Contents
From the study of subatomic particles to the laws of motion, Physics Topics offer insights into the workings of the world around us.
Human Eye – Definition, Structure, Function, Parts and Range of Vision
In the previous Chapter we have studied the refraction of light by lenses (convex lenses and concave lenses). The optical instruments such as cameras, microscopes, telescopes, film projectors and spectacles, etc., work on the refraction of light through various types of artificial lenses (man-made lenses) made of transparent glass.
The human eye works on the refraction of light through a natural convex lens made of transparent living material and enables us to see things around us. Our eye is the most important optical instrument gifted to us by the God. Without eye, all other optical instruments would have no value at all. In this chapter we will study the structure and working of the human eye.
We will also describe the common defects of eye (or defects of vision) and how they are corrected by using various types of lenses (in the form of spectacles). And finally, we will discuss the refraction of light through a glass prism, atmospheric refraction and scattering of light by atmosphere. Let us start with the human eye.
The main parts of the human eye are : Cornea, Iris, Pupil, Ciliary muscles, Eye lens, Retina and Optic nerve (see Figure). The eye-ball is approximately spherical in shape having a diameter of about 2.5 cm. We will now describe the construction and working of the eye.
Construction of the Eye
The front part of the eye is called cornea. It is made of a transparent substance and it is bulging outwards. The outer surface of cornea is convex in shape. The light coming from objects enters the eye through cornea. Just behind the cornea is the iris (or coloured diaphragm).
Iris is a flat, coloured, ring-shaped membrane behind the cornea of the eye. There is a hole in the middle of the iris which is called pupil of the eye. Thus, pupil is a hole in the middle of the iris. The pupil appears black because no light is reflected from it.
The eye-lens is a convex lens made of a transparent, soft and flexible material like a jelly made of proteins. Being flexible, the eye-lens can change its shape (it can become thin or thick) to focus light on to the retina. The eye-lens is held in position by suspensory ligaments. One end of suspensory ligaments is attached to the eye-lens and their other end is attached to ciliary muscles (see Figure 1).
Ciliary muscles change the thickness of eye-lens while focusing. In other words, the focal length of eye-lens (and hence its converging power) can be changed by changing its shape by the action of ciliary muscles. In this respect an eye differs from a camera. The focal length of the convex lens used in a camera is fixed and cannot be changed but the focal length of the convex lens present inside the eye can be changed by the action of ciliary muscles.
The screen on which the image is formed in the eye is called retina. The retina is behind the eye-lens and at the back part of the eye. The retina of an eye is just like the film in a camera. The retina is a delicate membrane having a large number of light sensitive cells called ‘rods’ and ‘cones’ which respond to the ‘intensity of light’ and ‘colour of objects’ respectively, by generating electrical signals.
At the junction of optic nerve and retina in the eye, there are no light sensitive cells (no rods or cones) due to which no vision is possible at that spot. This is called blind spot. Thus, blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye (see Figure).
When the image of an object falls on the blind spot, it cannot be seen by the eye. It should be noted that there is an eye-lid in front of the eye which is just like the shutter in a camera. When eye-lid is open, light can enter the eye but when eye-lid is closed, no light enters the eye.
The space between cornea and eye-lens is filled with a watery liquid called ‘aqueous humour’. And the space between eye-lens and retina is filled with a transparent jelly like substance called ‘vitreous humour’ which supports the back of the eye.
Working of the Eye
The light rays coming from the object kept in front of us enter through the cornea of the eye, pass through the pupil of the eye and fall on the eye-lens. The eye-lens is a convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. (Actually, the outer surface of cornea also acts as a convex lens due to which cornea converges most of the light rays entering the eye.
Only the final convergence of light rays is done by the eye-lens to focus the image of an object exactly on the retina). The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision. Actually, the retina has a large number of light-sensitive cells. When the image falls on the retina then these light-sensitive cells get activated and generate electrical signals.
The retina sends these electrical signals to the brain through the optic nerve and gives rise to the sensation of vision. Although the image formed on the retina is inverted, our mind interprets the image as that of an erect object. As far as physics is concerned, the eye consists of a convex lens (called eye-lens) and a screen (called retina). The eye-lens forms a real image of the objects on the retina of the eye and we are able to see the objects.
