You can demonstrate local adaptation of the retina by fixing

You can demonstrate \"local\" adaptation of the retina by fixing your eyes on the while dot in the center of the peace symbol in Figure 5 for 30 seconds. Then look at the black dot at the right of Figure 5. What do you see when you stare at the black dot? Explain Why. Why don\'t you see an afterimage like this when you are reading black print on a white page?

Solution

1) What do you see when you stare at the black dot:

Answer: A circle and a dot at the center.

Explain Why: This happens because of the local adaptation of Retina which results in the production of afterimages. Local adaptation means that Retina adapts to the ambient level of illumination and when there is a small change in the intensity of the light, then the optical system is able to compensate for that change immediately. However, if the change in light level is a lot, then the system takes relatively more time to readjust. There are several ways in which this adaptation is controlled - for e.g. switch-over from rod to cones, change in pupil size, adaptation in the photoreceptors as a result of photo pigment bleaching etc. When we stare at a image /color for a prolonged duration, then the cone which responds to that image are fatigued leading to the formation of an afterimage.

2) Why do not you see an after image like this when you are reading a black print on a white page?

Answer: During reading, we are not focusing on a particular character for a long time. Therefore the cones are not fatigued and we do not see any after image.