THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colours around us.
The main parts of the human eye include:
Cornea: Cornea is the transparent tissue covering the front of the eye that enables the light travel through them.
Iris: Iris a ring of muscles in the colored part of the eye that controls the size of the pupil.
Pupil: An opening in the center of the iris that changes size to control how much light is entering the eye is known as pupil.
Sclera: the white part of the eye that is composed of fibrous tissue that protects the inner workings of the eye is called Sclera.
Lens: Lens is located directly behind the pupil which focuses light rays onto the retina.
Retina: A very thin membrane at the back of the eye that changes light into nerve signals is known as retina.
Optic Nerve: A bundle of nerve fibers that carries messages from the eyes to the brain is called optic nerve.
Macula: A small and highly sensitive part of the retina responsible for central vision which allows a person to see shapes, colors, and details clearly and sharply is known as mascula.
Choroid: The choroid is a layer of blood vessels between the retina and sclera; it supplies blood to the retina.
Ciliary muscle: it changes the shape of the lens - (this is called accommodation). It relaxes to flatten the lens for distance vision; for close work it contracts rounding out the lens.
Aqueous humour: A water like fluid, produced by the ciliary body, it fills the front of the eye between the lens and cornea and provides the cornea and lens with oxygen and nutrients. It drains back into the blood stream through the canals of schlemm.
Vitreous humour: The space between the lens and retina filled with the gel like substance, called vitreous humor.
WORKING OF HUMAN EYE
Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball as shown in above figure. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. We find a structure called iris behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye. The eye lens forms an inverted real image of the object on the retina. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are, i.e. without inversion.
POWER OF ACCOMMODATION
The process by which the ciliary muscles change the focal length of an eye lens to focus distant or near objects clearly on the retina is called the accommodation of the eye.
How Does an Eye Focus Objects at Varying Distances?
To focus on distant objects the ciliary muscles relax making the eye lens thin. As a result the focal length of the eye lens increases and we see the distant objects. But to focus on nearby objects the ciliary muscles contract making the eye lens thick. As a result the focal length of the eye lens decreases and we see the nearby objects. In short it is the adjustment of the focal length of the eye lens which enables us to focus on objects situated at different distances.
Near point or Least Distance of Distinct Vision
Near point or least distance of distinct vision is the point nearest to the eye at which an object is visible distinctly. For a normal eye the least distance of distinct vision is about 25 centimetres. However, it varies with age of the person. For example, for infants it is only 5 to 8 cm.
Far Point
Far point of the eye is the maximum distance up to which the normal eye can see things clearly. It is infinity for a normal eye.
Range of Vision
The distance between the near point and the far point is called the range of vision.
DEFECTS OF VISION
A normal eye can see all objects over a wide range of distances i.e., from 25 cm to infinity. But due to certain abnormalities the eye is not able see objects over such a wide range of distances and such an eye is said to be defective. Some of the defects of vision are
- Hypermetropia or long sightedness
- Myopia or short sightedness and
- Presbyopia
- Astigmatism
HYPERMETROPIA
Hypermetropia is also known as far-sightedness. The defect of the eye due to which the eye is not able to see clearly the nearby objects though it can see the distant objects clearly is called hypermetropia Or hyperopia. The near point, for the person, is farther away from the normal near point (25 cm). Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a close by object are focussed at a point behind the retina as shown in below figure.
This defect arises either because
(i) the focal length of the eye lens is too long, or
(ii) the eyeball has become too small.
This defect can be corrected by using a convex lens of appropriate power. This is illustrated in below figure. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
MYOPIA
Myopia is also known as near-sightedness. A myopic person cannot see distant objects clearly because the far point of his eye is less than infinity. The defect of the eye due to which the eye is not able to see the distant objects clearly is known as myopia.
This defect may arise either due to
(i) excessive curvature of the eye lens, or
(ii) elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power. This is illustrated in below figure. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
PRESBYOPIA
Presbyopia occurs at the age of 40 years and its main symptom is reduced near vision. Difficulty in reading without glasses at 35-40 cm and fatigue after a short period of close work are present. Normally the lens is flexible enough to change its shape when focusing at close objects. Loss of its flexibility and elasticity known as loss of the eye's adjustment mechanism results in presbyopia. Presbyopia (which literally means "aging eye") is an age-related eye condition that makes it more difficult to see very close.
