Definition of Light Microscopy
A light microscope uses focused light and lenses to magnify a specimen, usually a cell. In this way, a light microscope is much like a telescope, except that instead of the object being very large and very far away, it is very small and very close to the lens.
Light microscopes send light through a path that first focuses the light into a tight beam and then passes that light through a sample, which creates an image. That image then passes through one or more lenses to magnify it until it reaches the user's eye or a camera. Because light needs to pass through the sample, it must be either very small or very thin. Most cells (bacterial or otherwise) are both small and transparent, and so light can easily pass through them.
Light microscopes can come in several forms. Simple light microscopes use a single lens to magnify an object and cannot reach high magnification. Compound light microscopes use two sets of lenses - an objective lens and an eyepiece - to produce images. Monocular microscopes have one eyepiece, while binocular microscopes have two eyepieces and reduce eye strain.
Uses of Light Microscopy
Microscopes are essential tools for scientists. They are used in microbiology, material science, mineralogy and medicine.
A combination of staining and light microscopy can allow scientists to identify different kinds of bacteria. Staining involves adding special dyes to a smear of cells. These stains are diagnostic for different kinds of cell membranes. Gram staining, for instance, uses crystal violet to stain Gram-positive bacteria and safranin to stain Gram-negative bacteria. These will show up in the light microscope as purple Gram-positive cells and pink Gram-negative cells.
Mineralogists also use light microscopy, typically with a special preparation of a sample called thin sections. As the name implies, thin sections are very thin slices of a rock. The sample needs to be thin enough for light to travel through from the light source to the user's eye. The thin section will allow the shape of different crystal grains to be seen. These shapes can tell the user what kinds of minerals are found in the sample.
Light microscopes are very much an 'easy to learn, hard to master' type of equipment. The microscope can be used with different techniques, like epifluorescence and phase contrast. Just about anyone can learn to use a microscope, but mastering techniques to produce the best quality image at high resolution can take years of training and practice.
Parts of a Light Microscope
Even though microscopes can look very different, they all use the same principles to magnify an object and have the same basic parts. The stage is the platform that holds the sample, usually a slide held in place with clips. This part of the microscope can move up and down to adjust focus. The condenser is a lens that focuses the main light source through the sample and into the objective lens. The user can adjust the light focusing with a diaphragm.
Objective lenses are the primary magnification lenses. Most modern compound microscopes will have several objective lenses mounted in a turret. This turret is sometimes called a nosepiece. The user can then go from low magnification to high magnification once they have found an area of interest in their sample. At very high magnifications, an objective lens may need to be paired with immersion oil, which helps minimize the bending of light to increase resolution.
The ocular lens is also called the eyepiece. This is the final lens and magnification step before the image reaches the user. Focus knobs generally include coarse and fine adjustment knobs. These move the stage up and down to bring an object into focus.
Light microscopes work by transmitting light through a very small or very thin object and magnifying the image that is created with a series of lenses. Light microscopes are widely used in a variety of applications, especially in the field of biology. The basic parts of a microscope include a stage to hold the sample, a light source and way to focus the light and a series of lenses. The best part about microscopes is that just about anyone can learn how to use one.
Light Microscopes Overview
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By working through this lesson on light microscopes, you could develop the ability to:
- Convey the purpose of a light microscope
- List some of the uses for a light microscope
- Outline the light microscope's parts
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