Name the four common defects of vision and state the cause of each defect. A person cannot see distinctly objects placed beyond 2 metres. State the nature and focal length of the lens which could be used to correct this defect.
The four common defects of vision and their causes are as follows :
- Myopia: It is caused when the eye ball becomes elongated or when the lens has excessive curvature.
- Hypermetropia: It is caused when the eye ball becomes small or when the lens has decreased curvature.
- Presbyopia: It is caused in old age when the power of accommodation decreases.
- Colour Blindness: It is caused when rod cells decrease in number or capacity.