The human eye is one of the most sensitive sense organs of sight which enables us to see the wonderful world of light and colour around us. It is like a camera having la lens system and forming an inverted, real image on a light sensitive screen inside the eye. The structure and working of the eye is as follows :
Structure and Working of Human Eye
The human eye has the following parts :
(i) Cornea : It is the transparent spherical membrane covering the front of the eye.
(ii) Iris : It is the coloured diaphragm between the cornea and lens.
(iii) Pupil : It is the small hole is the iris.
(iv) Eye lens : it is a transparent lens made of jelly like material.
(v) Ciliary muscles : These muscles hold the lens in positions.
(vi) Retina : it is the back surface of the eye.
(vii) Blind spot : it is the point at which the optic nerve leaves the eye. An image formed at this point is not sent to the brain.
(viii) Aqueous humor : It is clear liquid region between the cornea and the lens.
(ix) Vitreous humor : The space between eye lens and retina is filled with another liquid called vitreous humor.
In the eye, the image is formed on the retina by successive refractions at the cornera, the aqueous humor, the lens and the vitreous humor. Electrical signals then travel along the optic nerve to the brain to be interpreted. In good light, the yellow spot is most sensitive to detail and the image is automatically formed there.
Power of Accommodation
The image of the objects at different distances from the eye are brought to focus on the retina by changing the focal length of the eye-lens, which is composed of fibrous jelly-like material, can be modified to some extent by the ciliary muscles.
Near Point and Far Point
The nearest point at which a small object can be seen distinctly by the eye is called the near point. For a normal eye, it is about 25 cm and is denoted by the symbol D.
With advancing age, the power of accommodation of the eye decreases at the eye lens gradually loses its flexibility. For most of the old persons aged nearly 60 years, the near point is about 200 cm and corrective glasses are needed to see the nearby objects clearly.
The farthest point upto which our eye can seen objects clearly, without any strain on the eye is called the far point. For a person with normal vision, the far point is at infinity.
Least Distance of Distinct Vision
The minimum distance of an object from the eye at which it can be seen most clearly and distinctly without any strain on the eye, is called the least distance of distance of distinct vision. For a person with normal vision, it is about 25 cm and is represented by the symbol D, i.e.
least distance of distinct vision = D = 25 cm.
Persistence of Vision
The image formed on the retina of the eye does not fade away Instantaneously, when the object is removed from the sight. The impression (or sensation) of the object remains on the retina for about (1/16)th of a second, even after the object is removed from the sight. This continuance of the sensation of eye is called the persistence of vision.
Let a sequence of still pictures is taken by a move camera. If the sequence of these still pictures is projected on a screen at a rate of 24 images or more per second then the successive impression of the images on the screen appear to blend or merge smoothly into one another. This is because an image (or a scene) on the screen appears just before the impression of previous image on the retina is lost. Hence, the sequence of images blend into one another giving the impression of a moving picture. This principle is used in motion picture projection or in cinematography.
Colour – Blindness
The retina of our eye has large number of light sensitive cells having shapes of rods and cones. The rod- shaped cells responds to the intensity of light with different of brightness and darkness were as the cone shaped cells respond to colour. In dim light rods are sensitive, but cones are sensitive only bright. The cones are sensitive to red, green and blue colour of light to different extents.
Due to genetic disorder, some persons do not possess some cone-shaped cells that responds to certain specific colours only. Such persons cannot distinguish between certain colour but can seen well otherwise. Such persons are said to have colour-blindness. Driving licenses are generally not issued to persons having colour-blindness.
Colour Perception of Animals
Different animals have different colour perception due to different structure of rod shaped cells and core shaped cells. For example, bees have some cone-shaped cells that are sensitive to ultraviolet. Therefore bees can seen objects in ultraviolet light and can perceive colours which we cannot do.
Human beings cannot seen in ultraviolet light as their retina do not have cone-shaped cells that are sensitive to ultraviolet light.
The retina of chicks have mostly cone shaped cells and only a few rod shaped cells. AS rod shaped cells are sensitive to bright light only, therefore, chicks wake up with sunrise and sleep in their resting place by the sunset.