The human eye is like a camera. In the eye, a convex lens (called eye-lens) forms a real and inverted image of an object on the light-sensitive screen called retina whereas in a camera, the convex lens (called camera-lens) forms a real and inverted image of an object on the light sensitive photographic film.
The Function of Iris and Pupil
The iris controls the amount of light’entering the eyes. The iris automatically adjusts the size of the pupil according to the intensity of light received by the eye. If the amount of light received by the eye is large (as during the day time), then the iris contracts the pupil (makes the pupil small) and reduces the amount of light entering the eye (see Figure).
On the other hand, if the amount of light received by the eye is small (as in a dark room or during night), the iris expands the pupil (makes the pupil large) so that more light may enter the eyes (see Figure). Thus, the iris regulates (or controls) the amount of light entering the eye by changing the size of the pupil.
The iris makes the pupil ‘expand’ or ‘contract’ according to the intensity of light around the eye. If the intensity of the outside light is low, then the pupil expands to allow more light to enter the eye. On the other hand, if outside intensity of light is high, then the pupil contracts so that less light enters the eye.
It should be noted that the adjustment of the size of the pupil takes some time. For example, when we go from a bright light to a darkened cinema hall, at first we cannot see our surroundings clearly. After a short time our vision improves, and we can see the persons sitting around us.
This is due to the fact that in bright sunlight the pupil of our eye is small. So, when we enter the darkened cinema hall, very little light enters our eye and we cannot see properly. After a short time, the pupil of our eye expands and becomes large. More light then enters our eye and we can see clearly.
On the other hand, if we go from a dark room into bright sunlight or switch on a bright lamp, then we feel the glare in our eyes. This is due to the fact that in a dark room, the pupil of our eye is large. So, when we go out from a dark room into bright sunlight or switch on a bright lamp, a large amount of light enters our eyes and we feel the glare.
Gradually, the pupil of our eye contracts. Less light then enters our eye and we can see clearly. In this way, the iris also protects our eyes from the glare of bright lights.
Rods and Cones
The retina of our eye has a large number of light-sensitive cells. There are two kinds of light-sensitive cells on the retina : rods and cones.
(i) Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light.
Rods are the most important for vision in dim light (as during the night). We can see things to some extent in a dark room or in the darkness of night due to the presence of rod cells in the retina of our eyes.
Nocturnal animals (animals which sleep during the day and come out at night) like the owl have a large number of rod cells in their retina which help them see properly during the night when there is not much light (see Figure).
In fact, our night vision is relatively poor as compared to the night vision of an owl due to the presence of relatively smaller number of rod cells in the retinas of our eyes. Rod cells of the retina, however, do not provide information about the colour of the object.
(ii) Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light (or normal light). The cone cells of our retina also respond to colours. In other words, cone cells cause the sensation of colour of objects in our eyes.
The cone-shaped cells of the retina make us see colours and also make us distinguish between various colours (see Figure). Cone cells of the retina function only in bright light. The cones do not function in dim light. This is why when it is getting dark at night, it becomes impossible to see colours of cars on the road.
An Important Discussion
A normal eye can see the distant objects as well as the nearby objects clearly due to its power of accommodation. Before we discuss the power of accommodation of the eye, we should know the difference between the distant objects and nearby objects from the point of view of light rays received from them. This is described below.
(i) The rays of light coming from a distant object (at infinity) are parallel to one another when they reach the eye [see Figure(a)]. Actually, the rays of light given out by the distant object are diverging in the beginning but they become parallel when they reach the eye after travelling a large distance.
The parallel rays of light coming from a distant object need a convex eye-lens of low converging power to converge them or focus them to form an image on the retina of the eye. The convex eye-lens of low converging power is the one having a large focal length and it is quite thin.
(ii) The rays of light coming from a nearby object are diverging (or spreading out) when they reach the eye [see Figure (b)]. The diverging rays coming from a nearby object need a convex eye-lens of high converging power to converge them or focus them to form an image on the retina of the eye.
