Describe the defects that affect the mammalian eye and how they could be corrected.

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 Short-sightedness  (myopia);
-A condition  where  light  rays  from  a  distant  object  are  focused  in front  of  the  retina;  while  those  from  a  near  object  are  clearly  focused  on  the  retina;
-It  is  caused by  an  abnormally  elongated  eyeball;  or  too  much  refractive  power  of  the  eye  lens;  it  is  corrected by  wearing  concave/diverging  lenses;  which
diverge  light  rays;  or  reduce  the  refractive power  of  the  eye  before  they  reach  the  eye  lens;

Long-sightedness  (Hypermetropia);
-light  rays from  a  near  object  are  not  focused  by  the  time  they  reach  the  retina;  or  may  be  focused  behind the  retina;  while  the  rays  from  a  distant  object  are  sharply  focused;
 -The defect  is  caused  by  an eyeball  that  is  too  short;  or  an elongated/weak  lens  system  (distance  between  lens  and  the  retina  is  short/low refractive power);
-Corrected  by  wearing a converging/convex lens;  which  refracts  light  rays  before  reaching  the eye  lens;  this  enhances  refraction  resulting  in  rays  being  sharply  focused  onto  the  retina;

Astigmatism;
-Rays  from  an  object  are  brought  to  focus  on  different  planes;  due  to  unequal curvature  of  the  cornea/lens;  causing  unequal  refraction  of  light  entering  the  eye;  this  defect  is corrected  by  wearing  special  cylindrical  lens  in  front  of  the  eye;  the  lens  corrects  the  focus  in the  defective  planes;

Colour-blindness;
-A  genetic  defect;  in  which  an  animal  is  unable  to distinguish  between  colours  particularly  within  the  red-green  spectrum;  the  retina  lacks  cones; pigments  that  respond  to  colour  vision; genetically inherited condition that has no physical correction. 

Squintedness;
 -An eye  defect  in  which  extrinsic  muscles of  the  eye;  that  control  the  turning  of  the  eyeball  do  not  co-ordinate  accordingly  on  stimulation; it  affects  the  paired  rectus  muscles  that  move  the  eyeball  up  and  down;  and  the  lateral  rectus muscles  that  move  the  eyeball  left  to  right;  the  eyeballs  therefore  face  different  directions; making  focusing  and  accommodation  difficult  to  achieve;  corrected  by  specialized  surgery;

Old sight  (Presbyopia);
-Caused  by  old  age;  when  supplies  of  nutrients  and  oxygen  to  the  lens  is  far much  reduced;  hence  the  cells  of  the  lens  die;  the  lens’  elasticity  is  reduced;  and  hence  cannot adequately change  shape;  and  becomes  fixed  into  a  shape  that  is  not  suitable  for  distant  vision;  managed  by use  of  ‘reading  glasses  that  have  converging  lenses;  to  give  the  eyes  an  extra  refractive power;

 Cataracts;
-Associated  with  old  age;  but  may  also  be  caused  by  an  eye  injury  due  to  a blow;    or  complications  of  diabetes  mellitus;  the  eye  lens  become  cloudy;  blocking  transmission of  light  rays;  protein  fibres  become  denatured;  and  clump  together  making  the  lens  translucent/opaque; corrected  by  surgery;  to  replace  the  defective  lens  with  a  normal  one  from  a  donor;  or  use  of artificial  lens;  

Max.  20  mks 

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