At very high magnifications, the small amount of air between a specimen and the lens can scatter light rays before they are focused, compromising resolution due to the large difference in refractive indices between air and glass. To prevent this, a drop of immersion oil is applied to fill the space between the specimen and a specialized oil immersion lens. Because the oil has a refractive index very similar to that of glass, it increases the maximum angle at which light can enter the lens, thereby capturing more light and improving the resolution of the image.

Oil Immersion Lenses

The total magnification achieved by a compound microscope is determined by the product of the ocular lens magnification and the objective lens magnification. For instance, if an ocular lens magnifies by $${}10	imes$$ and the selected objective lens magnifies by $${}40	imes$$, the total magnification of the resulting image would be $${}400	imes$$.

Total Magnification

A compound light microscope consists of multiple interacting structural and optical components. The base houses an illuminator and a rheostat for lighting. An arm supports a stage equipped with x-y mechanical knobs to secure and position the specimen. Below the stage, a condenser and diaphragm regulate the incoming light. Above the stage, objective lenses on a revolving nosepiece and ocular lenses within the eyepieces provide magnification. Finally, coarse and fine focusing knobs on the arm adjust the stage's height to bring the specimen into sharp focus.

Parts of a Microscope

Brightfield microscopes can produce maximally sharp images at magnifications up to approximately $${}1000	imes$$. Increasing magnification beyond this point results in a larger image but does not provide any additional resolution. This specific magnification range allows observers to clearly see objects as small as bacteria, which become visible around $${}400	imes$$, but it is not powerful enough to visualize smaller structures like viruses.

Brightfield Microscope Magnification Limit

A darkfield microscope is a brightfield microscope modified with a small opaque light stop placed on the condenser. This component blocks most direct light from the illuminator, ensuring that the only light reaching the objective lens is light scattered, refracted, or reflected by structures in the specimen. Consequently, it creates high-contrast, high-resolution images showing bright objects on a dark background without the use of stains, making it exceptionally useful for viewing live, delicate specimens that might be killed by staining procedures.

Darkfield Microscopes

The brightfield microscope, considered the most commonly used type of microscope, is a compound microscope that employs two or more lenses to produce a dark image of a specimen against a bright background. These microscopes are illuminated by a light source focused by a condenser lens, and they can be monocular (possessing a single eyepiece) or binocular (possessing two eyepieces), with each eyepiece containing an ocular lens.

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-Brightfield Microscopes  
-Darkfield Microscopes  
-Phase Contrast Microscopes  
-Differential Interference Contrast Microscopes (DIC)    
-Fluorescence Microscopes  
-Confocal Microscopes  
-Two-Photon Microscopes  
-Electron Microscopy  
-Scanning Probe Microscopy

Types of Microscopes 

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Brightfield Microscopes

Phase-contrast microscopes are the oldest and simplest type of microscopes that create high-contrast, high-resolution images without staining by altering the wavelengths of light rays passing through the specimen. They utilize refraction and interference, causing structures that refract light to appear dark against a bright background of unrefracted light, while structures with different refractive indices appear in varying levels of darkness.

Phase Contrast Microscopes

A confocal microscope is a type of light microscope that uses a laser to successively scan multiple depths, known as z-planes, producing numerous two-dimensional, high-resolution images that a computer constructs into a three-dimensional model. Like fluorescence microscopes, they generally use fluorescent stains to increase contrast and resolution. By utilizing a confocal pinhole (a narrow aperture) to reject out-of-focus background light, confocal microscopes capture thin ($$<1$$ μm), unblurred optical sections. This capability makes them highly useful for examining thick, living specimens such as biofilms, which cannot be observed well using traditional light microscopy because slicing them to create thinner specimens might kill or disturb the microbial community. However, they still lack the resolution required for viewing extremely thick intact tissue samples.

Confocal Microscopes

Differential interference contrast (DIC) microscopes, also known as Nomarski optics, are instruments that utilize interference patterns to enhance the contrast between various features of a specimen. They operate by creating two beams of light with different directions of wave movement, known as polarization. After these beams pass through either the specimen or a specimen-free area, they are recombined. The specimen's characteristics alter the interference patterns formed during this recombination, which produces high-contrast images of living organisms with a distinct three-dimensional appearance. Because they do not require stains, DIC microscopes are particularly valuable for distinguishing internal structures within live, unstained specimens.

Differential Interference Contrast Microscopes (DIC)

A fluorescence microscope is a light microscope that transmits a short-wavelength excitation light, such as ultraviolet or blue light, toward a specimen. Fluorescent chromophores (fluorochromes) in the specimen absorb this energy and emit longer-wavelength visible light. The excitation light is then filtered out—partly to protect the eyes from harmful ultraviolet rays—allowing only the emitted visible light to pass through the ocular lens, producing brightly colored images of the specimen against a dark background.

Fluorescence Microscopes

A two-photon microscope is an advanced instrument that uses a scanning technique, fluorochromes, and long-wavelength light (such as infrared) to visualize specimens. Because the long-wavelength excitation light has low energy, two photons must strike a location simultaneously to excite the fluorochrome. This low-energy light is less damaging to living cells, and the long wavelengths penetrate deep into thick specimens, resolving the resolution limitations of confocal microscopes. They are exceptionally useful for examining living cells within intact tissues like brain slices, embryos, and whole animals, though their high costs (typically \$300,000 to \$500,000) restrict their use mostly to advanced clinical and research laboratories.

Two-Photon Microscopes

Electron microscopy utilizes short-wavelength electron beams, focused with magnets, to significantly increase magnification and resolution compared to visible light. While light microscopes are limited by visible light wavelengths, electrons behave as waves with much smaller wavelengths, allowing an electron microscope to produce sharp images with magnifications ranging from $${}20	imes$$ to $${}100,000	imes$$ or more. This resolving power enables the visualization of subcellular and molecular structures. However, electron microscopy cannot be used on living material due to the harsh methods required to prepare the specimens prior to viewing.

Electron Microscopy

Scanning probe microscopy is a technique that does not use light or electrons to observe a specimen, but rather utilizes very sharp probes that are passed over the specimen's surface and interact with it directly. This produces information that can be assembled into images with magnifications ranging from $${}100	imes$$ to $${}100,000,000	imes$$ or more, allowing for the observation of individual atoms. Because of this high resolution and complexity, scanning probe microscopes have primarily been used for research rather than for clinical diagnostics. The two main types of scanning probe microscopes are the scanning tunneling microscope (STM) and the atomic force microscope (AFM).

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