3. Visual angle and size perception. \"Size constancy\" refers to the fact that
ID: 3459140 • Letter: 3
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3. Visual angle and size perception. "Size constancy" refers to the fact that we tend to perceive that objects stay the same size even though their distance from us may vary. We must therefore use distance judgments along with visual angle to judge apparent sizes Emmert's Law. Suppose you form an afterimage by looking for several seconds at a bright object. The physical size of the afterimage on the retina remains constant. But the perceived size of the image may vary as you look at different distances a. Very briefly explain why the perceived size is larger at the farther viewing distance: Suppose you form an afterimage that has a visual angle of 5 deg. How large will the afterimage look if you fixate the book in front of you (at 1 m)? How large will the afterimage look if you instead fixate a far wall (at 10 m)?Explanation / Answer
3. Size Constancy
(a) Emmert's Law:
Emmert's law states that objects producing retinal images of the same size will look different in size if they appear to be located at different distances. We use judgments to estimate the size of an object based on its perceived distance from us, regardless of its actual physical size.
Why perceived size is larger at the further viewing distance:
There is a distinction between the size of an object in the world and the size the same object projects onto the retina of the human eye.
When we form an afterimage by looking at a bright object, the afterimage will appear to increase in size when projected to a greater distance. The physical size of the object remains same, but the brain perceives its size to be larger due to greater distance. This is because an object of constant size will project progressively smaller retinal images as its distance from the observer increases.
Which means, when an object close to your eyes takes up most of your field of vision, the brain concludes that the size is small, whereas when you see an object far away and it still takes up most of your field of vision, the brain concludes that the size is quite large.
Size of Afterimage when book is fixated at 1m:
Size = 2?Distance?tan(VisualAngle/2)
= 2?1?tan(5.000/2)
=0.0873 m
Size of Afterimage when far wall at 10 m is fixated:
Size = 2?Distance?tan(VisualAngle/2)
= 2?10?tan(5.000/2)
=0.8732 m
(b) Moon Illusion
The moon appears large on the horizon because our mind causes objects at the horizon to appear enlarged.
If the horizon moon appears 1.5 times larger at the horizon, the sky also appears 1.5 times closer at the horizon than directly overhead.
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