The document summarizes key aspects of the human eye and vision. It describes the structure of the eye, including the cornea, iris, pupil, lens, retina, and other parts. It explains how the iris controls the size of the pupil to regulate the amount of light entering the eye. It also discusses refractive errors like myopia and hyperopia, and how lenses are used to correct vision. Prisms and dispersion of light are described. Atmospheric effects like refraction, twinkling of stars, and why the sky appears blue are summarized.

1. Human Eye and the Colourful World

2. The human eye is one of the most valuable and sensitive sense organs.
It enables us to see the wonderful world and the colours around us.
It is impossible to identify colours while closing the eyes. Thus, of all the
sense organs, the human eye is the most significant one as it enables us
to see the beautiful, colourful world around us.
The beauty of Human Eye

3. Structure of Human Eye
Crystalline Lens

4. Various parts of the Human Eye
1. Cornea – Light enters the eye through a thin membrane called the cornea. It
forms a transparent bulge on front surface of the eyeball. Most of the refraction for
the light rays entering the eye occurs at the outer surface of cornea.
2. Sclera – It is the outer covering, protective tough white layer called the sclera
3. Eyeball- The eyeball is spherical in shape with a diameter of about 2.3 cm.
4. Aqueous Humour- It is a fluid which fills the space between cornea and the eye
lens.
5. Iris- Iris is a dark muscular diaphragm that controls the size of the pupil.
6. Pupil- Pupil is a small opening in the middle of the iris through which light enters
the eye. The pupil regulates and controls the amount of light entering the eye.

5. 7. Lens - Behind the pupil, there is a transparent and flexible jelly-like structure called a
lens. It is a convex lens. By the action of ciliary muscles, it changes its shape to focus light
on the retina, it becomes thinner to focus distant objects and becomes thicker to focus
nearby objects.
8. Ciliary muscles – The ciliary muscles are capable of modifying the curvature of the lens
and thereby affecting the focal length of the lens.
9. Vitreous Humour - The vitreous humour is a clear fluid which fills the eye between the
lens and the retina. This fluid helps the eye hold its shape, with light being transmitted
through it to the retina.
10. Retina - Retina is a light-sensitive screen on which the image is formed. It is a delicate
membrane having enormous number of light-sensitive cells. It converts the image formed
by the Lens into electrical impulses . These electrical impulses are then transmitted to the
brain through Optic nerves. It contains Rods and Cones.
11. Optic Nerve - These are the nerve which take the image to the brain in the form of
electric Signals

6. How Pupil Works ??
The iris makes the pupil expand or contract according to the intensity of light
around the eye.
i. When the light is bright : Iris contracts the pupil, so that less light enters the eye.
ii. When the light is dim : Iris expands the pupil so that more light enters the eye.
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 enter the hall
from the bright light, you won’be able to see but after some time you would be able to see.
Here the pupil of an eye provides a variable aperture, whose size is controlled by iris

7. Power of Accommodation
The ability of the eye lens to adjust its focal length is called power of
accommodation
Ciliary Muscles
Contract
Relax
Eye lens becomes thin
Increases the focal length
Enable us to see distant
object clearly
Eye lens becomes thick
Decreases the focal length
Enable us to see nearby object
clearly

8. A person with myopia can see nearby objects clearly but cannot see distant
objects distinctly. A person with this defect has the far point nearer than infinity.
Causes :
(i) excessive curvature of the eye lens.
(ii) elongation of the eyeball.
Corrections :
This defect can be corrected by using a concave lens of suitable power. A
concave lens of suitable power will bring the image back on to the retina and thus
the defect is corrected.
DEFECTS OF VISION AND THEIR CORRECTION
1. MYOPIA ( Near-Sightedness or Short Sightedness )

10. 2. HYPERMETROPIA ( Far-Sightedness or Long-Sightedness )
A person with hypermetropia can see distant objects clearly but cannot see nearby
objects distinctly. The near point, for the person, is farther away from the normal
near point (25 cm).
Causes :
(i) The focal length of the eye lens is too long.
(ii) The eyeball has become too small.
Corrections :
This defect can be corrected by using a convex lens of appropriate power. Eye-
glasses with converging lenses provide the additional focusing power required for
forming the image on the retina

