Chapter: The Human Eye and the Colourful World
1. The Human Eye
The human eye is like a camera. Its lens system forms an image on a light-sensitive screen
called the retina.
Key Parts and Functions:
:Cornea: A thin membrane through which light enters the eye. Most refraction happens here.
:Iris: A dark muscular diaphragm that controls the size of the pupil.
:Pupil: The small opening in the iris that regulates the amount of light entering the eye.
Bright light: Pupil becomes small.
Dim light: Pupil becomes large.
:Eye Lens: A crystalline lens that adjusts its focal length to focus objects on the retina.
:Ciliary Muscles: Muscles that hold the lens and help change its shape (curvature) to focus.
:Retina: The screen at the back of the eye. It has light-sensitive cells (rods and cones) that
generate electrical signals.
:Optic Nerve: Transmits the electrical signals from the retina to the brain.
Power of Accommodation
Definition: The ability of the eye lens to adjust its focal length to see both nearby and distant
objects clearly is called accommodation.
Near Point (Least Distance of Distinct Vision): The minimum distance at which an object can be
seen clearly without strain. For a normal adult eye, it is 25 cm.
Far Point: The farthest point up to which the eye can see clearly. For a normal eye, it is infinity.
2. Defects of Vision and Correction
Sometimes the eye loses its power of accommodation, resulting in blurred vision. There are
three common defects:
A. Myopia (Near-sightedness)
Definition: A person can see nearby objects clearly but cannot see distant objects distinctly.
Cause:
Excessive curvature of the eye lens.
Elongation of the eyeball.
Result: The image forms in front of the retina.
1. The Human Eye
The human eye is like a camera. Its lens system forms an image on a light-sensitive screen
called the retina.
Key Parts and Functions:
:Cornea: A thin membrane through which light enters the eye. Most refraction happens here.
:Iris: A dark muscular diaphragm that controls the size of the pupil.
:Pupil: The small opening in the iris that regulates the amount of light entering the eye.
Bright light: Pupil becomes small.
Dim light: Pupil becomes large.
:Eye Lens: A crystalline lens that adjusts its focal length to focus objects on the retina.
:Ciliary Muscles: Muscles that hold the lens and help change its shape (curvature) to focus.
:Retina: The screen at the back of the eye. It has light-sensitive cells (rods and cones) that
generate electrical signals.
:Optic Nerve: Transmits the electrical signals from the retina to the brain.
Power of Accommodation
Definition: The ability of the eye lens to adjust its focal length to see both nearby and distant
objects clearly is called accommodation.
Near Point (Least Distance of Distinct Vision): The minimum distance at which an object can be
seen clearly without strain. For a normal adult eye, it is 25 cm.
Far Point: The farthest point up to which the eye can see clearly. For a normal eye, it is infinity.
2. Defects of Vision and Correction
Sometimes the eye loses its power of accommodation, resulting in blurred vision. There are
three common defects:
A. Myopia (Near-sightedness)
Definition: A person can see nearby objects clearly but cannot see distant objects distinctly.
Cause:
Excessive curvature of the eye lens.
Elongation of the eyeball.
Result: The image forms in front of the retina.
