light microscopy

Cells are far too small to see with the naked eye. A typical animal cell is about a hundredth of a millimetre across, smaller than the dot at the end of this sentence. Light microscopy is the oldest and still most common way around this: you shine ordinary light through a thin, often stained, sample and use curved glass lenses to magnify the image until the cell is big enough to see. It is the tool that first revealed cells exist at all, in the seventeenth century.

In a light microscope, light passes through the specimen and then through a series of lenses that bend the rays so the image looks larger. Because it uses visible light, what you can resolve is limited by the wavelength of that light: structures closer together than about 0.2 micrometres (200 nanometres) blur into one. That is enough to see whole cells, the nucleus, and large structures such as chloroplasts, but not the fine detail of most individual organelles or single molecules. Living cells can often be viewed directly, which is a major advantage.

Light microscopy matters because it is cheap, fast, and gentle enough to watch living cells go about their business — dividing, crawling, swallowing food. It cannot reveal the smallest structures, and untreated cells are nearly transparent and hard to see, which is why staining and tricks like phase-contrast were invented. A common misconception is that a microscope's usefulness is set by its magnification; in reality it is set by its resolution, since magnifying a blurry image just gives you a bigger blur.