NCERT Class 10 The most important Basic concepts of chapter 11

The most Important points of The Human Eye and the Colourful World

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. Retina: 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 it's 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 accomodation: The power of accomodation of the eye is the maximum variation of its power for focussing on near and far objects. For a normal eye, the power of accomodation is about 4 dioptre.

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 (1/16th) 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 colours of light.

16. Colour blindness: A person who cannot distinguish between various colours but can see well otherwise, is said to be colour-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 (i) Myopia or near sightedness (ii) Hypermetropia or far-sightedness (iii) presbyopia (iv) astigmatism.

19. Myopia or near-sightedness: In this defect a person can see nearby object clearly but cannot see far off objects clearly. Here, either the eyeball becomes too short or the focal length of the eye lens become too short. It can be corrected by using a concave lens of suitable focal length.

Focal length of the correcting lens = Distance of the far point from the eye.

20. Hypermetropia or far-sightedness: In this defect a person can see the far off objects clearly but he cannot see nearby objects distinctly. Here, either the eyeball becomes too short or the focal length of the eye lens become too large. It can be corrected by using convex lens of suitable focal length.

  Focal length of correcting lens = y D / y - D

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 a 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

                   i + e = A + D

24. Dispersion of white light: The splitting of white light into its constituent colours 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 colours. 

25. Blue colour 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.