Light

INSIDE OF EYE:

The eye is probably the most important optical instruments. In principle, an eye can be compared to a photographic camera. In a camera, the aperture of the lens has to be adjusted. But the eye can regulate its aperature at its own. In a cam- era, the image of an object is focussed on the film by adjusting the distance between the lens and the film. But the eye can sharply focus the images of the objects lying at different distances on the retina by changing the focal length of its lens through muscle action.. In otherwords, it is far more delicate and perfect than the finest camera designed by man.


Parts of the eye. The human eye is about 2-5 cm in diameter and is nearly spherical in shape. It is maintained in this shape by the pressure of the fluid inside it. Fig 4.01 shows a vertical section of the eye. Following are the parts of the eye

Sclerotic: It is a dense, opaque and horny outer covering of the eye ball, part of which is visible as the white of the eye. Its function is to protect the eye.

Cornea: In front of the eye, the sclerotic merges into a transparent curved tissue, known as cornea. The light enters the eye through the cornea.

Choroid. It is the black coating of the sclerotic saturated with a black pigment and its function is to absorb the superfluous light, which otherwise will produce a general illumination in the interior of the eye.

Iris:  It is a circular diaphragm behind the cornea. It has a central hole, known as the pupil of the eye. The iris contains the pigment that determines the colour of the eye. Through the muscle action, the iris can change the area of the pupil from 2 to 8 mm in diameter so as to adjust and admit a suitable quantity of light intothe eye.

Crystalline lens: Behind the iris is a converging lens composed of microscopic glassy fibres. It is more convex at its back than in front. It is held behind the iris by the cilliary muscles. The function of the lens is to form real and inverted images of external objects on the retina. When tension is exerted on the lens by the attached muscles, the glassy fibres slide over each other, causing the shape of the lens to change and thus focussing the image properly.

 The line joining the centres of the cornea and the crystalline lens is called the optic axis of the eye.

Retina: It is a light sensitive membrane on the back interior wall of the eye- ball. It is composed of nerves and two types of light sensitive cells, called rods and cones. The rods are more sensitive to light and distinguish light from dark in low light intensities. On the other hand, the cones can distinguish frequency ranges of sufficiently intense light, which the brain interprets as light of different colours. There are two distinct spots in the retina, one the yellow spot, the most sensitive Key part of the renita and the other, the least sensitive part, known as the blind spot. Most of the cones are clustered together around a a small depression in the centre of the yellow spot, called the fovea centralis. The nerve, which conveys the light signals from retina to the brain, is called optic nerve.

 The line joining the centre of the crystalline lens and the fovea centralis is called the visual axis of the eye.

Aqueous humour and Vitreous humour: These are watery liquids of almost the same refractive index and filling up the anterior and posterior chambers of the eyeball, partitioned by the cystalline lens. Light passing through the eyeball travels through five different media with different indices of refraction: air (u=1:00), the cornea (u=1.38), the aqueous humour (u=1.33), the crystalline lens (u=1-40) amd the vitreous humour (u=1.34). The various refractions at the boundaries between these media participate in the formation of an image. Most of the refraction of light occurs at the front surface of the cornea, while the crystalline lens makes the fine adjustments needed to focus the image. The images of the objects lying at different distances are focussed sharply on the retina.

Accommodation: It is the process by which the eye adapts itself for objects at different distances. 

 The focussing is done by the eye by altering the curvature of the crystalline lens, which is caused by a change in the tension on the lens by the attached muscles. When the eye is at rest, distant objects are focussed on the retina. But as the objects move towards the eye, the muscles are put to tension and bring their images on the Key retina, which otherwise would have been focussed behind the retina. This process by which the eye sees objects at all distances is called the accommodation. The range of accommodation gradually decreases with the age. It  is because, the crystalline lens loses its elasticity, as one grows older.

The extremes of the range, over which distinct vision is possible, are known as the far point and the near point. The far point is the greatest distance beyond which the normal eye can not see the objects clearly and it is taken to be infinity. point is the closest distance up to which the objects can be seen clearly on the extent to which the curvature of the crystalline lens can be changed by accommodation. The near point gradually recedes with age. 
A normal and relaxed eye can see an object most distinctly, when it is brought close to the eye up to a distance of about 25 cm. This distance is called the least distance of distinct vision.

Dispersion:
dispersion  refers to the spreading of light into its individual colors as it passes through a prism. This occurs because different colors of light have different wavelengths and thus different refractive indices.


Braille:
"Braille," it is a system of raised dots that can be felt with the fingertips and is used by people who are blind or visually impaired to read and write. Braille characters are created using combinations of six dots arranged in two columns of three dots each. Different combinations of raised dots represent different letters, numbers, and symbols.
Louis Braille, a French educator, developed the Braille system in the 19th century. The system has since become a crucial tool for blind and visually impaired individuals, providing them with a means of literacy, communication, and access to information.