The human eye is one of the five sense organs of our body and is also one of its most important parts. We just cannot imagine what our life would have been without it, light then being quite unknown to us.
The human eye has a nearly spherical shape, of diameter about an inch. The front portion, which is more sharply curved in the outside, is the cornea. It is that portion which is visible as white from the outside. Behind the cornea is a space filled with a liquid known as aqueous humour. Further behind is a crystalline lens. Between the aqueous humour and the lens is the iris, a muscular diaphragm which has a small hole in it known as the pupil, so as to allow the passage of light. The pupil usually appears black, and controls the amount of light entering the eye. In dark conditions the iris expands so as to allow more light to enter the eye. In bright light it contracts to reduce the diameter of the pupil. This is known as light and dark adaptation.
The crystalline lens is hard in the middle and gradually softens towards the edge. Its curvature can be altered by the ciliary muscles to which it is attached. While focussing on distant objects the ciliary muscles are relaxed so that the focal length of the eye-lens has its maximum value. While focussing on near objects the ciliary muscles are strained and the focal length of the eye-length decreases. This is known as the power of accommodation of the eye. However, the muscles cannot be strained beyond a limit and hence, if an object is brought too closer to the eye, beyond a specific point, the focal length cannot be adjusted so as to form a clear image on the retina. This point is known as the near point of the eye and its distance from the lens is known as the least distance of clear vision.
Light, entering the eye forms an image on the retina which covers the inside of the rear part of the eyeball. It is composed of three layers, the first one of which contains about 125 million photoreceptors called the rods and cones, which receive the light signal. The second layer contains bipolar neurons with synapses for connection with the third layer, which has the ganglion cells with dendrites in contact with the optic nerve, which transmits the information to the brain. Rods and cones have essentially the same structure, but different functions - the rods respond to the intensity of light, whereas cones help us to differentiate between colors.
The space between the lens and the retina is filled with another liquid called the vitreous humour. The aqueous humour and the vitreous humour have approximately the same refractive index, about 1.336. The average refractive index of the eye-lens is 1.396. The eye focuses on different objects by changing the radius of curvature of the lens to change the refractive index. The cornea-lens-fluid system is equivalent to a single converging lens whose focal length can be adjusted by the ciliary muscles.
The eyeball is covered by a hard cover, known as the schlera, which is mainly responsible for protection. Between the schlera and the retina lies another layer, the choroid. The two eyes are protected inside two sockets in our skull.
