10 The Human Eye and the Colourful World

The human eye is a sensitive sense organs which enables us to see the wonderful world
The human eye is like a camera.
Diameter of human eye is 2.3
Parts of Human Eye : The eye contains three layers:
The outer layer, formed by the cornea and sclera
The middle layer, holding the primary blood supply for the eye and containing the iris and pupil which is called choroid
The inner layer, comprised of the retina
1. Cornea : The transparent part in front of the sclera is called the cornea.The Light enters the eye through the cornea. Most of the refraction of the light entering the eye occurs at the outer surface of the cornea.
2. Iris : The iris is situated behind the cornea. it is dark muscular diaphragm that control the size of pupil. The colour of the eye is due to the colour of the iris.
3. Pupil : it is window of eye. It is control the aperture of iris. It regulats and
controls the amount of light entering in eye.
4. Lens : It is composed of transparent and flexible fibrous jelly like material. It is located behind the pupil. it is a convex lens. The eye lens forms an inverted real image of the object on the retina. Its focal length can be adjusted with the help of ciliary muscles
5. Ciliary muscles: The ciliary muscles hold the lens in appropriate position. The ciliary muscles help in changing the focal length of the eye lens.
6. Ratina : it is a delicate membrane having enormous number of light senstive cells. Retina is innermost part of eye which is connected to the brain by optic nerves.
Retina is the screen on which image is formed by eye lens. Any image formed on the retina is converted into electrical signals with the help of light sensitive cells. 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.
7. Optic nerve: it transmit visual ilformation from ratina to brain
8. Aqueous humour : between the cornea and eye lens there is a space filled with transparent liquid is called aqueous humour which helps the refracted light to be focused on retina. It is also provides nutrition to eye
9. Vitrous humour : Space between lens and ratina filled with a jelly like liquid is called vitrous humour

Power of Accommodation : The ability of the eye lens to adjust its focal length is called accommodation.
To see an object clearly we have to focus on it and to do so we need to change the focal length of eye lens. These activities are performed by ciliary muscles. Focal length of eye lens can be modified by ciliary muscles.
If we have to see the far object then the muscles are relaxed, the lens becomes thin. and its focal length increases.
If we have to see the nearby object then the muscles are contract, the lens becomes thik. and its focal length decreases.
Near point : The minimum distance of the object from the eye, at which objects can be seen most clearly without strain, is called the least distance of distinct vision. It is also called the near point of the eye.
The near point of a normal human eye is at 25 cm from the eye.
Far point : The farthest point upto which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.
Range of vision: A normal eye can see objects clearly that are between 25 cm and infinity.
Cataract : The crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. This causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.
Defects Of Vision And Their Correction
Some times the human eye may loose its power of accommodation In such conditions, the person cannot see far or near object clearly.
There are mainly three common refractive defects of vision
(i) Myopia or near-sightedness,
(ii) Hypermetropia or far sightedness,
(iii) Presbyopia.
1. Myopia : Myopia is also known as nearsightedness. A person with myopia can see nearby objects clearly but cannot see far objects clearly.
The far point of myopic person shift from infinity to a distance of few meter. So the for point of myopic eye is less than infinity.
In a myopic eye, the image of a distant object is formed in front of the retina.

Cause of myopia
(i) Excessive curvature of the eye lens
(ii) Elongation of the eyeball.
Correction of myopia
This defect can be corrected by using a concave lens of suitable power.
2. Hypermetropia : Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects clearly.
The near point moves away from the normal near point. So the near point of Hypermetropiac eye is more than 25 cm.
The image of the object is formed beyond the retina.

Cause of Hypermetropia
(i) The focal length of the eye lens is too long, or
(ii) The eyeball has become too small.
Correction of Hypermetropia
This defect can be corrected by using a convex lens of appropriate power.
3. Presbyopia : The power of accommodation of the eye usually decreases with ageing due to it an old person can not see nearby object clearly.This defect of vision is called Presbyopia.
Caused of Presbyopia
The near point of an old person having Presbyopia become more than 25 cm.
It is also known as old age hypermetropia
(i) Weakening of the ciliary muscles
(ii) Diminishing flexibility of the eye lens
Correction of Presbyopia
This defect can be corrected by using a bifocal lens of appropriate power.
Some time a person may suffer from both myopia and hypermetropia such require bifocal lens.
Glass Prism : 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.
Refrection Of White Light By A Glass Prism.

