CHAPTER–11
THE HUMAN EYE AND THE COLOURFUL WORLD
HUMAN EYE:
Human eye is the optical instrument / sense organ which enables us to see.
It acts like a camera, enable us to capture the colourful picture of the surroundings.
An inverted, real image is formed on a light sensitive screen Retina
Parts of Eye and their Functions:
1. Eyeball: Spherical in shape, with a diameter of about 2.3 cm.
2. Cornea: It is a thin, transparent membrane through which light enters. It forms the transparent bulge on
the front of eyeball. Most of the refraction occurs at the outer surface of the cornea.
3. Aqueous humour is a transparent, watery fluid between cornea and lens.
4. Iris: It is the coloured part of the eye behind the cornea. It is a dark muscular diaphragm that
controls the size of the pupil and regulates the amount of light entering the eye.
5. Pupil: It is like a hole through which light enters the eye to pass through the lens. Light once entered
cannot exit. Black in colour.
6. Crystalline eye lens: It is composed of a fibrous, jelly like material. This is convex lens that converges
light at retina, provides real focus & inverted image of the object on the retina.
7. Ciliary muscles: It helps to change the curvature of eye lens and hence changes its focal length so that
we can see the object clearly placed at different positions.
8. Retina: It is a thin membrane with large no. of light sensitive cells. It is the screen for image formation. A
real and inverted image is formed here.
There are two types of photoreceptors in the human retina, rods and cones.
1. Rods are responsible for vision at low light levels.
2. Cones are active at higher light levels, are capable of colour vision.
When image is formed at retina, light sensitive cells gets activated and generate electrical signal. These
signals are sent to brain via optic nerve.
9. Optic Nerves: connects and transmits electrical signals from eye to brain. Brain analyse these signals
after which we perceive object as they are.
10. Vitreous humour is the clear gel that fills the space between the lens and the retina of the eyeball of
humans. Maintains spherical shape of eye.
11. Blind Spot: The portion on retina that does not have any rods or cones and image falling on this portion
cannot be seen.
Q) How does iris works?
a) The pupil of an eye provides a variable aperture, whose size is controlled by iris.
(a) When the light is bright: Iris contracts the pupil, so that less light enters the eye.
(b) When the light is dim: Iris expand the pupil, so that more light enters the eye. Pupil opens
completely, when iris is relaxed.
, Persistence of Vision: It is the time for which the sensation of an object continues in the eye. It is about
1/16th of a second.
Power of Accommodation
The ability of eye lens to adjust its focal length with the help of ciliary muscles is called power of
accommodation.
1. Curvature of lens decreases 1. Curvature of lens increases
2. Eye lens becomes thin 2. Eye lens becomes thick
3. Increases the focal length 3. Decreases the focal length
4. Enable us to see distant object clearly 4. Enable us to see nearby object clearly
Near point of the Eye Far point of the Eye
It is 25cm for normal eye. The minimum It is infinity for normal eye. It is the farthest
distance at which object can be seen most point upto which the eye can see objects clearly
distinctly without strain.
Defects of Vision and their C orrection
1. Cataract : A condition in which eye lens becomes milky/cloudy. So, the image cannot be seen clearly and
can cause blurred /complete or partial loss of vision.
TREATMENT: Surgical removal of extra growth (cataract surgery)/Laser treatment.
2. Myopia : (Near Sightedness/Short Sightedness)
Near vision is normal and far vision is defected.
Image formed in front of the retina.
The Reason of defect
1. Increase in the curvature of eye lens i.e Eye lens becomes thick and its focal length decreases.
2. Elongation of the eye ball.
CORRECTION
Corrected by using a Concave Lens of appropriate power (-ve).
THE HUMAN EYE AND THE COLOURFUL WORLD
HUMAN EYE:
Human eye is the optical instrument / sense organ which enables us to see.
It acts like a camera, enable us to capture the colourful picture of the surroundings.
An inverted, real image is formed on a light sensitive screen Retina
Parts of Eye and their Functions:
1. Eyeball: Spherical in shape, with a diameter of about 2.3 cm.
2. Cornea: It is a thin, transparent membrane through which light enters. It forms the transparent bulge on
the front of eyeball. Most of the refraction occurs at the outer surface of the cornea.
3. Aqueous humour is a transparent, watery fluid between cornea and lens.
4. Iris: It is the coloured part of the eye behind the cornea. It is a dark muscular diaphragm that
controls the size of the pupil and regulates the amount of light entering the eye.
5. Pupil: It is like a hole through which light enters the eye to pass through the lens. Light once entered
cannot exit. Black in colour.
6. Crystalline eye lens: It is composed of a fibrous, jelly like material. This is convex lens that converges
light at retina, provides real focus & inverted image of the object on the retina.
7. Ciliary muscles: It helps to change the curvature of eye lens and hence changes its focal length so that
we can see the object clearly placed at different positions.
8. Retina: It is a thin membrane with large no. of light sensitive cells. It is the screen for image formation. A
real and inverted image is formed here.
There are two types of photoreceptors in the human retina, rods and cones.
1. Rods are responsible for vision at low light levels.
2. Cones are active at higher light levels, are capable of colour vision.
When image is formed at retina, light sensitive cells gets activated and generate electrical signal. These
signals are sent to brain via optic nerve.
9. Optic Nerves: connects and transmits electrical signals from eye to brain. Brain analyse these signals
after which we perceive object as they are.
10. Vitreous humour is the clear gel that fills the space between the lens and the retina of the eyeball of
humans. Maintains spherical shape of eye.
11. Blind Spot: The portion on retina that does not have any rods or cones and image falling on this portion
cannot be seen.
Q) How does iris works?
a) The pupil of an eye provides a variable aperture, whose size is controlled by iris.
(a) When the light is bright: Iris contracts the pupil, so that less light enters the eye.
(b) When the light is dim: Iris expand the pupil, so that more light enters the eye. Pupil opens
completely, when iris is relaxed.
, Persistence of Vision: It is the time for which the sensation of an object continues in the eye. It is about
1/16th of a second.
Power of Accommodation
The ability of eye lens to adjust its focal length with the help of ciliary muscles is called power of
accommodation.
1. Curvature of lens decreases 1. Curvature of lens increases
2. Eye lens becomes thin 2. Eye lens becomes thick
3. Increases the focal length 3. Decreases the focal length
4. Enable us to see distant object clearly 4. Enable us to see nearby object clearly
Near point of the Eye Far point of the Eye
It is 25cm for normal eye. The minimum It is infinity for normal eye. It is the farthest
distance at which object can be seen most point upto which the eye can see objects clearly
distinctly without strain.
Defects of Vision and their C orrection
1. Cataract : A condition in which eye lens becomes milky/cloudy. So, the image cannot be seen clearly and
can cause blurred /complete or partial loss of vision.
TREATMENT: Surgical removal of extra growth (cataract surgery)/Laser treatment.
2. Myopia : (Near Sightedness/Short Sightedness)
Near vision is normal and far vision is defected.
Image formed in front of the retina.
The Reason of defect
1. Increase in the curvature of eye lens i.e Eye lens becomes thick and its focal length decreases.
2. Elongation of the eye ball.
CORRECTION
Corrected by using a Concave Lens of appropriate power (-ve).
