Transcript THE HUMAN EYE Cross Section through a

The Human Eye
And our
Colourful
THE HUMAN EYE
Cross Section through a Human Eye
Aqueous
humour
Pupil
Cross Section through a Human Eye
The Human Eye is Like a Camera:
• Light enters the eye through a thin membrane called the
cornea.
• Most of the refraction of light rays entering the eye occurs at
the outer surface of the cornea.
• Its lens forms an image on a light-sensitive screen called the
retina.
• The eye lens forms an inverted real image of the object on
the retina.
•The crystalline lens provides finer adjustment of focal length
required to focus objects at different distances on the retina.
• Iris is a dark muscular diaphragm behind the cornea and it
controls the size of the pupil.
• The pupil regulates and controls the amount of light entering
the eye.
Parts and functions of a Human Eye
• The eyeball is approximately spherical in shape, diameter approx 2.3 cm.
• The lens consists of layers of fibrous, protein-based tissues
• The lens is supported by ciliary muscles that change its shape by tensing
or relaxing; The change in curvature can thus change the focal length of
the lens.
• Retina: a delicate membrane having very many light- sensitive cells.
• The light-sensitive cells get activated and generate electric signals, nerve
impulses
• These signals are sent to brain via Optic Nerves.
• The brain intercepts these signals and finally processes the information
for our perception.
Power of Accommodation
The ability of the eye lens to adjust its focal length is called accommodation.
Because the lens is soft & flexible, the focal length can be changed as needed,
by contractions of the ciliary muscles.
When the muscles are relaxed, the lens becomes thin. The radius of curvature
and focal length increase. This enables us to see the distant objects clearly.
When we look at the objects closer to they, ciliary muscles contract,
decreasing the radius of curvature and focal length. This enables us to see the
nearby objects clearly.
Least Distance of Distinct Vision (LDDV):
The minimum distance, at which objects can be seen most distinctly without
strain, is called Least Distance of Distinct Vision. For a normal eye, LDDV is
25 cm.
Far Point:
The farthest point up to which the eye can see objects clearly is called far
point of the eye. Far point for a normal eye is infinity! A normal eye can see
objects clearly that are between 25 cm and infinity.
Visual Accomodation
DEFECTS OF VISION AND THEIR CORRECTION
Myopia (Short-sightedness or Near-sightedness)
A person with myopic eye can see nearby objects clearly but cannot
see distant objects clearly.
A person with this defect has a far point closer than infinity.
In a myopic eye, the image of a distant object is formed in front of the
retina and not on the retinal itself.
This defect may arise due to
(i) excessive curvature of the eye lens (short focal length of the lens)
or
(ii) Elongation of the eyeball.
Myopia can be corrected by using a concave lens.
Myopic Eye
O
I
Near Point
LDDV = 25 cm
O
I
LDDV = 25 cm
O
I
LDDV = 25 cm
O
I
LDDV = 25 cm
Myopic Eye corrected with Concave Lens
I
Correcting Vision with a Concave Lens:
Hypermetropia (Long-sightedness or Far-sightedness)
A person with hypermetropia can see distant objects clearly but
cannot see nearby objects clearly.
A person with this defect has the near point farther away from the
normal near point (25 cm).
Such a person may have to keep reading materials much beyond 25
cm from the eye for comfortable reading.
In a hypermetropic eye, the image of a nearby object is (or would be)
formed behind the retina and not on the retina itself.
This defect may arise due to
(i) long focal length of the eye lens or
(ii) Very small size of the eyeball.
Hypermetropia can be corrected by using a convex lens.
Hypermetropic Eye
O
I
Near Point
LDDV = 25 cm
O
I
LDDV = 25 cm
O
I
LDDV = 25 cm
O
II
LDDV = 25 cm
Hypermetropic Eye corrected with Convex Lens
Correcting Vision with a Convex Lens:
Presbyopia
The power of accommodation of the eye usually decreases with aging.
For most of the people, the near point gradually recedes away.
They can not see nearby objects comfortably and distinctly without corrective
eye-glasses.
This defect is called presbyopia.
It arises due to
(i) gradual weakening of the ciliary muscles and
(ii) diminishing flexibility of the eye lens.
* Sometimes, a person may suffer from both myopia & hypermetropia. Such
people require bi-focal lenses which consists of both concave and convex
lenses. The upper portion is concave for distant vision and the lower portion
is convex for near vision.
Summary of Vision Corrections:
The phenomenon of splitting a ray of white light into its constituent
colours (wavelengths) is called dispersion and the band of colours
from red to violet is called a spectrum:
ROYGBIV.
White
light
Screen
So, the colours are refracted at different
angles and hence get separated.
RAINBOW !!!!
A rainbow is a natural spectrum which is caused by dispersion of sunlight by
tiny water droplets present in the atmosphere after a rain shower.
The incident sunlight with suitable angle of incidence is refracted, dispersed,
internally reflected and finally refracted out by the rain drops.
Due to the dispersion and internal reflection, different colours reach the eye of
the observer.
A rainbow is always formed in a direction opposite to that of the Sun.There are
primary and secondary rainbows.
Rain drop
Sunlight
41º
Eye
43º
A line parallel to Sun’s ray
Twinkling of Stars:
The twinkling of a star is due to atmospheric refraction
of starlight.
Since the atmosphere bends starlight towards the
normal, the apparent position of the star is slightly
different from its actual position.
The star appears slightly higher (above) than its actual
position when viewed near the horizon.
This apparent position is not stationary, but keeps on
changing slightly, since the physical conditions of the
earth’s atmosphere are not stationary.
Apparent position
of the Star
Density of Atmosphere &
Refractive index increase
The atmospheric refraction occurs in a medium of
gradually changing refractive index.
Real
position
of the
Star
Since the stars are very distant, they approximate pointsized sources of light.
As the path of rays of light coming from the star goes
on varying slightly, the apparent position of the star
fluctuates and the amount of light entering the eye
flickers- the star sometimes appear brighter, and at
some other time, fainter which gives the twinkling effect.
Eye
Why Planets do not twinkle:
The planets are much closer to the earth, and are thus seen as extended
sources.
Since it is the collection of large number of point-sized sources of light, the
total variation in the amount of light entering into the eye from all the
individual sources will average out to zero, thereby nullifying the twinkling
effect.
Apparent position
of the Sun
Advance Sunrise
and Delayed
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.
Atmosphere
Horizon
Earth
Real position
of the Sun
Scattering of Light – Blue colour of the sky and Reddish appearance
of the Sun at Sun-rise and Sun-set:
Less Blue colour
is scattered
Horizon
Earth
Atmosphere
Other colours
mostly scattered
SCATTERING OF LIGHT BY TINY WATER DROPLETS IN THE MIST