Class 10 Science The Human Eye and the Colourful World Exam Notes

Human Eye

It is the most delicate and complicated natural optical instrument which enables us to see the wonderful world of light.

Structure :

Diagram shows the section of a human eye by a horizontal plane. It is a spherical ball of diameter about 2.5 cm. Its essential parts are described below :

Cornea : It is the front buldged out part of eye ball covered by transparent sclerotic.

Cornea of the eye-front view.

Iris : It is the coloured region under cornea formed by choroid. Its colour differs from person to person.

Pupil : It is central circular aperture in the iris. Its normal diameter is 1 mm but it can contract in excess light and expand in dim light, by means of two sets of involuntary muscular fibres.

Crystalline lens : It is a double convex lens L immediately behind iris. This is made of transparent concentric layers whose optical density increases towards the centre of the lens.

♦ Ciliary muscles : The lens is connected of the sclerotic by the ciliary muscles. These muscles change thickness of the lens by relaxing and exerting pressure.

♦ Aqueous humour : Anterior chamber is filled with a transparent liquid of refractive index. The liquid is called the aqueous humour.

♦ Vitreous humour : Posterior chamber is filled with a transparent watery liquid with little common salt having some refractive index. The liquid is called the vitreous humour.

♦ Retina : It forms innermost coat in the interior of the eye. It consists of a thin membrane which is rich in nerve fibres, containing two kinds of vision cells called rods and cones and blood vessels. It is sensitive to light, for it is a continuation of the optic nerves. It serves the purpose of a sensitive screen for the reception of the image formed by the lens system of the eye.
[The rods are responsible for colour vision in dim light (Scotopic vision). The cones are responsible for vision under ordinary day light (Photopic vision).

♦ Blind spot : The blind spot B. It is the spot where the optic nerves enter the eye. It is also slightly raised and insensitive to light, because it is not covered with choroid and retina.

◊ Working (Action of the eye) :

The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina. Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball. 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 retina is a delicate membrane having enormous number of light-sensitive cells. 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.

 POWER OF ACCOMMODATION

The ability of the eye lens to adjust its focal length is called accommodation. However, the focal length of the eye lens cannot be decreased below a certain minimum limit. To see an object comfortably and distinctly, you must hold it at about 25 cm from the eyes. The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye. For a young adult with normal vision, the near point is about 25 cm. 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.
You may note here a normal eye can see objects clearly that are between 25 cm and infinity.

◊ Defects of Vision

The major defects of vision are :

1. Short sightedness or myopia.

2. Long sightedness or hypermetropia.

3. Presbyopic

4. Astigmatism

1. Short sightedness or myopia

♦ Symptoms : Eye cannot see clearly beyond a certain distance. It means that the far point of the defective eye has shifted from infinity to a finite distance ahead.

♦  Reasons : It is so because the image of distant objects is formed in front of the retina. It is shown in fig.

Objective Questions

Question. 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
Answer : B

Question. 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.
Answer : A

Question. The image formed on the retina of human eye is
(a) virtual and erect
(b) real and inverted
(c) virtual an inverted
(d) real and erect
Answer : B

Question. The least distance of distinct vision for a young adult with normal vision is
(a) 25 m
(b) 20 m
(c) 25 cm
(d) 20 cm
Answer : C

Question. The light sensitive cell present on retina and is sensitive to the intensity of light is:
(a) cones
(b) rods
(c) both rods and cones
(d) none of these
Answer : B

Question. Which of the following colours is least scattered by fog, dust of smoke?
(a) Violet
(b) Blue
(c) Red
(d) Yellow
Answer : C

Question. The amount of light entering the human eye is controlled by
(a) Ciliary muscles
(b) Pupil
(c) Cornea
(d) Iris
Answer : B

Question. The component of white light with greatest wavelength is
(a) Violet
(b) Red
(c) Green
(d) Blue
Answer : B

Question. Colour blindness is caused due to
(a) lack of rod cells
(b) absence of optic nerve
(c) lack or cone cells
(d) none of these
Answer : C

Question. A student of class 10, is not able to see clearly the black board Question when seated at a distance of 5 m from the board, the defect he is suffering from is
(a) Myopia
(b) Hypermetropia
(c) Presbyopia
(d) Astigmatism
Answer : A

Question. The glass has greater refractive index for
(a) Violet light
(b) Green light
(c) Blue light
(d) Red light
Answer : A

Question. The eye defect represented by the figure is


(a) Myopia
(b) Hypermetropia
(c) Cataract
(d) Presbyopia
Answer : A

Very Short Answer Questions

Question. What is meant by power of accommodation of the eye?
Answer : The ability of the eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length of the eye lens is called power of accommodation.

Question. What is the far point and near point of the human eye with normal vision? 
Answer : The far point is infinity and the near point is 25 cm of the human eye with normal vision.

Question. Name the transparent membrane through which light enters first in the eye.
Answer : Cornea.

Question. Name the light sensitive part of the eye where image of an object is formed.
Answer : Retina

Question. What are light sensitive cells?
Answer : Rods and cones.

Question. Why is inverted image formed on the retina of human eye?
Answer : The inverted image is formed due to the eye lens which is convex in shape. Through it the light rays enters to form the real, inverted image.

Question. Which part of the human eye controls the amount of light entering the eye?
Answer : Pupil.

Question. What holds the crystalline lens in the human eye?
Answer : Ciliary muscles.

Question. Name the disease in which crystalline lens of human eye becomes opaque.
Answer : Cataract.

Question. Define least distance of distinct vision.
Answer : The minimum distance at which the objects can be seen clearly without any strain is called least distance of distinct vision. It is the near point of eye and is equal to 25 cm.

Question. Define the power of accommodation of human eye.
Answer : The ability of eye to see nearby as well as far off objects at the same time is called power of accommodation.

Question. In which type of eye defect near point of the eye becomes more than 25 cm?
Answer : Hypermetropia.

Question. What type of lens should be used to correct the presbyopia?
Answer : Bi-focal lens. Concave-convex lens. 

Question. Define angle of prism.
Answer : The angle formed due to two lateral faces of the prism is called the angle of prism.

Question. In visible spectrum which colour has longest wavelength.
Answer : Red.

Question. What is Tyndall effect?
Answer : Scattering of light in the nature due to small particles present in the atmosphere is called Tyndall effect.

Short Answer Questions

Question. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Ans. The focal length of a lens is given by (P = 1/f)
, f =1/P
(i) For distant vision
f = 1/−5.5D
= – 0.18
(ii) For near vision
f = 1/P
i.e., f = 1/1.5D
= 0.67 m

Question. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect?
Assume that the near point of the normal eye is 25 cm.
Answer :

Question. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer : The image distance in the eye remains the same. On increasing the distance of an object from the eye, the focal length of the eyes changes due to ciliary muscles which helps an eye to focus the object image on retina.

Question. Explain why the planets do not twinkle.
Answer : Planets are much closer to earth and behave like extended source.

Question. Why does the sky appear dark instead of blue to an astronaut?
Answer : In space there are no particles, air, gases, water droplets etc., present to scatter the light. So when the astronauts look at the sky in the space, there is no light entering our eyes, hence it appears dark.

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