Human Eye: The Colourful World
THE HUMAN EYE: Structure:
Pupil: Pupil is the round black spot in front of eye. It regulates the
amount of light entering the eyes. Pupil works like aperture of a camera. In
case of dim light pupil dilate to allow more light to enter the eyes. In case of
strong light pupil constrict allowing less light to enter.
Irish: Irish is made of muscles. They control the size of opening if pupil.
Lens: Lens lies just behind the pupil. Lens becomes thin to increase its
focal length. This enables us to see distant objects clearly. To focus on nearer
objects lens becomes thick to decrease its focal length. But there is a limit.
The minimum distance of clear vision is 25 cm. Below this distance we cannot see
Retina: Retina works like a screen or camera film. Retina is full of light
and colour sensitive cells. These cells, upon receiving image send electrical
signals to the brain, which processes these information to make a mental image
of what we see.
Benefits of two eyes: One eye is having a field of vision of about 150
degrees. Both the eyes enable us to see upto a field of 180 degrees. Moreover,
as two different images get juxtaposed in the brain, so we are able to see a
three dimensional view of the world.
Malfunctions of Eyes:
Cataract: In old age the cornea becomes cloudy. This reduces the vision in
old age. In early stages of the disease cataract can be cured by eye surgery.
Sometimes artificial lens is also transplanted during cataract surgery. This is
called Intra Ocular Lens Transplantation.
Myopia: Myopia is also known as near-sightedness. A person with myopia can
see nearby objects clearly but cannot see distant objects distinctly. In a
myopiceye, the image of a distant object is formed in front of the retina and
not at the retina itself. This defect may arise due to (i) excessive curvature
of the eye lens, or (ii) elongation of the eyeball. 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.
Hypermetropia: Hypermetropia is also known as far-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). Such a person has to keep a reading material much beyond 25 cm
from the eye for comfortable reading. This is because the light rays from a
closeby object are focussed at a point behind the retina. This defect arises
(i) the focal length of the eye lens is too long, or
(ii) the eyeball has become too small.
This defect can be corrected by using a convex lens of appropriate power.
Eye-glasses with converging lenses provide the additional focussing power
required for forming the image on the retina.
Presbyopia: The power of accommodation of the eye usually decreases with
ageing. For most people, the near point gradually recedes away. They find it
difficult to see nearby objects comfortably and distinctly without corrective
eye-glasses. This defect is called Presbyopia. It arises due to the gradual
weakening of the ciliary muscles and diminishing flexibility of the eye lens.
Sometimes, a person may suffer from both myopia and hypermetropia. Such people
often require bifocal lenses. A common type of bi-focal lenses consists of both
concave and convex lenses. The upper portion consists of a concave lens. It
facilitates distant vision. The lower part is a convex lens. It facilitates near
Twinkling of stars
The twinkling of a star is due to atmospheric refraction of starlight. The
starlight, on entering the earth’s atmosphere, undergoes refraction continuously
before it reaches the earth. 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 . Further, this apparent position of the
star is not stationary, but keeps on changing slightly, since the physical
conditions of the earth’s atmosphere are not stationary, as was the case in the
previous paragraph. 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 starlight entering the eye flickers – the star sometimes appears
brighter, and at some other time, fainter, which is 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. By actual
sunrise, we mean the actual crossing of the horizon by the Sun. The time
difference between actual sunset and the apparent sunset is about 2 minutes. The
apparent flattening of the Sun’s disc at sunrise and sunset is also due to the
SCATTERING OF LIGHT
The earth’s atmosphere is a heterogeneous mixture of minute particles. These
particles include smoke, tiny water droplets, suspended particles of dust and
molecules of air. When a beam of light strikes such fine particles, the path of
the beam becomes visible. The light reaches us, after being reflected diffusely
by these particles. The phenomenon of scattering of light by the colloidal
particles gives rise to Tyndall effect. This phenomenon is seen when a fine beam
of sunlight enters a smoke-filled room through a small hole. Thus, scattering of
light makes the particles visible. Tyndall effect can also be observed when
sunlight passes through a canopy of a dense forest. Here, tiny water droplets in
the mist scatter light. The colour of the scattered light depends on the size of
the scattering particles. Very fine particles scatter mainly blue light while
particles of larger size scatter light of longer wavelengths. If the size of the
scattering particles is large enough, then, the scattered light may even appear
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. The red light has a wavelength about 1.8 times
greater than blue light. Thus, 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. The sky appears dark to passengers flying at very high altitudes,
as scattering is not prominent at such heights. You might have observed that
‘danger’ signal lights are red in colour. The red is least scattered by fog or
smoke. Therefore, it can be seen in the same colour at a distance.