Compound: The Old Standby
The most-used microscope, particularly in schools, is the compound microscope, which uses visible light to illuminate a sample. Small organisms or cross-sections of organisms are placed on glass slides; you secure the slide on the microscope’s stage and choose one of the objective lenses – which have magnification ranges from 4 times to 100 times -- to select the magnification range. A light source near the base of the microscope shines light through the sample, which you view through the ocular lens, usually magnifying the sample an additional 10 times power. Typically, these microscopes have focus knobs to bring the object into focus. You use compound microscopes to look at a single cell layer or groups of cells or tissues.
What to Use for Larger Objects
The stereoscopic microscope is useful when looking at objects that won’t fit on a slide for a compound microscope. Stereoscopic, also called dissecting, microscopes have a low magnification range, around 2 to 30 times, but their field of view is much larger. These microscopes have an objective lens that allows magnification to be controlled by a knob, and the stage lies farther from the lens, which allows you to view larger objects. Rather than light passing through a glass slide, with stereoscopic microscopes, the light reflects on the surface of the object you view. With these microscopes, you can look at a large sample, such as a mineral or crystal, or living things like small animals or plants.
Looking at Objects in 3D
Electron microscopes let you see objects at higher magnification and greater resolution than other microscopes. They use a stream of electrons instead of light as an energy source. Electron microscopes have an objective lens and a condenser lens; objects are viewed through an eyepiece or projected onto a screen. The scanning electron microscope, or SEM, lets you look at the surface of objects at high resolution and with depth of field, which allows objects to look three-dimensional. You cannot look at living things with the SEM because samples must be dry, but you can use a CryoSEM to look at samples that contain liquid. SEM microscopes use a beam of electrons to scan the surface of a sample, but the transmission electron microscope, or TEM, lets the electron beam pass through the specimen. Transmission electron microscopes have the highest magnification and resolution of the electron microscopes. You can look at internal structures as opposed to just the SEM’s look at the surface of samples, and you can see extremely tiny structures like viruses with the TEM. Electron tomography allows you to build a 3D model from TEM data.
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