Optical microscopy is a technique employed to closely view a sample through the magnification of a lens with visible light. This is the traditional form of microscopy, which was first invented before the 18th century and is still in use today.
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An optical microscope, also sometimes known as a light microscope, uses one or a series of lenses to magnify images of small samples with visible light. The lenses are placed between the sample and the viewer’s eye to magnify the image so that it can be examined in greater detail.
Types of optical microscopes
There are many types of optical microscopes. They can vary from a very basic design to a high complexity that offers higher resolution and contrast. Some of the types of optical microscopes include the following:
- Simple microscope: a single lens to magnify the image of the sample, similar to a magnifying glass.
- Compound microscope: a series of lenses to magnify the sample image to a higher resolution, more commonly used in modern research.
- Digital microscope: may have simple or compound lenses, but uses a computer to visualize the image without the need for an eyepiece to view the sample.
- Stereo microscope: provides a stereoscopic image, which is useful for dissections.
- Comparison microscope: allows for the simultaneous view of two different samples, one in each eye.
- Inverted microscope: views the sample from underneath, which is useful to examine liquid cell cultures.
Other types of optical microscopes include petrographic, polarizing, phase contrast, epifluorescence, and confocal microscopes.
An optical microscope can generate a micrograph using standard light-sensitive cameras. Photographic film was traditionally used to capture the images.
Technological developments have now enabled digital images to be taken with CMOS and charge-couple device (CCD) cameras for optical microscopes. As a result, the image can be projected onto a computer screen in real time to examine a sample with these digital microscopes. This increases the convenience of use as eyepieces are no longer needed.
The power of magnification of a compound optical microscope depends on the ocular and the objective lenses. It is equal to the product of the powers of these lenses (e.g. for a 10x ocular lens and 100x objective lens used together, the final magnification is 1000x.)
Operation, applications and limitations
In order to use an optical microscope effectively, it is important to ensure that the microscope is set up correctly.
The objective lens should be brought close to the study sample to allow the light inside the tube of the microscope. This creates an enlarged, inverted image of the sample, which can be viewed through the eyepiece of the microscope.
Optical microscopy is commonly used in many research areas including microbiology, microelectronics, nanophysics, biotechnology and pharmaceutical research. It can also be useful to view biological samples for medical diagnoses, known as histopathology.
There are some instances when optical microscopy is not well suited to the task at hand due to limitations of the technique. For example, at very high magnifications airy disks may be visible, which are fuzzy discs surrounded by diffraction rings, which appear in place of point objects.
When the limitations of optical microscopy are significant, alternative types of microscopy may be more useful.
Alternative types of microscopy
There are several other types of microscopy that may be used as alternatives to optical microscopy. These include:
- Scanning electron microscopy
- Transmission electron microscopy
- Fluorescence microscopy
- Atomic force microscopy
- Scanning ion conductance microscopy
- Scanning tunneling microscopy
- Ultraviolet microscopy
- X-ray microscopy
Unlike optical microscopy, these types of microscopy do not use visible light to view the sample.
Reviewed by Afsaneh Khetrapal BSc (Hons)