The The Human Human

EyeEye

And ourAnd our

Colourful Colourful World!World!

THE HUMAN EYE

Cross Section through a Human Eye

Aqueous humour

Pupil

Cross Section through a Human Eye

• 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.

The Human Eye is Like a Camera:

• 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.

Parts and functions of a Human Eye

• 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 AccommodationThe 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.

LDDV = 25 cm

O

LDDV = 25 cm

I

O

LDDV = 25 cm

Myopic EyeO

I

I

O

LDDV = 25 cm

IO

LDDV = 25 cm

II

Near Point

Myopic Eye corrected with Concave Lens

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.

LDDV = 25 cm

Hypermetropic EyeO

Near Point

LDDV = 25 cm

IO

O

LDDV = 25 cm

I

I

LDDV = 25 cm

IO

LDDV = 25 cm

OII

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.

So, the colours are refracted at different angles and hence get separated.

White light

RO

YG

B I

V

Screen

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.

Eye

41º43º

Sunlight

Rain drop

A line parallel to Sun’s ray

Twinkling of Stars:

The twinkling of a star is due to atmospheric refraction of starlight.

The atmospheric refraction occurs in a medium of gradually changing refractive index.

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.

Since the stars are very distant, they approximate point-sized 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

Den

sity of A

tmo

sph

ere &

Refractive in

dex in

crease

Apparent position of the Star

Real position of the Star

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.

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.

Earth

AtmosphereHorizon

Apparent position of the Sun

Real position of the Sun

Earth Atmosphere

Horizon

Scattering of Light – Blue colour of the sky and Reddish appearance of the Sun at Sun-rise and Sun-set:

Other colours mostly scattered

Less Blue colour is scattered

SCATTERING OF LIGHT BY TINY WATER DROPLETS IN THE MIST