At the young age, the lens in your eye is soft and flexible. The lens of the eye changes its shape easily, allowing you to focus on objects both close and far away. After the age of 40, the lens becomes more rigid. Because the lens can’t change shape as easily as it once did, it is more difficult to read at close range. This normal condition is called presbyopia. Since nearly everyone develops presbyopia, if a person also has myopia (nearsightedness), hyperopia (farsightedness) or astigmatism, the conditions will combine. People with myopia may have fewer problems with presbyopia.
ASTIGMATISM
Astigmatism is an eye condition with blurred vision as its main symptom. The front surface of the eye (cornea) of a person with astigmatism is not curved properly - the curve is irregular - usually one half is flatter than the other - sometimes one area is steeper than it should be.
When light rays enter the eye they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.
Astigmatism is a type of refractive error. A refractive error means that the shape of the eye does not bend light properly, resulting in a blurred image. Light has to be bent (refracted) by the lens and the cornea correctly before it reaches the retina in order to see things clearly.
The two most common types of astigmatism are:
- Corneal astigmatism - the cornea has an irregular shape
- Lenticular astigmatism - the lens has an irregular shape
REFRACTION OF LIGHT THROUGH A PRISM
Prism is a transparent optical element, which refracts light. An optical object to be defined as prism must have at least two faces with an angle between them. A triangular glass prism has two triangular bases and three rectangular lateral surfaces.
These surfaces are inclined to each other. The angle between its two lateral faces is called the angle of the prism.
PE is the incident ray, EF is the refracted ray and FS is the emergent ray. A ray of light is entering from air to glass at the first surface AB. So, the light ray on refraction has bent towards the normal. At the second surface AC, the light ray has entered from glass to air. Hence it has bent away from normal. The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation. In above prism, D is the angle of deviation.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM
When a ray of light enters the prism, it bends towards the normal; because light is entering from a rarer medium to a denser medium. Similarly, when the light emerges from the prism, it follows the laws of refraction of light. Due to the angle of the prism and due to different wavelengths of different components of white light; the emergent ray gets segregated into different colours. Finally, a colourful band of seven colours is obtained. This phenomenon is called dispersion of white light by the prism.
RAINBOW FORMATION
A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop (see below figure). Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.
ATMOSPHERIC REFRACTION
Atmospheric refraction is the shift in apparent direction of a celestial object caused by the refraction of light rays as they pass through Earth’s atmosphere.
TWINKLING OF STARS
Stars emit their own light and they twinkle due to the atmospheric refraction of light. Stars are very far away from the earth. Hence, they are considered as point sources of light. When the light coming from stars enters the earth’s atmosphere, it gets refracted at different levels because of the variation in the air density at different levels of the atmosphere. When the star light refracted by the atmosphere comes more towards us, it appears brighter than when it comes less towards us. Therefore, it appears as if the stars are twinkling at night.
ADVANCE SUNRISE AND DELAYED SUNSET
The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction. By actual sunrise, we mean the actual crossing of the horizon by the Sun. The below figure shows the actual and apparent positions of the Sun with respect to the horizon. The time difference between actual sunset and the apparent sunset is about 2 minutes. The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon.
SCATTERING OF LIGHT
In the air, part of the sunlight is scattered. The small particles (molecules, tiny water droplets and dust particles) scatter photons the more, the shorter their wavelength is. Therefore, in the scattered light, the short wavelengths predominate, the sky appears blue, while direct sunlight is somewhat yellowish, or even reddish when the sun is very low.
TYNDALL EFFECT
The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air. When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect. This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest.
WHY IS THE COLOUR OF THE CLEAR SKY BLUE?
The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.
COLOUR OF THE SUN AT SUNRISE AND SUNSET
Light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes (see below figure).
However, light from the Sun overhead would travel relatively shorter distance. At noon, the Sun appears white as only a little of the blue and violet colours are scattered. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun.
NCERT EXERCISE QUESTIONS
1. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
(a) Presbyopia
(b) accommodation
(c) near-sightedness
(d) far-sightedness.
Ans: (b) Human eye can change the focal length of the eye lens to see the objects situated at various distances from the eye. This is possible due to the power of accommodation of the eye lens.
2. The human eye forms the image of an object at its
(a) cornea
(b) iris
(c) pupil
(d) retina
Ans:(d) The human eye forms the image of an object at its retina.