The convex eye- lens of high converging power is the one having a short focal length and it is quite thick. Keeping these points in mind, it will now be easy for us to understand the power of accommodation of the eye.
Accommodation
A normal eye can see the distant objects as well as the nearby objects clearly. We will now discuss how the eye is able to focus the objects lying at various distances. An eye can focus the images of the distant objects as well as the nearby objects on its retina by changing the focal length (or converging power) of its lens.
The focal length of the eye-lens is changed by the action of ciliary muscles. The ciliary muscles can change the thickness of the soft and flexible eye-lens and hence its focal length which, in turn, changes the converging power of the eye-lens. Let us see how it happens.
When the eye is looking at a distant object (at infinity), then the ciliary muscles of the eye are fully relaxed. The relaxed ciliary muscles of the eye pull the suspensory ligaments attached to the eye-lens tightly. The suspensory ligaments, in turn, pull the eye-lens due to which the eye-lens gets stretched and becomes thin (or less convex) [see Figure (a)].
The thin eye-lens has large focal length but its converging power is small. The small converging power of thin eye-lens is sufficient to converge the parallel rays of light coming from a distant object to form an image on the retina of the eye [see Figure (a)]. When the eye is looking at a distant object, the eye is said to be unaccommodated because it is the relaxed state of the eye.
The thin eye-lens is not powerful enough to converge the diverging light rays coming from the nearby objects onto the retina. So, to look at the nearby objects, the eye-lens has to change its shape and become thick (or more convex) to increase its converging power. This happens as follows :
When the same eye has to look at a nearby object, the ciliary muscles of the eyes contract. The contracted ciliary muscles make the suspensory ligaments loose. When the suspensory ligaments become loose, they stop pulling the eye-lens. The eye-lens bulges under its own elasticity and becomes thick (or more convex) [see Figure (b)], The thick eye-lens has small focal length but its converging power is large. Since the converging power of eye-lens increases, the thick eye-lens can converge the diverging light rays coming from the nearby object to form an image on the retina of the eye.
This is shown in Figure (b) in which an object O is near to the eye. It has been focused by the thick eye-lens to form an image I on the retina. When the eye-lens becomes more convex to focus the nearby objects, the eye is said to be ‘accommodated’. We can now say that : The ability of an eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length (or converging power) of its lens is called accommodation.
The maximum “accommodation” of a normal eye is reached when the object is at a distance of about 25 cm from the eye. After this the ciliary muscles cannot make the eye-lens more thick (or more convex). So, an object placed at a distance of less than 25 cm cannot be seen clearly by a normal eye because all the power of accommodation of the eye has already been exhausted.
Thus, a normal eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focused on the retina. The power of accommodation of the eye for a person having normal vision (normal eyesight) is about 4 dioptres.
Range of Vision of a Normal Human Eye
We will first understand the meaning of far point and near point of an eye. The farthest point from the eye at which an object can be seen clearly is known as the “far point” of the eye. The far point of a normal human eye is at infinity (see Figure). This means that the far point of a normal human eye is at a very large distance.
The nearest point up to which the eye can see an object clearly without any strain, is called the “near point” of the eye. The near point of a normal human eye is at a distance of 25 centimetres from the eye.
The near point of an eye is also known by another name as the least distance of distinct vision. The minimum distance at which an object must be placed so that a normal eye may see it clearly without any strain, is called the least distance of distinct vision. The least distance of distinct vision for a normal human eye is about 25 centimetres.
For example, to read a book clearly and comfortably without putting a strain on the eyes, it must be held at a distance of 25 centimetres from our eyes. If we try to read a book by holding it very close to our eyes, we will feel a lot of strain on the eyes and the image of printed matter of the book will also look blurred.
From the above discussion we conclude that the range of vision of a normal human eye is from infinity to about 25 centimetres. That is, a normal human eye can see the objects clearly which are lying anywhere between infinity to about 25 centimetres. Let us answer one question now.
Example Problem.
What happens to the image distance in the eye when we increase the distance of an object from the eye ?
Answer:
In the eye, the image distance (distance between eye-lens and retina) is fixed by the God which cannot be changed. So, when we increase the distance of an object from the eye, there is no change in the image distance inside the eye.