Have you ever noticed:
- When we enter a dark room from outside, immediately the things inside the room do not appear clear. Why?
- Why do we experience dazzling effect, when we come out of cinema hall after noon show?
Have you experienced above mentioned scenarios and wondered why does it
happen? How does our eyes adapt to these situations?
If yes, then this article on "Light and Dark adaptation of Human Eye"
will help to answer your curiosity.
Topics Covered:
A. Dark Adaptation in Human Eye
B. Light Adaptation in Human Eye
Let's begin to answer your curiosity about how does our vision differs in
brightly lighted area to the one from darkness and how does eye adapts to
transition when we move from lighted area to a darker area and
vice-versa.
A. Dark Adaptation in Human Eye
What is Dark Adaptation?
When we enter a dark room (such as theater) from a brightly lighted area,
we experience difficulty in seeing the objects in the room for a while.
But, after sometime our eyes get adapted to darkness and our vision
improves. This improvement of vision is due to a phenomenon called
dark adaptation.
Example:
Enter a theater from outside.
How does eyes gets adapted to darkness?
The dark adaptation of eyes takes place in two steps:
- Regeneration of Rhodopsin
- Dilation of pupil
1) Regeneration of Rhodopsin
Rhodopsin or vision purple is the pigment of rods which is
responsible for vision in dark or dim light.
In bright light, the rhodopsin gets broken down and
degenerates.
When we enter dark room from a brightly lighted area, through the process
of dark adaptation, the pigment Rhodopsin is regenerated which enables the
vision in dim light or dark.
2) Dilation of pupil
Pupil is the part of an eye from where
light enter the eyes. The amount of light entering the eyes is controlled by the size of the
pupil.
The size of the pupil is controlled by two muscles:
a) Circumferential sphincter muscle
b) Iris dilator muscle
In dim or dark light, pupil is dilated whereas in bright light,
pupil is constricted.
When we enter a dark room from the brightly lighted area, the pupil gets
dilated which allows more light to enter the eyes.
B. Light Adaptation in Human Eye
What is Light Adaptation?
When we enter a brightly lighted area from a darker area, we experience
a dazzling effect for a while. But, after few seconds our eyes get
adapted to light and our vision improves. This improvement of vision is
due to a phenomenon called light adaptation.
Example:
- Coming out of a cinema hall after the noon show
How does eyes gets adapted to light?
The light adaptation of eyes takes place in two steps:
- Degeneration of Rhodopsin
- Constriction of pupil
1) Degeneration of Rhodopsin
Rhodopsin or vision purple is the pigment of rods which is responsible
for vision in dark or dim light. Iodopsin is the pigment of cones which
is responsible for vision in light and colored vision.
In bright light, the rhodopsin gets broken down or bleached,
reducing its sensitivity.
2) Constriction of pupil
When we enter brightly lighted area from a darker area, the
pupil gets constricted which allows lesser amount of light to
enter the eyes.
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