum focal lengths of eye lenses are 2.5 cm and 2.27 cm respectively.
Sometimes the eye may gradually lose its ability for accommodation. In such conditions, the person cannot see an object clearly and comfortably. The vision becomes blurred due to defects of the eye lens. There are mainly three common defects of vision.

They are i) Myopia ii) Hypermetropia iii) Presbyopia

Myopia is also called nearsightedness. A person suffering myopia cannot see objects at a long distance but can see nearby objects clearly. To correct one's myopia by the bi-concave lens.
Hypermetropia is also known as "farsightedness". A person with hypermetropia can see the distant object clearly but cannot see objects at a near distance. The biconvex lens is used to correct the defect of hypermetropia.
Presbyopia is a vision defect. This happens due to the gradual weakening of ciliary muscles. To correct we need bi-focal lenses which are formed using both concave and convex lenses.

The refractive index of the material of the prism can be calculated by the formula

$$n=
\begin{matrix}
\frac{\left (Sin \frac{A+D}{2} \right )}{Sin\frac{A}{2}}
\end{matrix}$$

The splitting of white light into different colors (VIBGYOR) is called dispersion. The rainbow formation is the example of dispersion of light.
Sun appears red in color during sunrise and sunset due to scattering of light. The blue color of the sky is also due to scattering of light.