Ok so I have a Campaign that is focused in the Underdark where light is not always present. I have a caster that uses the Minor Illusion spell and casts an illusionary object without the presence of light. Would the illusion manifest itself in a place where there is no light to create the illusion? 

I say this based on the Rainbow effect- Without light there is no Rainbow and the Rainbow is simply an optical illusion of light. So really the heart of this is:

In Total Darkness can you still use the Minor Illusion spell (And any other visual based spell line for that matter)? 

Thank you again for taking a look at this objectively.  =o)

Illusion is not a trick of light. In D&D an illusion is a magical effect that targets the minds of creatures. It doesn't bend light or anything.

The object created would be a visual illusion in that space so you can think of the "spell" being energy in that space that can imprint the illusion of the object into the mind of any creature that would be capable of seeing that space. So, it could be entirely pitch black but a creature with darkvision can still see without any light and would therefore see your illusory object. If it was pitch black and the creature did not have darkvision then it would not see the object. But, the illusion is still there in effect just unseen.

You do not need to see the space yourself to create an illusion in it so even if you were in total darkness without darkvision you could still cast minor illusion of an object into a space you can choose like a big rock in front of you and you can then crouch down to hide behind it and if a creature did come along that had darkvision it would see the rock.

One of the many reasons why minor illusion is a fantastic cantrip.

Gotcha- so you are saying that the spell creates an optical illusion that is present in the mind of anyone looking into that space. So that light is really irrelevant to the hysteria of this thing being present and all onlookers seeing it. I say hysteria because it triggers a mental sensation that can be shared. I assume the same would hold true for audial sound as it would not generate a true sound which has a vibration but a mental concept of that sound for everyone in range to "Hear" it.

Ok so this begs the question- if that mind you place that object or sound into is unfamiliar with what the object or sound is, how would the mind translate it into the same idea presented by the caster?

Example: I make an illusion of a clock and a Trog comes up to where the illusion is and sees this thing it has never seen in its life how does it know how to construct this mental image in its own mind?  I am certain it might confuse it to say the least..   This is way different then you placing an illusion of a clock in the room using light as its a visual construct verses a mental projection.

I have been looking in the PHB and don't see it mention all illusions are mentally created only the most dangerous like a Phantasmal Killer for instance. Can you provide the page so I can find it. Thanks again for the discussion I really appreciate it.  example: page 203 reads that illusions deceive the senses or the minds of others. Only the most insidious of illusions plant images into the minds.    

You're looking at the same thing I am. Rather than thinking of it as in-game rationalising let me put it in a mechanic/rules way: if the spell required light it would say so, because it doesn't say so, it doesn't. A spell tells you what is needed and any restrictions. Since minor illusion doesn't mention needing light or even for you to see the space you cast into then these things are not required and the spell works as written regardless of light levels.

When I said it projects the illusion into minds I was rationalising given the spell not requiring light to function. The spell will go from you into the space and it is you who decides what the image appears as. The magic of the spell will imprint this into the perception/senses/mind/etc of any creature capable of "viewing" the space. It will see the object you create and it doesn't matter if it something it has never seen before because the construct of the illusion is based on you not the viewing creature. Seeing the illusion is no different than seeing a real object, just that it is possible to realise it is not real and move through it.

Only the most insidious spells go into a creature's mind directly. And for those it involves a saving throw (typically Wisdom). However, a spell like minor illusion may project things mentally but is not rooted in any creature's mind but if you have a hard time thinking of the "mind" bit then we can say the spell alters perceptions like sight, darkvision, etc if that is easier. The spell itself, the energy of it if you can call it that, is in the space you choose.

How about we simplify this: objects made by minor illusions are holograms that use magic instead of light and therefore work no matter how a creature might see. Sounds created by minor illusion also use magic not vibrating air and therefore work even in places where there is no air and could, if you want, be clearly audible even in situations where sound is normally distorted like underwater (you can use it to "speak" underwater if you wanted and have your allies hear "your" voice perfectly).

I hope this makes sense.

I like that idea- it makes good sense.. Thanks for the words of wisdom and I hope this post helps anyone else who was trying to rationalize this spell line as I was..  LOL   Thanks again Cybermind..