Light microscope definition
A light microscope, also called an optical microscope, is an instrument that forms an enlarged image of an object using visible light and refractive optics. Most laboratory instruments are compound microscopes: two lens systems, the objective and the eyepiece, work in sequence, so the total magnification is the product of the two. A 40× objective combined with a 10× eyepiece gives 400× on the eyepiece.
The name of each part and its role is covered in our guide to compound light microscope parts. This article focuses on the physics: what happens to the light between the lamp and your eye, and what ultimately limits the finest detail you can resolve.
How does a light microscope work: the light path
In a standard upright microscope used in transmitted light, light passes through the specimen from below. Following a single beam from the source to the observer shows the job of every component:
- Light source. A halogen lamp or, in most modern instruments, an LED produces the illumination.
- Collector lens and field diaphragm. The collector gathers light from the source; the field diaphragm in the base limits the illuminated area of the specimen.
- Condenser and aperture diaphragm. The condenser focuses light onto the specimen. Its aperture diaphragm controls the angle of the illuminating cone, which affects contrast, depth of field and resolution.
- Specimen. Light is absorbed, scattered, diffracted or phase-shifted by the sample. These changes carry the information that becomes the image.
- Objective. The most important lens in the system. It collects light leaving the specimen and forms the primary image.
- Tube lens. In modern infinity-corrected microscopes, light leaves the objective as parallel rays, and a tube lens focuses it into the intermediate image.
- Eyepiece or camera. The eyepiece magnifies the intermediate image for the eye, typically 10×, while a camera sensor can be placed directly in the intermediate image plane.
Finite and infinity-corrected optics
Older microscopes used a finite tube length, most often 160 mm: the objective itself formed the image at a fixed distance. Most current research microscopes are infinity-corrected. The parallel beam between objective and tube lens leaves room for filters, beam splitters and fluorescence cubes without degrading the image. The tube lens focal length differs between manufacturers, typically in the range of about 160 to 200 mm, which is why objectives from different systems are not always interchangeable.
Two sets of conjugate planes
A well-aligned microscope contains two interleaved sets of planes that are in focus with each other. The field planes include the field diaphragm, the specimen, the intermediate image and the retina or sensor. The aperture planes include the lamp, the condenser aperture diaphragm, the back focal plane of the objective and the exit pupil above the eyepiece. Anything sitting in a field plane, such as dust on a slide, appears sharp in the image; anything in an aperture plane is spread out evenly and invisible. This principle is the basis of Köhler illumination.
Köhler illumination: even light without an image of the lamp
Köhler illumination, introduced by August Köhler in 1893, is the standard way to set up a transmitted light microscope. The lamp filament or LED is imaged into the condenser aperture plane rather than into the specimen, so the field of view is evenly lit and free of a lamp image, and the illumination cone can be controlled independently of the illuminated area.
Setting it up takes a minute and should be repeated when you change objectives:
- Focus on a specimen with a 10× objective.
- Close the field diaphragm until you see its polygonal edge in the field of view.
- Raise or lower the condenser until the edge of the diaphragm is sharp.
- Centre the polygon with the condenser centring screws.
- Open the field diaphragm until it just disappears beyond the edge of the field.
- Adjust the aperture diaphragm. A common starting point is to open it to about 70 to 80 percent of the objective aperture, which you can check by removing an eyepiece and looking down the tube.
Do not use the aperture diaphragm to control brightness. Closing it too far adds contrast and depth of field but reduces resolution and creates diffraction fringes. Brightness should be adjusted with the lamp intensity or neutral density filters.
Objectives and numerical aperture
The engraving on an objective, for example 40×/0.65, gives its magnification and its numerical aperture (NA). Numerical aperture describes how wide a cone of light the objective can accept:
NA = n × sin θ, where n is the refractive index of the medium between the specimen and the front lens, and θ is half of the acceptance angle.
