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It seems to me that "optical" relates more to the mechanics of light and vision, whereas "visual" is a much broader term.

For example, hallucinations are classed as "visual" or "auditory", rather than "optical" or "acoustic". Other examples are visual puns, visual cliches, visual metaphors, etc. I notice on the Wikipedia page on optical illusions uses the terms interchangeably.

Is there a semantic reason why the term "optical illusion" is used instead of visual?

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    Because an optical illusion is about tricking the eye into believing something that is not true. A (visual/auditory) hallucination is something totally unrelated, it is the person's mental health that leads him or her to believing something that is completely imaginary as being real. – Mari-Lou A Jul 18 '15 at 9:05
  • @dn3s good point. But this is what language is - not always precise. – Konrad Gajewski Jul 18 '15 at 11:27
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    "Optical" got there first, and claimed that territory. – Hot Licks Jul 19 '15 at 1:49
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After doing a bit of background searching on the words "optical" "optical illusion," "visual" and "optics," I have put together some ideas as to why there are two expressions. Please keep in mind that I am not an expert on optics, the history of optics, or Greek and Latin.

I'm going to use the expression "optical illusion" for this discussion.

Optical illusions were known to the ancient Greeks (http://www.opticalspy.com/spy-blog/a-brief-history-of-optical-illusions) At that time, anything having to do with seeing and light was lumped into the category "optics." According to the article, there were two ideas about optical illusions, one stating that our sensory organs are to blame, another saying it was the fault of the environment (think mirage.)

The science of optics was mainly written about in Greek. The Arabs took over and made many discoveries, and wrote in Arabic but the main sources of information were written in Greek. Possibly some in Latin, although I am not expert in this so this is a guess.

The study of optics made it's way to Europe and it is there were ideas flourished. However, optics still talked about lenses, the air, light, and the human eye, and people still didn't really know how people see. So if you said you were studying optics, this meant you were interested in how the eye worked, rainbows, lenses, and why sticks in water "bend." Today, if you say you are studying optics, the eye is not included.

The word "optics" comes from Greek. The word "illusion," however comes from Latin. It is here where I am not certain where to go further because Greek actually has a word for "optical illusion" (ophtalmarati) which seems to mean "eye trickery." The word "Optical Illusion" wouldn't have been used by the Greeks. It is possible that in early writing about this subject many expressions were used and someone who knew both Greek and Latin and had read the ancient manuscripts and current books (written in Latin) decided to take Optics and merge it with Illusion, changing Optics to the adjective with an "al" ending, making "optical illusion."

In light of what we know about optical illusions a better expression might be "visual illusion." I say this because Optics is the study of the properties and phenomenon of light, and isn't really related to the study of our eyes and our visual system, so somewhere along the line, "visual illusion" became used more. A google Ngram search (https://books.google.com/ngrams/graph?content=Optical+illusion%2C+Visual+Illusion&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2COptical%20illusion%3B%2Cc0%3B.t1%3B%2CVisual%20Illusion%3B%2Cc0) shows that around 1900 Visual Illusion started being used and then took off in the 1940s. I am not a historian of Neuroscience, so possibly someone with that background can explain the spike in "visual illusion" at that time. Since then, the words appear to be used the same, with "optical illusion" being more popular.

Hopefully people with the necessary backgrounds can comment on this answer and will help me to edit it into something of more substance. People with a background in the science of optics and also the coining of English words (why Greek and Latin words are mixed together) are encouraged to contribute.

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They are the same thing as far as I know. I guess it is the person using it or the difference between a magician using it or a scientist.

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Think of it in steps

It is visual and optical illusion.

First, it is an illusion

Illusion

then it is a

Visual Illusion

and then it is an

Optical Illusion

You could probably even get away with writing optical visual illusion

Optics is the science behind the eye. It isn't just about vision. For instance, an optic nerve aids you visually... not optically.

An optical illusion means that the optic mechanics of your eye/brain are illuded.

  • Optics is not about the science behind the eye, it is about how light passes through mediums such as air, glass, water, etc. Ophthalmology deals with the eye. – michael_timofeev Sep 17 '15 at 4:35
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Words are only very rarely invented by a committee.

Occasionally an individual is solely responsible for inventing/selecting the word and somehow "injecting" it into the argot. Sometimes this individual will consider, eg, Latin roots when constructing the word, sometimes it's just a whim.

More often, the need for the word exists and various people use different terminology to describe the concept, often combining and recombining the terms, sometimes borrowing from another language or discipline, until some word or phrase begins to be used more frequently and slowly achieves "blessed" status.

Inventing a new word is a messy business, and it's unreasonable to expect every word to "make sense" from the standpoint of Latin roots, scientific terminology, or some other standard. You get what you get.

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