Solution:

The figure shows a simplified horizontal cross-section of the human eye. $\rightarrow$ The eye is nearly a sphere with an average diameter of approximately $20 \mathrm{~mm}$.

Three membranes enclose the eye the cornea s sclera outer cover, the choroid \& the retina.

The cornea is a tough; transparent tissue that covers the anterior surface of the eye Continuous with the cornea, the sclera is an opaque membrane that encloses the remainder of the optic globe.

The choroid lies directly below the sclera. This membrane contains a network of blood vessels that serve as the major source of nutrition for the eye.

The choroid coat is heavily pigmented and hence helps to reduce the amount of extraneous light entering the eye. and the backscatter within the optic lobe.

As its anterior is extreme, the choroid is divided into the ciliary body s the iris. The lens is colored by a slightly yellow pigmentation that increases with age.

The visible light spectrum, with relatively higher absorption at shorter wavelengths. Both infrared s ultraviolet light are absorbed appreciably by proteins within the lens structure.

When the eye is properly focused, light from an object outside the eye is imaged on the retina.

There are two classes of receptors: cones and 8 rods.

The cones in each eye number between 3 million and are located primarily in the central portion of the retina called the fovea and are highly sensitive to color.

Muscles controlling the eye rotate the eyeball until the image of an object of interest falls on the foveal.

Cone vision is called photopic or bright light vision.

The number of rods is much larger: some 75 to 150 million are distributed over the retinal surface.

The larger area of distribution and the fact that several rods are connected to a single nerve end reduce the amount of detail discernible by these receptors.

The larger area of Rods serves to give a general, overall picture of the field of view They are not involved in color vision and ? are sensitive to low levels of illumination.