PE – Incident ray Ði – Angle of incidence
EF – Refracted ray Ðr – Angle of refraction
FS – Emergent ray Ðe – Angle of emergence
ÐA – Angle of prism Ðd – Angle of deviation
Dispersion Of White Light By A Glass Prism
When white light is passed through a prism, it splits into its seven component colours. The splitting of white light into its component colours is called dispersion. Seven component colours are violet, indigo, blue, green, yellow, orange and red. (VIBGYOR).

The band of the coloured components of a light beam is called its spectrum.
Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most.
Recombination of the spectrum of white light.
Newton was the first to use a glass prism to obtain the spectrum of sunlight. When white light is passed through a prism, it disperses, and when another prism is kept inverted, the dispersed light returns to white light. This is referred to as light spectrum recombination. This is caused by the different velocities of different colors passing through the prism.

Any light that gives a spectrum similar to that of sunlight is often referred to as white light.
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. 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 Due to the dispersion of light and internal reflection, different colours reach the observer’s eye. A rainbow is always formed in a direction opposite to that of the Sun.
Atmospheric refraction : Refraction of light caused by the Earth’s atmosphere due to change in the refractive indices of different layers is called atmospheric refraction . The two phenomena which can be explained on the basis of atmospheric refraction are:
Twinkling of Stars ·
Advance Sunrise and Delayed Sunset
It is caused due to the varying optical densities of different layers of earth's atmosphere.
Twinkling of stars : The twinkling of a star is due to atmospheric refraction of starlight. There are various rarer or denser layers of air in our atmosphere. The stars are very far away and are appeared as a point -size source of light. So when the light from the stars enter the atmosphere it pass through rarer to denser mediums. Due to this starlight bends towards the normal. As a result the star appears slightly above than its actual position.This apparent position of the star is not stationary, but It changes slightly all the time , since the physical conditions of the earth’s atmosphere are not stationary, As a result, the star sometimes appears brighter and sometimes dull, which is the twinkling effect.
Why don’t the planets twinkle?
Planets do not twinkle because The planets are much closer to the earth compared to the stars. So they act as extended source of light . If we consider a planet as a collection of a large number of point-sized sources of light. So the total variation in the amount of light entering our eye from all the individual point-sized sources will average to zero, thereby nullifying the twinkling effect.
Early 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.
Before sunrise, the sun is below the horizon. The rays coming from the sun travel from more rare to denser layers of air. Becaus of atmosferic refraction of light to an observer on the earth light rays appears to come from above the horizen. Thus the Sun appears to us earlier than the actual sunrise. Same reason the Sun is visible to us after the actual sunset
Scattering of light.
When white light from sun enters the earth's atmosphere, the light spreads in all directions by the smoke, tiny water droplets, suspended particles of dust and molecules of air present in the atmosphere. This phenomenon is called scattering of light.
Example
Red colour of sun at sunrise and sunset.
White colour of sky at noon.
Blue colour of sky.
Red colour used as danger signa
Tyndall effect
“The phenomenon of scattering of a beam of light by colloidal particles, when passed through a colloidal solution, is called the Tyndall effect
When a beam of light passes through colloidal particles (smoke, tiny water droplets, suspended particles of dust and molecules of air), the path of the beam becomes visible.
Tyndall effect can be observed when sunlight passes through a canopy of a dense forest.
Tyndall effect can be observed when a fine beam of sunlight enters a smoke-filled room through a small hole.
The colour of the scattered light depends on the size of the scattering particles.

1. Name the essential parts of human eye?
   Retina (b) Eye lens (c) Cornea (d) Pupil (e) Iris
   
2. Which liquid fills the space behind the   cornea?
   Aqueous humour is filled in the space behind the cornea.
   
3. Which part of the eye controls the amount of light entering the eye?( function/ role of pupil)
   Pupil controls the amount of light entering the eye.
   
4. What is eye lens made of ?
   Eye lens is made of transparent and flexible jelly like material.
   
5. An old person is unable to see clearly nearby object as well as distant objects, What defect of vision is he suffering from? 
   Old person is suffering from  presbyopia.
6. Name the part of the human eye that helps in changing the focal length of the eye lens.
   Ciliary muscles help in changing the focal length of   the eye lens.
   
7. Name the part responsible for the power of accommodation of the  eye.( function/ role of ciliary muscles.)
   Ciliary muscles help for the power accommodation of the eye.
8. How is the sense of vision carried from the eye to the brain?
   Through optical nerve, sense of vision is carried from eye to the brain.
   
9. What is the number of receptors contained in the retina?
   Retina contains more than 125 millions receptors which are of two types, rods and cones.
   
10. Which liquid fills the space between eye lens and retina?
    Vitreous humour.
11. What is  power of accommodation of the eye?
    The ability of the eye lens to adjust its focal length is called accommodation.
    
