we will go through activity 16.9 class 8 science chapter 16 Light in which we will discuss the blind spot on the retina. You should also go through activity 16.6 to get more about reflection.
Activity
· Make
a round mark and a cross on a sheet of paper with the spot to the right of the
cross.
· The
distance between two marks may be 6-8 cm.
· Hold the sheet of paper at an arm’s length
from the eye.
· Close
your left eye.
· Look
continuously at the cross.
· Move the sheet slowly towards you, keeping
your eye on the cross.
· What
do you find?
· Does
the round mark disappear at some point?
· Now
close your right eye.
· Look at the round mark now and repeat the
activity.
· Does
the cross disappear?
· The
disappearance of the cross or the round mark shows that there is a point on the
retina that cannot send messages to the brain when light falls on it.
Observation and explanation
There is a thin layer of neural tissues inside the eye
which called the retina. It has numerous lights detecting cells called
photoreceptors.
When light falls on the retina where it is converted into
electric impulses. These electric impulses to the brain through the optic nerve.
The blind spot is the area where the optic nerve
connects to the brain and it has no light-sensitive cells, so we can’t see
anything if the light coming from an object falls on the blind spot.
When we move the sheet keeping eye on a cross or round
mark, we cannot see these ‘ cross or round marks’ at a certain position.
Why this
happens?
Because at a certain position, light from the object
falls on the blind spot where no light-detecting cells are present so the light is
not converted into electric impulses.
Conclusion
The blind spot is a region on the retina that does not
have any light-detecting cells so images formed on this part of the retina cannot be
seen.
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