Assume the players are in an area of natural darkness (no magic involved). Can I use silent image to create a bright illusion such as a torch or maybe a miniature sun to illuminate our way?
A good answer should be based on rule-text, not on "a good DM would...".
Yes, but not for creating real illumination. The illusion's details are completely invented by you, so you wouldn't be able to use it as a light source to reveal anything unknown to you.
Even using it as a deceptive illusion, it wouldn't be very useful. The image would only add the appearance of illumination within a 7½-foot radius of the illusory torch. That includes both "bright" light and shadows. Silent image has a 15-foot cube maximum area of effect, so you'd have to have a torch with "illuminated" surroundings limited to that size of sphere.
That is... not a very convincing torch. A real torch illuminates an 80-foot diameter sphere (bright light in a 20-foot radius, plus another 20 feet of dim light; PHB p. 153). That's five times too large to create a convincing image of a burning torch plus illuminated surroundings. You'd be limited to making an image of a guttering torch moments away from burning out, a torch inside 15-foot-or-smaller rooms (so long as nobody looks at the non-light spilling out a doorway), or using it at a very great distance from the observers so that a mere pinpoint of light would be convincing as... something emitting light far away.
For an intuitive understanding of the disparity in size, even the illumination of a candle (20-foot diameter sphere) is too large to fit correctly inside the area of effect of silent image. You'd have to make an image of a candle burning low before silent image was adequate to pull off this trick in a large space.
So yes, you can create the appearance of illuminated surfaces surrounding an illusory torch, in strict principle. But it's not real light, and even as an illusion it'll be a terrible illusion and the DM would be quite justified in giving observers a free "investigate" check to see through it every round, if the DM wanted to. (That's up to "a good DM" of course, but you can't dismiss it as relevant to the RAW because deciding the impact of illusions on NPC behaviour is inherently within the DM's sphere of authority.)
(That, without even getting into the issue of whether you accurately reproduced the lit-up appearance of the surroundings if your observers know it better than you do; also something that's just up to "a good DM", but is a likely hurdle for using silent image in this way under some circumstances.)
I would also say no, but with another explaination.
The Silent Image text says
The image is purely visual; it isn't accompanied by sound, smell, or other sensory effects.
I read this passage as you create the image of the torch or mini-sun but can not create it sensory effect - the shining of light.
There is another problem, if you are in complete darkness and can't see the surrounding, you can make up the image of any surrounding you think of, but it would be luck if your imagination of the surrounding and the real surrounding would be the same.
Sadly, no.
From page 203 of the PHB:
Illusion spells deceive the senses or minds of others.
When you create an illusion, you're not making an actual image out of light (like a hologram), you're causing viewers to see something that doesn't exist.
Even if this worked, the illusion created by Silent Image is limited in size to a 15 foot cube, so the illumination would abruptly end at the edges.
Silent image is the illusion of an image, not the real thing. You can create the illusion of light, but to illuminate an area, you would need to know what the area looks like to begin with, because you not lighting the area, you’re creating the image of a lighted area. If you know an area well, maybe you could do it passably, but things you miss may poke through, or you may conceal that pot hole you forgot was there. Either way, the illumination is not real, just a 3D projection of what you think the area looks like or want it to look like. No one actually sees anything, they see the image you created.
