Rahul is sitting in a park during a bright sunny day of winters for 3 continuous hours. He got a call
from his mother to immediately come home to fix a new bulb in his room as it has stopped working,
making his room very dim less illuminated. After returning, Rahul experienced some dizziness in his
eyes and was not able to see properly. After half an hour he managed to see things clearly around him.
Then his mother gave him a new bulb to place in the room. His mother continuously stared at that
bulb and after handed it over to Rahul, she suddenly made a mental image of that bulb with a patch of
Grey and white color of that bulb for some time.
13 (i) identify the type of visual adaptation taken place here. (1+1+1+1=4)
13 (ii) Name the chemical substance present in the rods of the eyes.
13 (iii) Rahul’s mother had made which type of after image here?
13 (iv) Identify the nature of stimulus received by Rahul in this example.
Answers
(i) The type of visual adaptation that takes place here is Dark adaptation
(ii) The chemical substance present in the rods of the eyes is called Rhodopsin
(iii) Rahul’s mother had made positive after image
(iv) The nature of stimulus received by Rahul in this example is visual stimulus
Details:
(i) Dark adaptation refers to the process of adjusting to a dimly lit environment after exposure to bright light. This may take half an hour or even longer depending on the previous level of exposure of the eye to light.
(ii) The rods have a photo-sensitive chemical substance called rhodopsin. It is also called visual purple.
(iii) Rahul's mother had made positive after image because positive after images resemble the original stimulus in terms of hue, saturation and brightness, and usually occur after a brief but intense stimulation of dark adapted eyes.
(iv) Rahul received a visual stimulus because the bulb and its light are things that could be seen.