Sometimes due to the formation of a membrane over the crystalline lens of some people in the old age, the eye lens becomes hazy or even opaque. This is called cataract. It results in decrease or loss in vision of the eye. Cataract can be corrected by surgery leading to normal vision.
Defects of Vision and their Correction
People with normal vision can focus clearly on very distant objects. We say their far point is at infinity.
People with normal vision can focus clearly on near objects upto a distance of 25 cm. We say their near point is at a distance 25 cm from the eye.
But there are some defected due to eye irregularities which are as follows :
Short Sightedness (or Myopia):
A person who can seen the near objects clearly but cannot focus on distant objects in short sightedness. The far point of a short-sighted person may be only a few metres rather than at infinity. This defect occurs if a person’s eyeball is larger that the usual diameter. In such a case, the image of a distant object is formed in front of the retina as shown in the figure. It is because the eye lens remain too converging, forming the image of the object in front of the retina.
To correct short-sighted vision, a diverging lens (concave lens) of suitable focal length is place din front of the eyes as whose in figure. The rays of light from distant object are diverged by the concave lens so that final image is formed at the retina. If the object is very far off (i.e. u ≅ ∞ ), then focal length of the concave lens is so chosen that virtual image of the distant object is formed at the far point F of the short-sighted eye. Therefore rays of light appear to come the image at the far point F of the short-sighted eye and not from the more distant object.
Note that focal length of the lens for a short-sighted person is equal to the negative value of the person’s far point.
Far Sightedness (or Hyperopia or Hypermetropia)
A person who can seen distant objects clearly but cannot focus on near objects is farsighted, whereas the normal eye has a near point of about 25 cm. A farsighted person may have a near point several metres from the eyes. This defect may occur if the diameter of person’s eyeball is smaller than the usual or if the lens of the eye is unable to curve when ciliary muscle contract. In such a case, for an object placed at the normal near point (i.e. 25 cm from eye), the image of the object is formed behind the retina as shown in the figure (i). It is because the lens of the eye is to sufficiently converging to focus the object located at the normal nearer point.
A farsighted person has the normal far point but needs a converging lens in order to focus objects which are as close as 25 cm. The converging lens of correct focal length will cause the virtual image to be formed at the actual near point of the farsighted person’s eye.
This defect arises with aging. A person suffering from this defect can see neither nearby objects nor distant objects clearly/distinctly. This is because the power of accommodation of the eye decreases due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
This defect can be corrected by using bi-focal lenses. Its lower part consists of a convex lens and is used for reading purpose whereas the upper part consists of a concave lens and in used for seeing distant objects.
A person suffering from this defect cannot simultaneously focus on both horizontal and vertical lines of wire gauze.
This defect arises due to the fact that the cornea is not perfectly spherical and has different curvatures for horizontally and vertically lying objects. Hence, objects in on direction are well focused whereas objects in the perpendicular direction are not well focused. This defect can be corrected by using cylindrical lenses. The cylindrical lenses are designed in such a way so as to compensate for the irregularities in the curvature of cornea.
Objective Practice Questions
- The focal length of eye lens controlled by-
(C) Ciliary muscles
(D) Optic nerve
- A white lights falls on a glass prism, the least deviated colour is –
- Blue colour of sky is due to –
(A) dispersion of light
(B) scattering of light
(C) refraction of light
(D) reflection of light
- Rainbow is formed due to –
(A) reflection and dispersion of light through a water droplet
(B) Total internal reflection, refraction and dispersion of light through a water droplet
(C) only dispersion of light
(D) only refraction of light
- Power of accommodation (max. variation in power of eye lens) of a normal eye is about –
- Dispersion of light by a prism is due to the change in –
(A) frequency of light
(B) speed of light
(D) none of these
- Least distance of distinct vision of a long-sighted man is 40 cm. He wish to reduce it to 25 cm by using a lens, the focal length of the lens is –
(A) + 200/3 cm
(B) − 200/3 cm
- Which of the following colour has the least wave length ?
- Convex lens of suitable focal length can correct –
(A) short sightedness
(B) long sightedness
- The focal length of human eye lens is –
Subjective Practice Questions
- What are the causes of near sightedness ?
- How is the amount of light entering our eye is controlled
- Which colour bends the maximum from its path when a beam of white light is incident on it ?