light microscope
Meaning of light microscope: The instrument behind nearly every pathology slide, and one whose limit is set by the wavelength of light itself.
Definition of light microscope
A light microscope magnifies small objects using visible light and a series of glass lenses. Light passes through or reflects off a thin specimen, and the lenses spread that image until detail invisible to the naked eye can be resolved. It remains the standard instrument for examining stained tissue sections, blood films, urine deposits and cultured cells.
How does it work?
A lamp beneath the stage sends light up through the specimen. A condenser gathers that light into a cone so the sample is evenly lit. The objective lens sitting just above the slide does most of the magnifying, and the eyepiece enlarges the image the objective has formed. Turning a turret swaps one objective for another to change magnification. At the highest power a drop of oil is placed between lens and slide, because oil bends light similarly to glass and stops rays being lost at the air gap.
What limits how much it can show?
Magnification and resolution are different things. Enlarging an image further does not add detail once the limit is reached, it only makes a blur bigger. That limit comes from the wavelength of visible light: two points closer together than roughly half a wavelength merge into one. In practice this means bacteria are visible, while individual viruses, ribosomes and the layers of a cell membrane are not.
How does it compare with an electron microscope?
| Light microscope | Electron microscope | |
|---|---|---|
| What forms the image | Visible light | A beam of electrons |
| Smallest detail | Around the wavelength of light | Far finer, down to structures inside a cell |
| Specimen | Can be alive and in color | Must be dried, fixed and viewed in vacuum |
| Image | Colored by stains | Shades of gray |
An electron microscope resolves far more because electrons behave as waves of a much shorter wavelength. The cost is that nothing living survives the process.
Are there variations on the standard design?
Several, each solving a different problem. A dark-field microscope lights the specimen from the side so only scattered light enters the lens, making unstained organisms glow against black. Phase contrast converts small differences in thickness into differences in brightness, which allows living cells to be watched without staining. Fluorescence versions excite a dye attached to a chosen molecule and image only the light it gives back. Polarized light reveals crystals, which is how gout and pseudogout are told apart in joint fluid.
Also known as
Used in a sentence
Sections were examined by light microscope after staining with hematoxylin and eosin.
Good to know
Beyond a certain point, extra magnification adds no detail, because resolution is capped by the wavelength of the light being used.
Word origin
Where “light microscope” comes from
Microscope combines the Greek mikros, meaning small, with skopein, meaning to look at.