3. The least distance of distinct vision for a young adult with normal vision is about
(a) 25 m
(b) 2.5 cm
(c) 25 cm
(d) 2.5 m
Ans: (c) The least distance of distinct vision is the minimum distance of an object to see clear and distinct image. It is 25 cm for a young adult with normal visions.
4. The change in focal length of an eye lens is caused by the action of the
(a) pupil
(b) retina
(c) ciliary muscles
(d) iris
Ans: (c) The relaxation or contraction of ciliary muscles changes the curvature of the eye lens. The change in curvature of the eye lens changes the focal length of the eyes. Hence, the change in focal length of an eye lens is caused by the action of ciliary muscles.
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Ans: A normal eye is unable to clearly see the objects placed closer than 25 cm because the ciliary muscles of eyes are unable to contract beyond a certain limit. If the object is placed at a distance less than 25 cm from the eye, then the object appears blurred and produces strain in the eyes.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: Since the size of eyes cannot increase or decrease, the image distance remains constant. When we increase the distance of an object from the eye, the image distance in the eye does not change. The increase in the object distance is compensated by the change in the focal length of the eye lens. The focal length of the eyes changes in such a way that the image is always formed at the retina of the eye.
10. Why do stars twinkle?
Ans: Stars emit their own light and they twinkle due to the atmospheric refraction of light. Stars are very far away from the earth. Hence, they are considered as point sources of light. When the light coming from stars enters the earth’s atmosphere, it gets refracted at different levels because of the variation in the air density at different levels of the atmosphere. When the star light refracted by the atmosphere comes more towards us, it appears brighter than when it comes less towards us. Therefore, it appears as if the stars are twinkling at night.
11. Explain why the planets do not twinkle?
Ans: Planets do not twinkle because they appear larger in size than the stars as they are relatively closer to earth. Planets can be considered as a collection of a large number of point-size sources of light. The different parts of these planets produce either brighter or dimmer effect in such a way that the average of brighter and dimmer effect is zero. Hence, the twinkling effects of the planets are nullified and they do not twinkle.
12. Why does the Sun appear reddish early in the morning?
Ans: During sunrise, the light rays coming from the Sun have to travel a greater distance in the earth’s atmosphere before reaching our eyes. In this journey, the shorter wavelengths of lights are scattered out and only longer wavelengths are able to reach our eyes. Since blue colour has a shorter wavelength and red colour has a longer wavelength, the red colour is able to reach our eyes after the atmospheric scattering of light. Therefore, the Sun appears reddish early in the morning.
13. Why does the sky appear dark instead of blue to an astronaut?
Ans: The sky appears dark instead of blue to an astronaut because there is no atmosphere in the outer space that can scatter the sunlight. As the sunlight is not scattered, no scattered light reach the eyes of the astronauts and the sky appears black to them.
THE HUMAN EYE AND THE COLOURFUL WORLD
QUESTIONS BANK SET – 1
1. A man can read the number of a distant bus clearly but he finds difficulty in
reading a book. From which defect of the eye is suffering from?
2. What type of spectacles should be worn by a person having the defects of myopia
as well as hypermetropia? How does it help?
3. The sun near the horizon appears flattened at the sun set and sun rise. Explain
why.
4. Explain why and when the sun is overhead at noon it appears white
5. A boy uses spectacles of focal length -50 cm. Name the defect of vision he is
suffering from. Compute the power of this lens.
6. Give the meaning of the term ‚ VIBGYOR‛ with which phenomenon is it
connected?
7. Explain the following terms connected with the eye. (i) Ciliary muscles (ii)
Accommodation.
8. What is meant by spectrum of white light?
9. What will be colour of the sky in the absence of atmosphere?
10. Why are the traffic light signals (or danger signals) of red colour?
11. Why does the sky appear dark and black to an astronaut instead of blue?
12. Explain why, when the sun is overhead at noon, it appears white?
13. What is Atmospheric Refraction?
14. A person with myopic eye cannot see objects beyond 1.2metre distinctly. What
should be the nature of corrective lenses to restore proper vision?
15. The far point of a myopic person is 80 cm in front of the eye. What is the nature
and power of the lens required to correct the problem?
16. The far point of myopic person is 80 cm in front of the eye. What is the nature and
power of the lens required to enable him to see very distant objects distinctly?
