I was going to point out that one of the balancing factors between Fire Bolt and other cantrips like Acid Splash is that Fire Bolt suffers disadvantage if an enemy gets adjacent to you before you cast it, while the 'save or take damage' cantrips can be cast while an enemy is adjacent without penalty. Then I saw this:
There's an arcane tricker in one of my games who uses silent image to create "an opaque, black hemisphere with a diameter of 15 feet."
Oh, the glorious odor of cheese. Sadly, it works a little differently than the writer envisions.
First off, the various Image spells create 'objects, creatures, or visible manifestations'. In a later post, the writer compares the created illusion with Darkness, but the intent of the Image definition is that the illusion be something tangible, and darkness is not tangible ("An illusory object made by minor illusion is meant to be like a stool or a rock, not an atmospheric effect....The system doesn't treat light or darkness as an object. An object is something tangible.", Jeremy Crawford, 16 Sep 2015 (from Twitter))
The caster gets around this problem by proposing an opaque dome rather than a 'zone of darkness', but that just creates other problems...
Because he cast the spell and therefore knows that it is an illusion, he can see through it.
As far as I'm aware, there is nothing in the rules, either in the description of the Image spells or in the general spellcasting rules, that says that a caster is immune to the effect of his own spells, including illusion spells. You can automatically Dispel your own spells (see Dispel Magic), but not automatically ignore the effects of them. Since the caster would not be able to spend an action to interact with the illusion until his next round, the caster would be unable to see through it on the round he cast it.
You might argue that this violates common sense, and that someone who creates an illusion should be aware that the creation is an illusion -- but if a caster is immune to his own illusion spells, would that mean he can ignore the effect of his own Silence spell (a 2nd level illusion spell)?
Note as well the phrases "[p]hysical interaction with the image reveals it to be an illusion...If a creature discerns the illusion for what it is, the creature can see through the image...." As noted by other posters, the first time something physically interacts with the illusion, the illusion is revealed and it stops limiting every creature in the combat (including the caster) at that point. Simply having an enemy walk through the illusionary dome would reveal it to everyone in the combat, and it would cease having any effect on the combat immediately.
General rule of thumb: if the interpretation of a low-level spell seems way out of balance for its power level, then the interpretation is wrong.
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Pauper