12. What is the range of vision for a normal human eye?
    Range of vision is 25 cm to  infinity.
    
13. What is twinkling of stars due to?
    Twinkling  of star is  due  to atmosphere refraction.
14. Write the value of least distance /near point of distinct vision for normal eye.
    For a normal human eye the value of near point of distinct vision is 25 cm.
15. A person is advised to wear spectacles with concave lenses. What type of defect of the vision is he suffering from?

    Myopia.
16. How would the sky  appear  in  the  absence  of  earth's  atmosphere?

    Black.
17. On what factor the colour of the scattered light depends?

    The colour of the scattered light depends on the size of scattered particles.
18. Name the condition resulting due to the eye lens becoming cloudy.
    Cataract
19. A person suffering from an eye defect uses lenses of power 1D. Name the defect he is suffering from and  the nature of lens  used.

    He uses a concave lens (negative power) because he is suffering from myopia.
20. What is meant by far point?
    The farthest point upto which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.
    
21. List any two natural phenomenon based on scattering  of light ?
    Two natural  phenomenon  based  on  scattering  are (i) Reddish colour of sky during sunrise and sunset, (ii) Blue colour of  sky.
22. What kind of lens will be required to see clearly the nearby as well as distant objects? Give reason.
    He cannot see near objects as well as distant objects, so he has to use bifocal lenses which consists of concave and convex lenses  both.
23. Name the two  phenomena involved in  the formation  of rainbow.

    The phenomena involved in formation of rainbow are dispersion and total internal reflection and refraction
24. Why sky appears dark to the passengers flying at high altitudes?

    Or Why  does the sky appear dark in space?

    Due to absence of atmosphere in the space, there  is no scattering of light and hence space appears black/ dark.
25. Does the least distance increase or decrease for long sighted eye (hyperopic)? 
    Least distance of distinct vision increases for long sighted eye. 
    A hypermetropic eye can see upto infinite distance, which is more than 25 cm.
26. Which colour of white light suffers (i) least deviation and (ii) maximum deviation when a beam of  white light passes through a glass prism?

    Longer wavelength light (red) suffers least deviation and shorter wavelength (violet) suffers maximum deviation when white light is passed through a prism.
27. List two causes leading to myopia of the eye. 
    Causes of Myopia
    (a) Excessive curvature of cornea
    (b) Elongation of eye ball.
28. Describe an experiment to discuss the recombination of dispersed light.

    When white light is passed through a prism, it disperses, and when another prism is kept inverted, the dispersed light returns to white light. This is referred to as light spectrum recombination. This is caused by the different velocities of different colors passing through the prism.
29. Name the part of the eye where image is formed by the eye lens. What is the nature of the image formed? How is this image sent to the brain?
    Retina is the screen on where image is formed by eye lens.  
    Any image formed on the retina is  inverted and real. 
    Any image is converted into electrical signals with the help of light sensitive cells. These signals are sent to the brain via the optic nerves. 
30. What is meant by scattering of light? 

    When white light from sun enters the earth's atmosphere, the light spreads in all directions by the smoke, tiny water droplets, suspended particles of dust and molecules of air present in the atmosphere. This phenomenon is called scattering of light.
31. What is meant by least distance of distinct vision/ Near point? How does this vary between the very young and old people?
    The minimum distance of the object from the eye, at which objects can be seen most clearly without strain, is called the least distance of distinct vision. It is also called the near point of the eye.
    In young, the least distance of distinct vision is lesser and for old people it is  more.
32. Draw a diagram, to show the dispersion of white light by a glass  prism.
    
33. When you enter from a bright sunshine into a dark room, you are not able to see clearly for sometime, why?
    In bright sunlight the pupil of our eye is small. So, when we enter the dark lit room, very little light enters our eye and we are unable to see properly. Afte.r some time, the pupil of our eye expands and becomes large so more light enters our eye because of which we can see properly
34. What is myopia ?
    A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
    A person with this defect has the far point nearer than infinity. In a myopic eye, the image of a distant object is formed in front of the retina  and not at the retina itself.
35. Why is the colour of the clear Sky Blue?

    Scattering of light is responsible for the blue colour of the sky.

    When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red.  Thus the scattered blue light reaches  our eyes.  As a result, the sky appears as blue to us
36. Define dispersion of white light and name the colours of white light in order.

    The splitting of white light into its component seven colours when passed through the prism is called dispersion of light

    When white light is passed through a prism, it splits into its seven component colours. Seven component colours are violet, indigo, blue, green, yellow, orange and red. (VIBGYOR).
37. Why does the sun appear reddish at the time of sunrise and sunset.? 

