Monday, December 24, 2012

CBSE_NCERT Chapter 11 Human Eye and Colorful World class 10th

Gist of Lesson for Quick Revision (By JSUNIL)
1. An eye is the most important organ of the human body. It is associated with vision and acts as a vital optical instrument.

2. An eye is spherical in shape of about 2.3 cm diameter and is covered externally by a hard thick and opaque layer called Sclerotica. The outermost layer of the eye is sclerotic that protects and holds the eye.

3. Cornea is transparent part bulging out of eye and allows the light to enter in the eye. Cornea is also known as the window or aperture of the eye.

4. Iris is a circular diaphragm having a hole in its centre .this hole is called pupil

5. Iris has muscles and coloured pigments that provide different colour y eyes

6. Iris controls the amount of light entering the eye through the pupil.

7. The pupil becomes small when bright light falls on the eye. However, it becomes wide when there is dim light.

8. The eye lens is a crystalline convex lens made up of transparent and flexible tissues behind the pupil and held by the muscles called ciliary muscles. It focuses inverted the images of objects on the retina of the eye.

9. Ciliary muscles control the focal length of the eye lens by contracting and relaxing. This ability

10. When ciliary muscles contract lens become thin thus the focal length of the lens increases. 

11. When ciliary muscles expand lens become thick, and decrease the focal length of the lens. This helps us to see nearby objects clearly.

12. The ability of the eye lens to adjust its focal length is called accommodation.

13. The focal length of the eye lens cannot be decreased below a certain minimum limit this is why a normal eye not able to see clearly the objects placed closer than 25 cm.

14. The image distance in the eye remains same when we increase the distance of an object from the eye as distance between eye lens and retina is constant.

15. The least distance of distinct vision for a young adult with normal vision is about 25 m. it is called near point

16. The furthest distance of distinct vision for a young adult with normal vision is at infinity. it is called far point.

17. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to accommodation.

18. A normal eye can see objects clearly that are between 25 cm and infinity.

19. In old age eye lens of people 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.

20. One eyes of human being has a horizontal field of view of about 150° So for  a horizontal field of view of about 180° We have two eyes.
21. Retina acts as a screen to obtain the image of the object. It contains number of cells in the form of rods and cones which are sensitive to light. These cells convert light energy into nerve impulses or signals.

22. Optic nerve carries nerve impulses or signals to the brain. The brain finally interprets the signal and visualize erect image.

23. Rods and cones contain photoreceptors that transfer light to vision centers in the brain.

24. Rods and cones first convert light into chemical energy, and then into electrical energy, before sending it through the optic nerve to the brain.

25. Rods are specialized to perceive dim light and are the first receptors to detect movement and register shapes. There are about 126 million rods in each eye.

26. Cones function best in bright daylight. They allow people to see colors and fine detail.

27. Cones are divided into three different types, which absorb wavelengths of light in the short (blue), middle (green), and long (red) ranges. There are about 6 million cones in each eye.

28. Color blindness occurs when a person is missing one type of cone.

29. The aqueous humor fills the space behind the cornea. The space between eye lens and retina is filled with another liquid called Vitreous humour.

30. Chicken’s have large no of rods cells in their eye which help detects the slightest intensity of light and hence chicken or roosters wake up so early.

31. Blind spot is point on retina where optic nerves leave retina .An image formed at this point is not sent to the brain due to absence of rod and cones.

32. Choroids contain black pigment, which avoids internal reflections within the eye.

33. Myopia defect of the eye in which we are unable to visualize clearly the distant objects, but can see nearby objects. Causes of Myopia:
(a) Increase in the length of the eyeball as if distance of the retina from the eye has increased.
(b) Decrease in focal length of eye lens when the eye is fully relaxed.

34. This defect is due to the high converging power of the eye lens, which is created by the insufficient relaxation of the ciliary muscles as a result the image of distant object is formed in front of the Retina.

35. Myopia is corrected by using spectacles containing concave lenses of suitable power.

36. Concave lenses for myopic eyes diverges the incident light rays coming from the distant object and helps the lens to focus the image on the right spot of the Retina and so that the eyes can it clearly.

37. Hypermetropia or Far-sightedness a defect in which an eye is unable to see the nearby objects perfectly though it can see the distant objects clearly.

38. Hypermetropia is caused due to the low converging power of the eye lens, which is mainly due to weakening of the ciliary muscles. As a result the image of nearby  object is formed behind of the Retina.

39. Hypermetropia arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small.

40. Hypermetropia can be corrected by using a convex lens of appropriate power that  provide the additional focusing power required for forming the image on the retina.

41. Presbiopia is defect of vision in which we cannot see the near object as well as far off (or distant) objects clearly.  This defect arises due to the ageing of a person.

42. Presbiopia arises due to the gradual weakening of the ciliary muscles and decrease in flexibility of the eye lens.

43. Presbiopia can be corrected by using a bi-focal lens.  The upper surface of bi-focal lens (i.e the concave lens) enables the person to see distant objects clearly and the lower surface of the bi-focal lens (i.e., convex lens) enables the person to see the near objects.

44. Astigmatism: The defect, by which the person is notable to differentiate horizontal and vertical position, is called astigmatism. It can be rectified by using cylindrical lenses. 

