CHAPTER – 11
The Hyman Eye and the Colourful World
In this chapter we will study Human eye that uses the light and enable us to see the
objects.
We will also use the idea of refraction of light in some optical phenomena in nature
i.e. Rainbow formation, twinkling of star, blue and red colour of sky etc.
Human Eye : A Sensitive sense organ
It acts like a camera, enable us to capture the colourful picture of the surroundings.
It forms an inverted, real image on light sensitive surface Retina
The Various parts of eye and their functions
1. Cornea : It is a thin 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.
2. Eyeball : it is approximately spherical in shape, with a diameter of about
2.3cm.
3. Iris : It is a dark muscular diaphragm that controls the size of pupil. It is
behind the cornea.
4. Pupil : It regulates and control the amount of light entering the eye. It is the
black opening between aqueous humour & lens.
5. Crystalline eye lens : Provide the focussed real & inverted image of the
object on the retina. It is composed of a fibrous, jelly like material. This is
convex lens that converges light at retina.
114 X-Science
, 6. Ciliary muscles : It helps to change the curvature of eyelens and hence
changes its focal length so that we can see the object clearly placed at different
positon.
7. Retina : Thin membrane with large no. of sensitive cells.
8. When image formed at retina, light sensitive cells gets activated and generate
electrical signal. These signals are sent to brain via optic nerue. Brain analyse
these signals after which we perceive object as they are.
How pupil works ?
Example : You would have observed that when you come out of the cinema hall
after watching movie in the bright sun light, your eyes get closed . And when you
entered the hall from the bright light, you won't be able to see and after some time
you would be able to see.
Here the pupil of an eye provide 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 din : Iris expand the pupil, so that more light enters the eye.
Pupil open completely, when iris is relaxed.
Persistence of Vision : It is the time for which the sensation of an object continue
th
in the eye. It is about 1/16 of a second.
Power of Accommodation :
The ability of eye lens to adjust it focal length is called accommodation with the
help of ciliary muscles.
Ciliary Muslces
Relaxed Contract
1. Eye lens become thin 1. Eye lens become thick
2. Increases the focal length 2. Decreases the focal length
3. Enable us to see distant object clearly 3. Enable us to see nearby object clearly
Near point of the Eye For point of the Eye
It is 25cm for normal eye. The It is infinity for normal eye. It is the
minimum distance at which object can farthest point upto which the eye can
be seen most distinctly without strain. see object clearly.
115 X-Science
The Hyman Eye and the Colourful World
In this chapter we will study Human eye that uses the light and enable us to see the
objects.
We will also use the idea of refraction of light in some optical phenomena in nature
i.e. Rainbow formation, twinkling of star, blue and red colour of sky etc.
Human Eye : A Sensitive sense organ
It acts like a camera, enable us to capture the colourful picture of the surroundings.
It forms an inverted, real image on light sensitive surface Retina
The Various parts of eye and their functions
1. Cornea : It is a thin 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.
2. Eyeball : it is approximately spherical in shape, with a diameter of about
2.3cm.
3. Iris : It is a dark muscular diaphragm that controls the size of pupil. It is
behind the cornea.
4. Pupil : It regulates and control the amount of light entering the eye. It is the
black opening between aqueous humour & lens.
5. Crystalline eye lens : Provide the focussed real & inverted image of the
object on the retina. It is composed of a fibrous, jelly like material. This is
convex lens that converges light at retina.
114 X-Science
, 6. Ciliary muscles : It helps to change the curvature of eyelens and hence
changes its focal length so that we can see the object clearly placed at different
positon.
7. Retina : Thin membrane with large no. of sensitive cells.
8. When image formed at retina, light sensitive cells gets activated and generate
electrical signal. These signals are sent to brain via optic nerue. Brain analyse
these signals after which we perceive object as they are.
How pupil works ?
Example : You would have observed that when you come out of the cinema hall
after watching movie in the bright sun light, your eyes get closed . And when you
entered the hall from the bright light, you won't be able to see and after some time
you would be able to see.
Here the pupil of an eye provide 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 din : Iris expand the pupil, so that more light enters the eye.
Pupil open completely, when iris is relaxed.
Persistence of Vision : It is the time for which the sensation of an object continue
th
in the eye. It is about 1/16 of a second.
Power of Accommodation :
The ability of eye lens to adjust it focal length is called accommodation with the
help of ciliary muscles.
Ciliary Muslces
Relaxed Contract
1. Eye lens become thin 1. Eye lens become thick
2. Increases the focal length 2. Decreases the focal length
3. Enable us to see distant object clearly 3. Enable us to see nearby object clearly
Near point of the Eye For point of the Eye
It is 25cm for normal eye. The It is infinity for normal eye. It is the
minimum distance at which object can farthest point upto which the eye can
be seen most distinctly without strain. see object clearly.
115 X-Science
