12th Zoology

Errors of refraction

Errors of refraction

In a normal eye focused for distant objects, parallel rays of light come
to a sharp focus exactly on the retina. It can accommodate for clear vision of
objects from infinity (far point) down to 25 cm (near point). This ideal
refractive state is called emmetropia. A deviation from emmetropia is called
ametropia. The important forms of ametropia are myopia,
hypermetropia, astigmatism, presbiopia. Ametropia results from an
imbalance between the length of the eye ball and its refractive power.  (Errors of refraction)

Myopia (Short Sightedness)

Myopia results if the lens curvature is too great or the entire eyeball
becomes elongated. Light rays entering the eye are refracted more than is
necessary. Consequently light is focused in front of the retina. The image
perceived is thus blurred. The condition is called short-sightedness as
objects near the eye are clearer than those further away. Myopia can be
corrected by placing a concave lens in front of the eye. The surface of the
concave lens refracts light rays in such a way that the rays diverge slightly
from their original path. The lens of the myopic eye now refracts the
diverged light rays in to focus on the retina.  (Errors of refraction)

Hypermetropia (Long Sightedness)

Hypermetropia results when the curvature of the eye lens is not great
enough. Light rays are not refracted enough and would thus be focused
behind the retina. The condition is called long-sightedness because distant
objects are clearer than near ones. This happens because light rays from
distant objects require less refraction than rays from near objects.
Correction of hypermetropia requires placing a convex lens in front of the
eye. The lens converges light rays before they enter the eye so that the eye’s
focuses the light correctly on the retina.  (Errors of refraction)


Astigmatism occurs if either the cornea or lens is distorted. One part
of the focusing mechanism then refracts light rays too much, or too less.
Usually most of the images perceived is out of focus. Light rays from part of
the object are focused in front of the retina, as in myopia. Rays from other
parts would be focused behind the retina, as in hypermetropia. Astigmatism
can be corrected by placing a lens in front of the eye. The curvature of this
lens varies from one part to another to compensate for the eye’s
deficiencies. (Errors of refraction)


This is the result of a reduction in the amplitude of accommodation
with age due to hardening and loss of plasticity of the lens. Hence it becomes
less capable of being moulded into a more complex form. Presbiopia begins
at about 40 years of age. The remedy is convex lenses for reading. Any
defect in the eye should be consulted immediately with the optometrist.  (Errors of refraction)

Related Topics in Zoology:

Bio Zoology All Important Topics

  1. Human Physiology Introduction

  2. Nutrition

  3. Carbohydrates Poly hydroxyaldehydes (or) ketones

  4. Proteins (Polypeptides)

  5. Lipids

  6. Vitamins – Functions Of Vitamins

  7. Deficiency of Vitamin

  8. Minerals – Water – Role of water

  9. Balanced diet

  10. Obesity

  11. Digestive System

  12. Dental Caries (Tooth decay)

  13. Root Canal Treatment

  14. Peptic ulcer

  15. Hernia and Types

  16. Appendicitis (Appendix)

  17. Gall Stones

  18. Hepatitis

  19. Fractures – Types of fractures

  20. Mechanism of fracture

  21. Dislocation of joints

  22. Arthiritis

  23. Rickets and Osteomalacia – Orthopedics

  24. Muscles

  25. Mechanism of muscle contraction

  26. Types of muscle contraction

  27. Myasthenia Gravis

  28. Respiration – Process of pulmonary respiration

  29. Mechanism of Breathing

  30. Regulation of Respiration

  31. Pneumonia Tuberculosis Symptoms Treatment

  32. Bronchitis – Acute bronchitis, Chronic Bronchitis Causes

  33. Circulatory System – Functioning of Human heart

  34. Cardiac Cycle

  35. Coronary blood vessel and its significance

  36. Myocardial infarction

  37. Angina pectoris

  38. Angiogram – Angioplasty

  39. Atherosclerosis

  40. Heart block Echo cardiography Heart Valves

  41. Rheumatic Heart Disease (RHD), ICCU – (Intensive Coronary Care Unit)

  42. Blood Pressure

  43. Heart transplantation

  44. Pulse rate

  45. Cardio Pulmonary Resuscitation (CPR)

  46. Blood – Composition of plasma – Blood cells

  47. Clotting of Blood or Haemostasis

  48. Thrombosis

  49. Nervous system Co-ordination systems

  50. The Brain – Fore Brain, Midbrain, Hindbrain

  51. Memory

  52. Sleep – Types of sleep

  53. Stroke – Brain haemorrhage

  54. Alzheimer – Meningitis (Brain fever)

  55. Conditioned reflex

  56. Electroencephalography EEG

  57. Right and Left brain concept

  58. Spinal cord functioning

  59. Chemical co-ordination – Functions of Endocrine glands

  60. Hypothalamus

  61. Pituitary gland – hormone

  62. Hormones of Neurohypophysis – vasopressin

  63. Thyroid gland

  64. Parathyroid Gland

  65. Pancreas

  66. Adrenal gland

  67. Gonads

  68. Receptor Organs – Eye

  69. Photochemistry of Retinal visual Pigments

  70. Errors of refraction

  71. Optometry – Retinopathy

  72. Cataract – Lens Replacement – Glaucoma – Nyctalopia

  73. Eye Infections and Eye Care

  74. Ear

  75. Mechanism of hearing

  76. Defects of the ear

  77. Hearing Aid – Noise pollution

  78. Skin and functions of skin

  79. Melanin functions

  80. Effects of solar radiation / UV radiation – Skin grafting

  81. Dermatitis

  82. Tongue – Mechanism of Stimulation

  83. Excretion Ureotelism Nephron

  84. Mechanism of urine formation

  85. Renal Failure, Dialysis, Kidney Machines

  86. Kidney stone – Kidney transplantation

  87. Diabetes mellitus

  88. Functioning of male reproductive system

  89. Functioning of female reproductive system

  90. Ovulation and fate of the ovum – Menstrual cycle

  91. Fertilization

  92. Birth control

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