Human Eye and Correction of Defects

The Human Eye

• The human eye is a sense organ that enables us to see the world around us, with the ability to identify colors.
• It is like a camera, with a lens system that forms an image on a light-sensitive screen called the retina.

Structure of the Eye

• The eye has a transparent bulge on the front surface called the cornea, where light enters the eye.
• The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
• The iris is a dark muscular diaphragm that controls the size of the pupil, regulating the amount of light entering the eye.
• The crystalline lens provides finer adjustment of focal length to focus objects at different distances on the retina.

Power of Accommodation

• The eye lens is composed of a fibrous, jelly-like material that can change its curvature to modify its focal length.
• The ciliary muscles can modify the curvature of the eye lens to focus on objects at different distances.
• The ability of the eye lens to adjust its focal length is called accommodation.
• The focal length of the eye lens cannot be decreased below a certain minimum limit.

Least Distance of Distinct Vision

• The minimum distance at which objects can be seen most distinctly without strain is called the least distance of distinct vision or near point of the eye.
• For a young adult with normal vision, the near point is about 25 cm.

Far Point of the Eye

• The farthest point up to which the eye can see objects clearly is called the far point of the eye.
• For a normal eye, the far point is infinity.

Vision Defects

• Sometimes, the crystalline lens of people at old age becomes milky and cloudy, leading to defects in vision.
• Lenses used in spectacles can correct defects of vision.