In air, n is 1.0, so a dry objective cannot exceed an NA of 1.0 and in practice reaches about 0.95. Immersion oil with a refractive index of about 1.515, close to that of glass, prevents light from being bent away at the coverslip surface, allowing NA values of about 1.4 and slightly above. Water and glycerol immersion objectives sit in between and are preferred for living samples, where the refractive index of the specimen is closer to water.
| Typical objective | Numerical aperture | Lateral resolution (0.61 λ / NA, λ = 550 nm) | Useful total magnification (500–1000 × NA) |
|---|---|---|---|
| 4× dry | 0.10 | about 3.4 µm | 50–100× |
| 10× dry | 0.25 | about 1.3 µm | 125–250× |
| 20× dry | 0.40 | about 0.84 µm | 200–400× |
| 40× dry | 0.65 | about 0.52 µm | 325–650× |
| 100× oil (achromat) | 1.25 | about 0.27 µm | 625–1250× |
| 100× oil (plan apochromat) | 1.40 | about 0.24 µm | 700–1400× |
The table shows why a higher NA matters more than a larger magnification number. A 100× objective with an NA of 1.40 resolves noticeably finer detail than one with 1.25, even though both magnify the same.
Correction classes
Objectives are also classified by how well they correct optical errors. Achromats correct chromatic aberration for two wavelengths and are common in teaching microscopes. Fluorites (semi-apochromats) offer better colour correction and higher NA. Apochromats correct for three or more wavelengths and reach the highest NA values. The prefix plan means the image is flat to the edge of the field, which matters for photography and image analysis.
Light microscope resolution and the Abbe diffraction limit
Resolution is the smallest distance at which two points can still be seen as separate. Because light behaves as a wave, a point in the specimen is never imaged as a point but as a small diffraction pattern, the Airy disk. When two disks overlap too much, they merge into one.
Ernst Abbe described this limit in 1873. In its common form, the smallest resolvable distance is:
d = λ / (2 NA)
For green light at 550 nm and an oil objective with an NA of 1.4, d is about 196 nm. The related Rayleigh criterion, d = 0.61 λ / NA, gives about 240 nm for the same conditions. Either way, the practical lateral resolution of a light microscope is about 200 to 250 nm. Resolution along the optical axis is worse, typically several hundred nanometres even with the best objectives.
Magnification versus resolution
Magnification makes the image larger; resolution decides whether there is any new detail to see. Beyond about 1000 times the NA, extra magnification is empty magnification: the image grows, but blurs, and no new structures appear. That is why most light microscopes stop at about 1000× to 1500× total magnification.
How to get closer to the limit
- use the objective with the highest NA suitable for your sample;
- match the condenser NA to the objective and set up Köhler illumination;
- use oil on both the objective and, for the highest NA, the condenser;
- use coverslips of the thickness the objective is designed for, usually 0.17 mm;
- use shorter wavelengths: blue light at about 450 nm improves resolution compared with red light.
Key takeaway: a light microscope cannot separate structures much closer than about 200 nm, no matter how high the magnification. To improve the image, invest in numerical aperture and correct illumination first, and only then in magnification.
Techniques that break this limit, such as STED, SIM and single-molecule localisation, are explained in our overview of super resolution microscopy.
Contrast methods in light microscopy
Resolution is useless without contrast. Many biological specimens, such as unstained cells, are almost transparent and barely change the intensity of light. Light microscopes use several contrast methods to make them visible:
| Method | What it uses | Best for | Limitation |
|---|---|---|---|
| Brightfield | absorption of light by the specimen | stained tissue sections, blood smears | poor contrast for unstained cells |
| Darkfield | only scattered light reaches the objective | bacteria, small particles, edges | dust and debris also scatter brightly |
| Phase contrast | phase shifts converted into brightness differences | living, unstained cells in culture | halo artefacts around edges |
| DIC | gradients in optical path length | relief-like images of cells and embryos | not suitable with plastic dishes |
| Fluorescence | light emitted by fluorophores | specific molecules and structures | photobleaching, requires labelling |
Phase contrast deserves special mention because it made routine observation of living cells possible. Its principle is described in detail in our article on phase contrast microscopy.
Common mistakes when using a light microscope
- using the aperture diaphragm as a brightness control instead of the lamp intensity;
- leaving the condenser in the lowest position, which wastes NA and resolution;
- switching to a dry objective after oil without cleaning the slide, which smears the front lens;
- using a slide upside down, so a high-NA objective cannot focus through the thick glass;
- chasing magnification with high eyepieces while the objective NA is the real limit.
Conclusion
A light microscope works by collecting light that has interacted with a specimen and forming a magnified image through the objective, tube lens and eyepiece. Köhler illumination gives even, controllable light, and the numerical aperture of the objective decides how much detail reaches the image. Because of diffraction, lateral resolution stops at about 200 nm, so careful alignment and the right objective matter far more than the largest magnification number.