"AKC SCIENCE CLASSES"
CLASS 10 TH (CBSE AND MP BOARD)
CHAPTER 11
THE HUMAN EYE AND THE COLORFUL WORLD
BASIC CONCEPTS
1) Human eye :- It is most important and sensitive sense organ. The essential parts of a human eye are sclerotic, cornea, choroid, iris, pupil, crystalline lens, ciliary muscles, aqueous humour, vitreous humour and retina.
2) Cornea :- It is the transparent membrane on the front portion of the eyeball through which light enters the eye.
3) Iris :- It is an opaque circular diaphragm that controls the size of the pupil.
4) Pupil :- It is a small central hole in the iris which becomes smaller in bright light and opens up in dim light.
5) Eye lens :- It is a converging lens situated behind the iris. It is made of fibrous, jelly-like material.
6) Reina :- It is a delicate light sensitive membrane on the back wall of the eyeball on which images are formed and transmitted to the brain.
7) Accommodation :- It is the ability of the eye lens due to which it can change its focal length so that images of objects at various distances can be formed on the same retina.
8) Range of normal vision :- The distance between infinity and 25 cm point is called the range of normal vision.
9) Least distance of distinct vision (D) :- The minimum distance from the eye, at which the eye can see the objects clearly and distinctly without any strain is called the least distance of distinct vision. For a normal eye, its value is 25 cm.
10) Near point :- The nearest point from the eye, at which an object can be seen clearly by the eye is called its near point. The near point of a normal eye is at a distance of 25 cm.
11) Far point :- The farthest point from the eye, at which an object can be seen clearly by the eye is called the far point of the eye. For a normal eye, the far point is at infinity.
12) Power of accommodation :- The power of accommodation of the eye is the maximum variation of its power for focussing on near and far objects. For a normal eye, the power of accommodation is about 4 dioptres.
13) Persistence of vision :- The phenomenon of the continuation of the impression of an image on the retina for sometime even after the light from the object is cut off is called persistence of vision. The impression of the image remains on the retina for about (1/16)th of a second. Cinematography works on the principle of persistence of vision.
14) Rods :- These are rod-shaped cells of the retina that are sensitive to the intensity of light.
15) Cones :- These are the cone-shaped cells of the retina that are sensitive to the colors of light.
16) Color blindness :- A person who cannot distinguish between various color but can see well otherwise, is said to be color-blind. It is due to lack of some cones in the retina of the eyes.
17) Cataract :- It is due to the development of hazy or opaque membrane over the eye lens which results in the decrease or loss of vision. It can be cured by surgery.
18) Common defects of vision :- There are mainly four common defects of vision which can be corrected by the use of suitable eye glasses. These are :-
- myopia or near sightedness
- hypermetropia or far-sightedness
- presbyopia
- astigmatism.
Focal length of the correcting lens = Distance of the far point from the eye.
20) Long-sightedness or hypermetropia :- In this defect a person can see the far off objects clearly but he cannot see nearby objects directly. Here, either the eyeball become too short or the focal length of the eye lens becomes too large. It can be corrected by using convex lens of suitable focal length.
Focal length of correcting lens
Where y = Distance of the near point from the defective eye.
21) Presbyopia :- In this defect, a person in old age cannot read correctly due to the stiffening of the ciliary muscles and the decrease in flexibility of the eye lens.
22) Astigmatism :- It is a defect of vision in which a person cannot simultaneously see both the horizontal and vertical views of an object with the same clearity. It is due to the irregular curvature of the cornea. It can be corrected by using a cylindrical lens.
23) Prism :- A prism is a portion of a transparent medium bounded by two plane faces inclined to each other at a certain angle. A ray of light, after suffering refraction through prism, bends towards the base of the prism.
For any ray of light,
Angle of incidence + Angle of emergence
= Angle of prism + Angle of deviation
or i + e = A + D
24) Dispersion of white light :- The spitting of white light into its constituent color when it passes through a glass prism is called dispersion. The dispersion of light occurs because refractive index of prism material is different for light of different colors.
25) Rainbow :- It is a beautiful spectrum of light produced by refraction, dispersion and internal reflection of sunlight by spherical raindrops. It is observed when the sun shines on raindrops after a rainshower. An observer standing with his back towards the sun observes it in the form of concentric circular arcs of different colors in the horizon.
26) Twinkling of stars :- The apparent position of a star is slightly different from the actual position due to refraction of starlight by the atmosphere. Due to the variations in the atmospheric conditions, the amount of light from a particular star changes randomly with time. This gives rise to a twinkling effect of the star.
27) Advance sunrise and delayed sunset :- Due to atmospheric refraction, the sun is visible to us about two minutes before the actual sunrise and two minutes after the actual sunset.
28) Scattering of light :- The scattering of light by the atmospheric molecules causes the blue color of sky and the reddening of the sun at sunrise and sunset.
29) Tyndall effect :- When a beam of light is passed through a colloidal solution, placed in a dark room, the path of the beam becomes illuminated or visible. This effect is called Tyndall effect. The path of a beam of light is not visible through a true solution.
30) Blue color of the sky :- The atmospheric molecules scatter blue light of shorter wavelength more strongly than the red light of longer wavelength. When this scattered light reaches our eyes, it contains blue light in larger proportion. That is why the sky appears blue.
-----------------------------------✖----------------------------------
INTEXT QUESTIONS AND ANSWERS
NCERT INTEXT QUESTIONS (PAGE NO. 190)
Q.01:- What is meant by power of accommodation of the eye?
Ans:- The power of accommodation of the eye is the maximum variation of its power for focussing on near and far (distant) objects. For a normal eye, the power of accommodation is about four dioptres.
Q.02:- A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Ans:- Diverging or concave lens.
Q.03:- What is the far point and near point of the human eye with normal vision?
Ans:- For human eye with normal vision, the far point is at infinity and near point is at 25 cm from the eye.
Q.04:- 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?
Ans:- The child is suffering from myopia. He should use concave lenses of suitable focal length.
-----------------------------------✖----------------------------------