    During morning and evening time, sun is at the horizon and sunlight has to travel longer distance in atmosphere. Violet and blue colours of shorter wavelength scatter in the space only. The longer wavelength red colour reaches to the observer on earth and sun appears reddish at the time of sunrise and sunset.
38. What is Presbyopia ?

    Presbyopia is a defect of vision in which person find difficult to see the nearby object as well as distinct objects clearly. The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.
39. Sushil went to an eye specialist for check up. He prescribed him to use spectacle lens of + 0.5D power.

    (i) Name the defect of vision he is suffering from.

    Hypermetropia.

    (ii) Find the focal length of spectacle lens.

    Power of spectacle P = + 0.5D

                               P = 1/f

                                f = 1/P

                                  = +1/.5

                                  = + 2 m

    Positive sign means it is a convex lens.
40. Draw a neat diagram to show the refraction of a light ray through a glass prism, and label on it the angle of incidence and angle of deviation. 
    

    PE – Incident ray                   ∠i – Angle of incidence
    EF – Refracted ray                 ∠r – Angle of refraction
    FS – Emergent ray                 ∠e – Angle of emergence
    ∠A – Angle of prism             ∠d – Angle of deviation
41. Why do stars twinkle? Explain.

    The twinkling of a star is due to atmospheric refraction of starlight. There are various rarer or denser layers of air in our atmosphere. The stars are very far away and are appeared as a point -size source of light. So when the light from the stars enter the atmosphere it pass through  rarer to denser mediums. Due to this starlight bends towards the normal. As a result the star appears slightly above than its actual position.This apparent position of the star is not stationary, but It changes slightly all the time , since the physical conditions of the earth’s atmosphere are not stationary, As a result, the star sometimes appears brighter and sometimes dull.
42. State the function/ role of ciliary muscles iris, pupil, and cornea.
    Ciliary muscles help in accommodation of power  of eye lens./

    Ciliary muscles help in changing the focal length of the eye lens. 

    Iris: It controls the size of the pupil.

    Pupil: The pupil regulates and controls the amount of light entering the eye.

    Cornea: The  Light enters the eye through the cornea. 

    Cornia refracts the light on the lens.
43. Study the diagram and answer the questions the follows :
    
    (a) Identify the defect of vision represented by this diagram.
    (b)List the two possible causes of this defect.
    (c)With the help of ray diagram explain how this defect can be corrected.
    (a)The defect of vision is hypermetropia.
    (b) (i) The focal length of the eye lens is too long, or
    (ii) The eyeball has become too small.
    (c) Hypermetropia can be corrected by using convex lens.
    
44. A person cannot see objects farther than 12 m from the eye clearly. Name the defect of vision he is suffering from and the lens that should be used for correction of this defect. Illustrate with the help of a diagram, how this lens will correct the defective
    Or
    A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
    Defects of vision: Myopia
    
    
    Myopia can be corrected by using concave lens.A 
45. person can see distant signboards clearly but cannot read clearly a book which is at 25 cm from his eye. Name the defect. How can it be corrected? Draw ray diagrams for (i) defective eye (ii) corrected eye and explain them.
    The person is suffering from long sightedness/ hypermetropia. It can be corrected by using convex lens of suitable focal length.
    
46. Draw a neat labelled diagram of the structure of the human eye and mention the functions of any four parts.
    1. Cornea  : The transparent part in front of the sclera is called the cornea.The  Light enters the eye through the cornea. Most of the refraction of the light entering the eye occurs at the outer surface of the cornea.
    2. Iris : The iris is situated behind the cornea. it is dark muscular diaphragm that control the size of pupil. The colour of the eye is due to the colour of the iris.
    3. Pupil : it is window of eye. It is control the aperture of iris. It regulats and controls the amount of light entering in eye. 
    4. Lens : It is composed of transparent and flexible fibrous jelly like material. It is located behind the pupil. it is a convex lens. The eye lens forms an inverted real image of the object on the retina. 
    The ciliary muscles hold the lens in appropriate position. The ciliary muscles help in changing the focal length of  the eye lens. Between the cornea and lens filled with a transparent liquid is called aqueous humour. The aqueous humour helps the refracted light to be focused on retina and provides nutrition to eye.
    5. Ratina :  it is a delicate membrane having enormous number of light senstive cells. Retina is innermost part of eye which is connected to the brain by optic nerves. Retina is the screen on which image is formed by eye lens. Between lens and ratina filled with a jelly like liquid is called vitrous humour