Can you use Minor Illusion to create an illusion of a working mirror, i.e. with a working reflection?
According to the description:
You create a sound or an image of an object within range that lasts for the duration. The illusion also ends if you dismiss it as an action or cast the spell again.
If you create a sound its volume can range from a whisper to a scream. It can be your voice, someone else's voice, a lion roaring, a drum beating or any other sound you choose. The sound continues unabated throughout the duration or you can make discreet sounds at different times before the spell ends.
If you create an image of an object such as a chair, muddy footprints or a small chest-it must be no larger then a 5 Foot cube. The image can't create sound, light, smell or any other sensory effect. Physical interaction with the object reveals it to be an illusion, because things can pass through it.
With this in mind - I would say no. Minor Illusion is very limited (e.g. static), and cannot produce any side effects (e.g. no light, or other sensory effects). I expect this also means the light would pass through the illusion, and that maintaining the same properties as the physical version of the object is beyond the scope of the illusion. This is mostly because its not actually a real object (e.g. it does not behave like a mirror), it only looks like one.
Yes, because physics.
The simple fact that you can see an illusion means it reflects light, whether it is the illusion of a mirror or not [edit: see below].
Note that this does not violate the rule that “The image can't create sound, light, smell or any other sensory effect.” The image is not emitting light, it is reflecting it. An illusion of a mirror would act no differently than the illusion of anything else - it just reflects the light perfectly, allowing the viewer to see an image.
Objects that do not reflect light (at all) look like a perfectly flat, black surface. So if illusions are to be useful in any way, they must reflect light.
[Edit]: I will concede that if illusions are a purely mental construct, like an hallucination, that physics will not apply; however, in that case my answer is still yes, because if we're tricking the mind we can surely trick it thoroughly.
There are two ways you could think about the reflection. You could imagine it as an effect created by the caster - a constant modification in reaction to the movement of objects around the "mirror" - but that would imply the constant attention of the caster. But that stands in contradiction to the fact that the spell does not require concentration. (It doesn't seem that any motion in the visual images is allowed, although some flexibility with sound illusions is given.)
Alternatively, you could imagine that the image has the property of being able to reflect light - that is, it is a physical mirror. But this is a physical interaction, and "Physical interaction with the image reveals it to be an illusion, because things can pass through it." This may seem a little strange, since we intuitively assume that we can perceive illusions in the same physical way that we perceive actual objects - that is, ambient light reflects from the surface of the illusion and reaches our eyes. But this intuition is apparently wrong. Illusions do not reflect ambient light - if we discern the illusion it becomes faint to our sight, so clearly most or all ambient light actually passes through the illusion. Furthermore, different observers may see different images, depending on whether they discern the illusion, so the process cannot be as deterministic as conventional science would demand. The physics are not explained, but clearly they are different from what we are used to in the mundane world.
I am going to say no, and here is why.
Minor illusion allows you to mimic simple things. However it does not duplicate effects. Although a mirror is a simple thing, the images it reflects are or could be very complicated. Additionally, they'd need to mimic what's being reflected in mirror image.
I think you could use minor illusion to mimic a portrait, but s mirror I think is too complicated.
Ok, I am pulling out the boom to refine my answer, but that's this is still the basis.
Update: nevermind, shadow players answer is good.
No, one cannot create a mirror illusion with this spell, because reflection requires matter interacting with light-stuff (called photons in our reality) and the collision causes them to change direction (which we call either reflection or refraction depending on how focused the effect is). While several of the answers above point out that illusions are visible, I would like to humbly point out that illusions are a form of magic often relegated to a school centered on light-production and light-manipulation, not creating matter that then interacts with light on its own.
If I conjure a brown wooden table, then that table reflects brown light while absorbing other hues. When the lights go out the table has no light to reflect, so it is, for all intents and purposes, impossible to see. However, an illusory table is not solid like the conjured one; it is a preprogrammed construction of light that is specifically defined as impossible for matter to interact with. That would mean it cannot reflect or absorb light; light passes through it and it is technically transparent. The only reason we can see it is because it is producing light.
The way I read the description, at the time of casting the spell, the caster must decide exactly what they wish to present, and that is the amount of continuous light the object produces. Not only would such an illusion be easily spotted due to a lack of reflection, any change in the lighting of the room would not cause the object to lighten or darken – it would stand out because it is fundamentally incapable of reacting to its environment in the way actual matter is. In a dark room therefore it would glow.
This does however raise several mechanical issues. Any and all light that would come into contact with the object instead passes through it. So if I simulate a five foot box then I can see within the box using light that passes through the illusion from outside, but I cannot see what is outside. Likewise if I light a torch within the box then those outside will see the box emitting torchlight from within. I believe a similar issue arose when my group contemplated the possibility of an invisible light source. You see exactly what the spell says you can see (in this instance whatever the caster decided upon at casting time) and then apply lighting effects as if the spell were not in effect.