The Human Eye
The Human Eye
The eye is probably the most important optical instrument.
The eye is probably the most important optical instrument. In principle, an eye can be compared to a photographic camera. In the camera, the aperture of the lens has to be adjusted. But the eye can regulate Its aperture on its own. In the camera, the image of an object is focused on the film by adjusting the distance between the lens and the film. But the eye can sharply focus the image of the objects lying at different distances on the retina by changing the focal length of its lens through muscle action. In other words, it is far more delicate and perfect than the finest camera designed by man. (The Human Eye)
Parts Of the Eye
The human eye is nearly spherical in shape and its diameter is about 2.5 cm. The pressure of the fluid inside the eye keeps its shape maintained nearly spherical. The vertical section of the eye is shown in [Fig. 4.001]. The human eye is found to consist of the following parts. (The Human Eye)
Sclerotic
Its prime function is to provide protection to the eye. It is an opaque, horny, and dense outer covering of the eyeball and is visible as the white part of the eye. (The Human Eye)
Cornea:
It is a transparent bulging outwards surface in front of the eyeball. It is the primary refracting surface of the eye and the light enters the eye through this part. The refractive index of this part is 1.37. Most of the refraction of light takes place at the outer surface of the cornea. (The Human Eye)
Pupil:
It is a hole having a diameter in the range between 3 mm to 7 mm. It is the pupil through which the light passes into the eyes. (The Human Eye)
Iris:
It is a circular diaphragm behind the cornea and it contains the pigment that determines the color of the eye. Through muscle action, the iris can change the area of the pupil from 2 mm to 8 mm in diameter so as to adjust and admit a suitable quantity of light into the eye. It, therefore, controls the size of the pupil. (The Human Eye)
Crystalline Lens:
Behind the iris, is a converging lens composed of microscopic glassy fibers. It has more convexity at its back than its front. It is held behind the iris by the ciliary muscles. The crystalline lens makes real and inverted images of the external objects at the retina. When tension is exerted on the crystalline lens by the attached muscles, the glassy fibers slide over each other, causing the shape of the lens to change and thus, the focus of the image is done properly.
Thus, the lens refracts the light rays so that a clear image is formed at the retina. Its refractive index lies between 1.38 and 1.40 The optic axis of the eye is the line joining the centers of the cornea and the crystalline lens. (The Human Eye)
Retina:
It is a light-sensitive membrane on the back interior wall of the eyeball. Rays of light after refraction by the cornea and lens are focused on the retina. It is made of nerves and two types of light-sensitive cells just like non-digital camera films. These are called rods and cones.
The rods, on one side, are more sensitive to light and distinguish light from dark in low light intensities. While on the other hand, the cones are capable of distinguishing frequency ranges of quite intense light, which the brain interprets as different colors. There are two distinct spots in the retina, namely called the yellow spot which is the most sensitive part of the retina, and the other is called blind spot which is the least sensitive part. Most of the are clustered together around a small depression in the center of the yellow spot, called the fovea centralis. (The Human Eye)
The nerve, which conveys the light signals from the retina to the brain, is called the optic nerve. The visual axis of the eye is the line joining the center of the crystalline lens and the fovea centralis. (The Human Eye)
Aqueous humor and vitreous humor:
Aqueous humor is a salty fluid that fills the space between the cornea and the eye lens having a refractive index of 1.337. Vitreous humor is a jelly-like fluid that fills the space between the retina and the eye lens having a refractive index of 1.437. The light passing through the eyeball travels through the five different media of different refractive indices.
The various refractions at the boundaries between these media, participate in the image formation of an object. Most of the refraction of light takes place at the front surface of the cornea, while the lens makes the fine adjustments required necessarily to focus the image. In this way, the images of the objects lying at different distances are focused sharply on the retina. (The Human Eye)
Working of Human Eye:
The light from an object is first received by the protective sheet called the cornea. The cornea bends the light rays in such a way that they pass freely through the pupil. The iris has the ability to enlarge and shrink, depending on the intensity of light that enters the eye. The light then moves into the lens, which focuses the rays onto the retina of the eye. The retina is covered with millions of light-sensitive receptors or cells. The light-sensitive cells convert the light reaching them into electrical signals. These electrical signals are carried by optic nerves to the brain, which interprets the signals and helps us to perceive the image.
Power of Accommodation:
The ability of the eye lens to adjust its focal length for objects at different distances is called accommodation. The focusing is done and the accommodation of the eye is achieved by altering the curvature of the crystalline eye lens. The curvature of the eye lens can be modified to some extent by the ciliary muscles.
When the muscles are relaxed and the eye is at rest, the lens becomes thin. So, its curvature increases. It results in an increase in the focal length of the lens. So, we can see distant objects. On the other hand, when ciliary muscles contract the lens becomes thick. So, the curvature of the eye lens decreases. The islands then become thicker as the focal length of the eyelash decreases. It helps us to see the nearby objects clearly.
Some Important Terms
Near Points:
The nearest point from the eye at which an object can be seen clearly by the eye is called the near point. For a normal eye, the near point lies at a distance of 25 cm. (The Human Eye)
Table for approximate Near Point age-wise
Far point:
The far point of the eye is defined as a point at the farthest point from the eye at which an object can be seen clearly seen. The far point lies at infinity for a normal eye. (The Human Eye)
Least distance of distinct vision:
The least distance of distinct vision is defined as the minimum distance from the eye at which the eye can observe the objects distinctly and clearly without any strain on the eye. For a normal eye, its value is 25 cm. (The Human Eye)
Range of Vision:
The distance between the near point and the far point of an eye is called the range of vision. For an adult normal eye, the range of vision varies from 25 cm to infinity. (The Human Eye)
Power of accommodation:
It is defined as the maximum variation in the power of the eye for focusing on near and for objects. For a normal eye, the power of accommodation is nearly about 4 diopters. (The Human Eye)
Persistence of vision:
The impression of the image on the retina remains for nearly 1/16th of a second even after the removal of the object from the eye. Thus, the phenomenon of the continuation of the impression of an image on the retina for some time, even after the light from the object is cut off is called the persistence of vision. (The Human Eye)
Rods:
Rods are the photoreceptors that provide vision during dim light or night also known as scotopic vision. Rods are generally rod-shaped light-sensitive cells of the retina which are responsible for twilight vision (black and white). These cells are very sensitive to the intensity of the light incident from the objects. Therefore, the rods are incapable of distinguishing between different colors.
Cones:
These are the photoreceptors capable of providing vision during the time of day or during the time of bright light. These are known as photopic vision. These are generally, cone-shaped light-sensitive cells of the retina that are responsible for color vision. The different cones are sensitive to different colors and so is their responsibility toward the colors. The R-cones, G-cones, and B-cones are sensitive to red, green, and blue light. The cones become active only in bright light. This is the reason that we cannot see the colors in dim light.
Color Blindness:
A person is said to suffer from color blindness when he is unable to distinguish between various colors but can see well otherwise. Color blindness is primarily due to the absence of either one type or two types or all three types of cones in the retina of the eyes.