45. A boy uses spectacles of focal length -50 cm.  Negative sign of focal length means he is using concave lens. Thus he has myopic vision defect. The power of lens is given by power of a lens in dioptre = 1/f(m) = 1/0.5 = -2 dioptre

46. The image of an object seen persists on the retina for 1/16 second even after the removal of the object. This continuance of sensation of eye for some timed is called persistence of vision.

47. The focal length of the convex lens used to correct hypermetropic eyes  is given by f=(x d) / (x-d) where x is the distance of near point of defective eye d is the distance of near point of normal eye (25 cm)

A person with hyper me tropic eye can see objects only at 1 m in front of eyes meter. 
Then we choose the convex lens of focal length= f=(x d) / (x-d) = f=(100x25) / (100- 25) =100/3 =1/3m power will be P = 1 / f = 1 /(1/3) = 3D

48. The focal length of the concave lens used to correct Myopic eyes is given by f= -X where x is the distance of far point of defective eye.
A person with myopic eye cannot see objects beyond 80 cm meter distinctly. We use concave lens of focal length F = -x = -o.80 m and power will be P = 1 / f = 1 / -0.80 = -1.25D

49. Light ray gets refracted through a rectangular glass slab. The emergent ray is parallel to the incident ray because of parallel refracting surfaces.

50. A triangular glass prism has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. Therefore the emergent ray displaced laterally towards base.

51. The angle between its two lateral faces is called the angle of the prism.

52. Angle between Incident ray and emergent ray is called the angle of deviation of prism.

53. A ray of light is entering from air to glass on refraction has bent towards the normal.

54. The light ray has entered from glass to air. Hence it has bent away from normal.

55. When white light passes through a glass prism, we get a band of seven colors on a white screen called spectrum.

56. The phenomenon of splitting white light into seven colors (VIBGYOR) is known as dispersion.

57. Refractive index and wave of different colors is different. As a result, different colors undergo different deviations on passing through the prism.

58. The formation of rainbow is due to the dispersion of white light from the sun and the phenomenon of total internal reflection of light from the water droplets suspended in the air after a shower.

59. A rainbow is always formed in a direction opposite to that of the Sun.

60. The red colour has longer wavelength and violet is shortest.

61. If wave length of a colour is large, the speed of the colour is also large. Therefore, the speed of red colour in a medium is the highest and the speed of violet colour is the least.

62. Total internal reflection is a phenomenon that occurs- when light ray travels from denser to rarer medium in such a way incident angle is greater than critical angle, it will be totally reflected back into the denser medium.

63. When light ray travels from denser to rarer medium, the angle of incident for which angle of refraction is 90 degree. This angle of incident is called Critical angle.

64. The refraction of light taking place in the atmosphere is known as atmospheric refraction. This occurs due to different refractive index of the different layers of atmosphere.

65. As we go higher and higher, the density of layers and refractive index decreases progressively.

66. When the light from a star enters the upper most layer of the atmosphere, it bends towards the normal as it enters the next layer. This process continues till the light enters our eyes. So due to refraction of light, the apparent position of the star is different from the actual position of the star.

67. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers and causes the twinkling effect.

68. Plants are very close to the earth as compared to the stars. So the intensity of light we receive from the planet is very large. Therefore, the variation in the brightness of the planets is not detected. Hence, planets do not twinkle.

69. Advance sunrise and delayed sunset are caused by atmospheric refraction.

70. In the sunrise, the light from the Sun got refracted by the atmospheric layers and reaches the observer by making apparent position positions of the Sun with respect to the horizon even it is below horizon. It takes about 2 min to cross horizon. Thus the Sun is visible to us about 2 minutes before the actual sunrise

71. The sun sets when it is below the horizon in the evening. The rays of light from the sun below the horizon reach our eyes because of refraction of light. These rays appear to come from the apparent position of the sun which is about the horizon. Hence we can see the sun fore few minute (about 2 minutes) even after it has actually set.

72. Scattering of light colors in atmosphere caused by large number of molecules of dust partials and water drop lets.

73. Intensity of scattered light (Is) varies inversely to the 4th power of the wave length (λ) of incident light.
µ 1/λ4

74. The sky appears blue because the blue colour of shorter wave length is scattered much more strongly.

75. The clouds are at lower height so colour of all the wave lengths are scattered equally hence it appears white.

76. The sun looks reddish at the time of sun rise and sun set because the rays from the sun have to travel large part of atmosphere. Since wave length of  blue color is less than  red  so most of the blue light is scattered away. Only red colors which is least scattered enter our eye

77. Danger signals are Red because wave length of red colour is large and intensity of scattering of the red light is least and can be seen from maximum distance.

78. Sky appears dark to an Astronaut because at such huge heights there is nothing to scatter sun light. Therefore, the sky appears dark. 

79. When the sun is overhead at noon, then the light coming from the sun has to travel a relatively shorter distance through the atmosphere to reach us. As a result, only a little of the blue colour of the white light is scattered (most of the blue light remains in it). Since the light coming from the overhead sun has almost all its components colors in the right proportion, therefore, the sun appears white.

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