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Comments on Lecture 21 "Pictures That Stand Up"

Comment 1. Answer to the question"


The object appear to be floating in the air because we perceive that its shadow is separated. This illusion occurs only when the true shadow is invisible. Therefore, we have to choose an object and the illumination in such a way that the true shadow of the object cannot be seen. A cone-like object is suitable for this purpose, because its shadow is completely hidden below the object when we illuminate them from above. The natto containers placed in an upside-down posture have this property.

Comment 2. Picture of the shadows used for the natto containers

The picture of the shadows used in the lecture is shown in the left. This is made by blurring the picture of dark gray squares. Note that they are not general rectangles or parallelograms. This is because the shadow cast by a square parallel to the desk surface is also a square. If we use a blurred image of general parallelograms, the object, if it appears floating, will be perceived in a slanted posture.

The picture of the shadows used in the lecture is posted here. If you are interested in, please download it, print it, and enjoy the floating-object illusion by yourself.