This way, each wave, quantum, point of light, will slowly separate from its neighbour - taking with it a point of image detail from the specimen. The cone is aimed and shaped so that the top of the cone hits the objective at the widest angle. This creates a limitation. If you create a short wide cone too quickly, yer have to get that objective lens very, very, close to the specimen to pick up all the detail and points of light before they whizz past the lens. A concave mirror produces a very narrow cone of light.
Most microscopes therefore have a lens called a condensor just below the specimen slide.
Light from yer concave mirror or other in-built light source is beamed through the condensor to produce an evenly dispersed bright cone of light arriving at the objective lens. By moving the condensor up or down you can determine where the top end of the cone ultimately falls on the objective. We could maybe make a black hole right in the middle of that cone of light. Different, isn't is A bit like looking at a relief map rather than a road map. When we stop light in the middle of the cone from hitting the specimen, the light at the edge of the cone whacks through the specimen bouncing off the bits with the most extreme angles to the light cone edge.