The Human Eye and the Colourful
World Summary
Accommodation of the Eye
The process by which the ciliary muscles change
the focal length of an eye lens to focus distant or
near objects clearly on the retina is called the
accommodation of the eye.
Power of Accommodation
The ability of the eye to focus objects lying at
different distances is called the power of
accommodation of the eye.
Near point or Least Distance of Distinct Vision
Near point or least distance of distinct vision is
the point nearest to the eye at which an object is
visible distinctly. For a normal eye the least
distance of distinct vision is about 25
centimetres. However, it varies with age of the
person. For example, for infants it is only 5 to 8
cm.
Far Point
Far point of the eye is the maximum distance up
to which the normal eye can see things clearly. It
is infinity for a normal eye.
Range of Vision
The distance between the near point and the far
point is called the range of vision.
Refractive Errors or Defects of Vision
The abnormalities in the normal vision of the eye
due to the change in focal length of the lens or
due to the deformation of cornea are known as
refractive errors or defects of vision.
Myopia
Myopia is an eye defect due to which the eye is
not able to see distant objects clearly. This
occurs when the light rays entering the eye
converge in front of the retina in the vitreous
body.
Hypermetropia
Hypermetropia or hyperopia is an eye defect in
which distant vision is clear while near vision is
blurred. This occurs when the light rays entering
the eye converge behind the retina.
Presbyopia
Presbyopia is caused when the centre of the eye
lens hardens making it unable to accommodate
near vision.
Astigmatism
Astigmatism is the most common vision problem
resulting in distorted images, as light rays are
prevented from meeting at a common focus.
Dispersion
The phenomenon by which, a ray of light splits
into its constituent colours, when passed through
a transparent medium, is known as dispersion.
The band of colours obtained due to the
dispersion of light is called the spectrum.
Atmospheric Refraction
Atmospheric refraction is the shift in apparent
direction of a celestial object caused by the
refraction of light rays as they pass through
Earth�s atmosphere.
Scattering
Scattering is a general physical process whereby
some forms of radiation, such as light or moving
particles, for example, are forced to deviate from
a straight trajectory by one or more localized
non-uniformities in the medium through which it
passes.
Tyndall Effect
Tyndall effect is the visible scattering effect of
light on particles along the path of a beam of
light passing through a colloid system.
