The pupil regulates and
controls the amount of light entering the eye.
When the light
is very bright, the iris contracts the pupil to allow less light to enter the
eye. However, in dim light the iris expands the pupil to allow more light to
enter the eye. Thus, the pupil opens completely through the relaxation of the
iris.
The change in
the curvature of the eye lens can thus change its focal length. When the
muscles are relaxed, the lens becomes thin. Thus, its focal length increases.
This enables us to see distant objects clearly. When you are looking at objects
closer to the eye, the ciliary muscles contract. This increases the curvature
of the eye lens. The eye lens then becomes thicker.
Consequently,
the focal length of the eye lens decreases. This enables us to see nearby
objects clearly.
The ability of
the eye lens to adjust its focal length is called accommodation.
To see an object
comfortably and distinctly, you must hold it at about 25 cm from the eyes. The
minimum distance, at which objects can be seen most distinctly without strain,
is called the least distance of distinct vision.
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.
Some animals, usually
prey animals, have their two eyes positioned on opposite sides of their heads
to give the widest possible field of view. But our two eyes are positioned on
the front of our heads, and it thus reduces our field of view in favour of what
is called stereopsis
Myopia is also
known as near-sightedness. 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.
In a myopic eye,
the image of a distant object is formed in front of the retina.
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.
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 close by object are focussed at
a point behind the retina
This defect
arises either because (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.
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 vision.
The splitting of
light into its component colours is called dispersion.
Isaac Newton was
the first to use a glass prism to obtain the spectrum of sunlight.
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.
The apparent
random wavering or flickering of objects seen through a turbulent stream of hot
air rising above a fire or a radiator. The air just above the fire becomes
hotter than the air further up. The hotter air is lighter (less dense) than the
cooler air above it, and has a refractive index slightly less than that of the
cooler air. Since the physical conditions of the refracting medium (air) are
not stationary, the apparent position of the object, as seen through the hot air,
fluctuates. This wavering is thus an effect of atmospheric refraction (refraction
of light by the earth’s atmosphere) on a small scale in our local environment.
The twinkling of
a star is due to atmospheric refraction of starlight.
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.
The blue colour
of the sky, colour of water in deep sea, the reddening of the sun at sunrise
and the sunset are some of the wonderful phenomena that follows scattering of
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 white.
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.
