THE HUMAN EYE & COLOURFUL WORLD
Our eye is the most important natural optical instrument. The eye resembles a camera in many ways. It has nearly a spherical shape.
What are the essential parts of the eye and their functions
The essential parts of the eye are shown in Fig. and are described below:
(a) Cornea: The front of the eye is covered by a transparent spherical membrane called the cornea. Light enters the eye through cornea. The space behind the cornea is filled with a clear liquid called aqueous humor.
(b) Iris and pupil: The iris and pupil form the variable aperture system of the eye. Iris is a dark coloured muscular diaphragm which has a small circular opening in its middle. The central circular aperture of iris is called pupil.
The iris regulates the amount of light entering the eye by adjusting the size of the pupil. This is explained below:
(i) When the light is very bright (as on a sunny day), the iris makes the pupil to contract. As a result, the amount of light entering the eye decreases.
(ii) When the light is dim (as in a dark room), the iris makes the pupil to expand. As a result, more light can enter the eye.
(c) Eye lens: The eye lens is a convex lens (converging lens) made of a transparent jelly-like proteinaceous material. The eye lens is hard at the middle and gradually becomes soft towards the outer edges. The eye lens is held in position by ciliary muscles. The curvature and therefore the eye lens may be changed by the action of the ciliary muscles.
(d) Retina: The inside surface of the rear part of the eye ball where the light entering the eye is focused is called retina. The surface of retina consist: of about 125 million light sensitive receptors. These receptors are of two types - rods and cones. When the light falls on these receptors, they send electrical signals to the brain through the optic nerve.
The space between the eye lens and retina is filled with another liquid called vitreous humor. The image formed on the retina is retained by it for about 1/ 16th of a second.
(e) Blind spot: There are no rods and cones at the point where optic nerve leaves the eye ball to go to the brain. So, if any image is formed in this part of the retina, then no signal is sent to the brain. As a result the object is not seen. This part is therefore called the blind spot of the eye.
HOW DOES THE EYE WORK?
The light coming from an object enters the eye through cornea and the pupil. The eye lens converges these light rays to form a real, inverted and high diminished image on the retina. The surface of retina consists of a large number of light sensitive cells.
When light falls on them, they get activated and generate electrical signals. These signals are then sent to the brain by the optic nerves, and the observer sees the actual sized, erect image of the object.
Ex. Why does it take some time to see objects in a dim room when you enter the room from bright sunlight outside?
Sol. In bright sunlight, iris makes the pupil to become small, so that only a small amount of light is able to enter the eye. When someone enters a dim room, very little light is available to the eye. The iris then makes the pupil to gradually expand (open up) to allow more light to enter the eye. This process takes some time. That is why the pupil takes a little time to adjust itself to dim light.
What is meant by the far point, near point, and the least distance of distinct vision?
(a) Far point: The farthest point up to which an eye can see clearly is called the far point of the eye. For a normal eye, the far point is at infinity.
(b) Near point: The closest (nearest) point up to which an eye can see clearly is called the near point of the eye. The near point for a healthy normal eye of an adult lies about 25 cm from the eye.
(c) Least distance of distinct vision: The minimum distance up to which an eye can see clearly without any strain is called the least distance of distinct vision (denoted by d or D). The least distance of distinct vision is thus equal to the distance between the eye and its near point. For a normal eye of an adult, the least distance of distinct vision is about 25 cm. This distance usually increases with age.
POWER OF ACCOMMODATION OF THE EYE:
What is meant by accommodation of the eye?
A normal eye can see both the distant and the nearby objects clearly. For a clear vision, the image of any object must fall on the retina. For a person, the distance between the retina and the eye lens is fixed. So, the distance of the image (v) from the eye lens is fixed. For objects at different distances, the values of u are different. So, to get the images at the same v, the focal length of the eye lens should be different.
The eye can focus the images of all the objects, distant or nearby, at the same place (on the retina) by changing the focal length of its lens. The eye lens can change its focal length by changing its thickness with the help of its ciliary muscles.
The property due to which eye lens is able to change its focal length is called accommodation of the eye.
When the eye is focussed on distant objects (objects at infinity), the ciliary muscle is fully relaxed. When the ciliary muscle is in the relaxed state, the thickness of the lens is minimum and the focal length maximum: equal to the distance of the retina from the eye lens. Therefore, the parallel rays coming from any distant object are focussed on the retina, and the object is seen clearly.
When the eye is focussed on a nearby object, the ciliary muscle gets strained (tense). Tension in the ciliary muscle decreases the focal length of the eye lens by slightly increasing its thickness in such a way that the image is formed on the retina. Thus, the eye focuses on the nearby objects by tensing the ciliary muscle.
These adjustments in the focal length of the eye lens take place so fast that we do not realize such changes.
What is meant by the limit of accommodation?
The eye can accommodate only up to a limit. A normal eye can accommodate up to the least distance of distinct vision: about 25 cm for a healthy adult. The objects which are very close to the eye produce blurred image. Thus, a normal eye can accommodate objects lying between infinity and the least distance of distinct vision.
What is the range of vision?
The range of distance over which the eye can see clearly is called its range of vision. A normal human eye can see objects clearly which lie between infinity and the least distance of distinct vision. So, the range of vision of a normal healthy human eye is from infinity to the least distance of distinct vision, i.e. from infinity to about 25 cm from the eye.
What is meant by the persistence of vision?
The image formed on the retina of the eyes is not permanent. It also does not fade away instantaneously after the object is removed or we have stopped seeing the object. The image formed on the retina persists for 1/16th of a second even after we have stopped looking at the object. The ability of the eye to retain the image (or the sensation caused by the light coming from the object) for about 1/16th of a second even after we have stopped seeing the object is called persistence of vision.
The phenomenon of persistence of vision is made use of in motion-picture projection (or cinematography). A sequence of still pictures are recorded on a film by a movie camera. This recorded film is projected on a screen at the speed of about 24 pictures per second. Due to the persistent of vision, the successive images on the screen merge smoothly into one another giving an impression of continuity. In this way, we are able to see the pictures in motion.
How do we see the colours?
We see an object only when its image is formed at the retina.
Our retina has a large number of light-sensitive cells. These cells are of two shapes - rods and cones .
The cone-shaped cells become active only in bright light. That is why, we can't differentiate between colours in dim light. There are in fact three different kinds of the cone-shaped cells: one sensitive to red, second to green and the third to blue colour. So, depending upon the colour of the light entering the eye, one or more kinds of the cone-shaped cells get activated.
How do certain animals including birds differ in their colour perception?
The structure and the number of rod-shaped and cone-shaped cells are different for different animals/birds. As a result, therefore, their colour perceptions are also different. For example,