12. As we become old, the power of accommodation of the eye usually
decreases, the near point gradually recedes away. This defect is
called Presbyopia.
Person may suffer from both myopia and hypermetropia.
3. Presbyopia
Reason of defect : Gradual weakening of ciliary muscles and decreasing
the flexibility of the eye lens.
Correction : Using Bifocal lens with appropriate power.Bifocal lens
consist of both concave and convex lens

13. 4. Cataract
Sometimes, the crystalline lens of people at old age becomes
milky and cloudy. This condition is called cataract. This causes
partial or complete loss of vision. It is possible to restore vision
through a cataract surgery.

14. REFRACTION OF LIGHT THROUGH A PRISM
PE – Incident ray
∠i – Angle of incidence
EF – Refracted ray
∠r – Angle of refraction
FS – Emergent ray
∠e – Angle of emergence
∠A – Angle of the prism
∠D – Angle of deviation

16. The splitting of white light into its component colours when it passes
through a glass prism is called dispersion of light. The various colours are
violet, indigo, blue, green, yellow, orange, red. The sequence of colours
remembers as VIBGYOR.The band of seven colours is called the spectrum.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM

17. Recombination of Dispersed Light

18. A rainbow is a natural spectrum appearing in the sky after a rain shower It is caused
by dispersion of sunlight by tiny water droplets, present in the atmosphere. A
rainbow is always formed in a direction opposite to that of the Sun. The water
droplets act like small prisms. They refract and disperse the incident sunlight, then
reflect it internally, and finally refract it again when it comes out of the raindrop
Due to the dispersion of light and internal reflection, different colours reach the
observer’s eye.
Rainbow Formation

19. It is due to atmospheric refraction of starlight.
The temperature and density of different layers of atmosphere keeps varying.
Hence we have different medium.
Distant star act as point source of light. When the starlight enter the earth’s
atmosphere it undergoes refraction continuously, due to changing refractive
index i.e. from Rarer to denser, it bends towards the normal.
Due to this the apparent position of the star is different from actual position.
The star appears higher than its actual position.
ATMOSPHERIC REFRACTION
The refraction of light caused by the Earth’s atmosphere ( having air layers of
varying optical densities ) is called atmospheric refraction.
Twinkling of stars

21. Actual sunrise happens when the sun is below the horizon in the morning. the
rays of the light from the sun below the horizon reach our eyes because of
refraction of light. similarly , the sun can be seen about few minutes after the
actual sunset. The sun is visible to us about 2 minutes before the actual sunrise,
and about 2 minutes after the actual sunset because of atmospheric refraction.
Advance sunrise and delayed sunset

22. SCATTERING OF LIGHT
The scattering of light is the phenomenon by which a beam of light is redirected in
different directions on interacting with the particles present in the medium
Tyndall Effect
Tyndall Effect deals with the phenomenon of scattering of light by colloidal particles.
(1) When a fine beam of sunlight enters a room, the particles present in the room become visible
due to scattering of light by these particles.
(2) When sunlight passes through a canopy of a dense forest, tiny water droplets in the mist
scatter light.
The colour of the scattered light depends on the size of the scattering particle. Very fine particles
scatter mainly blue light while particles of larger size scatter light of longer wavelength. If the
size of the scattering particle is large enough, then the scattered light may even appear white.

24. Why is the colour of the clear Sky Blue?
The molecules of air and other fine particles in the atmosphere have size smaller
than the wavelength of visible light. These are more effective in scattering light of
shorter wavelengths at the blue end than light of longer wavelengths at the red end.
When sunlight passes through the atmosphere, the fine particles in air scatter the
blue colour (shorter wavelengths) more strongly than red. The scattered blue light
enters our eyes. If the earth had no atmosphere, there would not have been any
scattering. Then, the sky would have looked dark.

26. Thanks For Watching