Bright field microscopes focus light from the light source through a condenser lens, from where it will pass through the specimen on the stage. Dark field microscopes place a disk between the light source and the condenser lens so the specimen appears bright on a dark background. Fluorescence microscopes illuminate the specimen with ultraviolet or certain wavelengths of visible light, exciting electrons in fluorescent dyes added to the specimen so they fluoresce and emit light of a particular color.
Some simple light microscopes use a mirror you can tilt to reflect sunlight so it passes through the specimen on the stage. Most light microscopes incorporate a built-in light source --- usually an incandescent bulb. The light source can often be adjusted to provide different intensities of light or a differently shaped beam.
When using a light microscope, always adjust the light source to provide the right amount of light -- enough to illuminate the specimen but not so much that you hurt your eyes. The condenser lens is often adjustable, and many microscopes come equipped with an aperture diaphragm that restricts the diameter of the light beam so you can have more complete control over the light quality.