17. The far point of a myopic person is 150 cm in front the eye. Calculate the focal
length and power of a lens required to enable him to see distant objects clearly.
18. How is the eye lens held in its position?
19. What is meant by near point?
20. What is meant by least distance of distinct vision?
21. Which part of the eye controls the amount of the light entering the eye?
22. Which liquid fills the space behind the cornea?
23. Why is blind spot so called?
24. What is meant by the accommodation of the eye?
25. What is the least distance of distinct vision of a normal human eye?
26. Name the defects of vision of human eye?
27. What is the other name of near sightedness?
28. Where is the image formed in an eye suffering from near sightedness?
29. What is the other name of long sightedness?
30. Where is the image formed in an eye suffering from long sightedness?
31. How is long sightedness corrected?
32. A person has to use a concave lens in his spectacles. What defect of vision is he suffering from?
33. What is the other name of Presbyopia?
34. What is the twinkling of stars due to?
35. Give one example of source of white light.
36. Which scientist first explains the dispersion of light?
37. Name the delicate membrane in the eye having enormous number of light sensitive cells.
38. What kind of lens is used in the spectacles of a person suffering from Myopia (near sightedness)?
39. On what factor the colour of the scattered light depends?
40. What is a function of choroids?
41. Why does sky appear blue on a clear sky?
42. What happens to the lens and the ciliary muscles when you are looking at nearby
objects?
43. In an experiment the image of a distant object formed by a concave mirror is
obtained on a screen. To determine the focal length of the mirror, you need to
measure the distance between the:-
(a) Mirror and the screen
(b) Mirror and the object
(c) Object and the screen
(d) Mirror and the screen and also between the object and the screen.
44. The image formed by concave mirror is real. The position of the screen should be
(a) behind the mirror
(b) on the same side of object between focus and infinity
(c) on the same side of object between focus and pole
(d) none of these
45. In the experiment to determine focal length of a convex lens, a student obtained a sharp inverted image of a distant tree on the screen behind the lens. She then removed the screen and looked through the lens in the direction of the object. She will see:-
(a) An inverted image of the tree at the focus.
(b) No image as the screen has been removed.
(c) A blurred image on the wall of the laboratory.
(d) An erect image of the tree on the lens.
46. While performing the experiment for determination of focal length of a convex lens by using the sun as a distant object a student could not find a screen with stand. Which one of the following methods he should adopt safely? He should see:-
(a) The image of sun directly through convex lens.
(b) Focus the image of sun on his hand
(c) Focus the image of sun on his nylon shirt.
(d) Focus the image of sun on the wall of the room.
47. In an experiment to determine the focal length of a convex lens, the image of a distant tree is obtained on the screen. To determine the focal length of the lens, you are required to measure the distance between the :-
(a) Lens and the tree only
(b) Lens and the screen only
(c) Screen and the tree only
(d) Screen and the tree and also between the screen and the lens
48. For performing an experiment, a student was asked to choose one concave mirror and one convex lens from a lot of mirrors and lenses of different kinds. The correct procedure adopted by her will be :-
(a) To choose a mirror and lens which can form an enlarged and inverted image of an object.
(b) To choose a mirror which can form a diminished and erect image and a lens which can form an enlarged and erect image of the object.
(c) To choose a mirror and lens which can form an enlarged and erect image of an object.
(d) To choose a mirror and a lens which can form a diminished and erect image of an object.
49. Your school laboratory has one large window. To find the focal length of a concave mirror using one of the walls as the screen, the experiment may be perfomed.
(a) Near the wall opposite to the window.
(b) On the same wall as the window
(c) On the wall adjacent to the window
(d) Only on the table as per the laboratory arrangement
50. A students obtains a blurred image of a object on a screen by using a concave mirror. In order to obtain a sharp image on the screen, he will have to shift the mirror :–
(a) towards the screen
(b) away from the screen
(c) either towards or away from the screen depending upon the position of the object
(d) to a position very far away from the screen
QUESTIONS BANK SET - 2
MULTIPLE CHOICE QUESTIONS
1. A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
(a) + 0.5 D
(b) – 0.5 D
(c) + 0.2 D
(d) – 0.2 D
2. A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his text book. Which of the following statements is correct?
(a) The near point of his eyes has receded away
(b) The near point of his eyes has come closer to him
(c) The far point of his eyes has come closer to him
(d) The far point of his eyes has receded away
