Illusion preventing folks from trusting their own eyes? (7)

I believe the answer is:

optical

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But I can't explain it at all! Could you help me understand?

(Other definitions for optical that I've seen before include "connected to light" , "Using visible light" , "Relating to sight" , "Sort of illusion" , "Involving light" .)

I've seen this clue in The Hindu.

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