the optical microscope
The optical microscope is the everyday workhorse that magnifies a polished specimen with visible light and glass lenses — the same basic idea as a biology-class microscope, but built to look at opaque metal by shining light down through the objective and catching what reflects back (a reflected-light or metallurgical microscope), instead of shining light through a thin transparent slide.
Light passes through an objective lens, is magnified, and reaches your eye or a camera; typical useful magnifications run from 50x to about 1000x. The hard limit is diffraction: you cannot resolve two features closer than roughly half the wavelength of light, about 0.2 micrometres (200 nanometres). Cranking magnification past that just gives 'empty magnification' — a bigger but blurrier image, not more detail. That 0.2 micrometre floor is exactly why electron microscopes exist.
For grain size, phase distribution, porosity, cracks, coating thickness and the general health of a microstructure, the optical microscope is fast, cheap and enough. It is usually the FIRST look; only when you need to see sub-micron features — fine precipitates, dislocations — do you move up to the SEM or TEM.
At 500x an annealed brass shows clear equiaxed grains with straight twin bands crossing them; the analyst measures a mean grain diameter of about 30 micrometres — comfortably above the 0.2 micrometre resolution limit, so the optical microscope is the right tool.
Reflected light plus glass lenses: enough to read most microstructures, blind below about 0.2 micrometres.
More magnification is not more information. Past about 1000x with visible light you hit diffraction and get empty magnification — bigger blur, not finer detail. To see smaller, you must change the probe (electrons), not the lens.