Main Difference – Light Microscope vs. Electron microscope
Light microscopes (optical microscopes) and electron microscopes are both used to look at very small objects. The main difference between light microscope and electron microscope is that light microscopes use beams of light to illuminate the object under examination while the electron microscope uses beams of electrons to illuminate the object.
What is a Light Microscope
Light microscopes illuminate their specimen using visible light and utilise lenses to produce a magnified image. Light microscopes come in two varieties: single-lens and compound. In single-lens microscopes, a single lens is used to magnify the object whereas a compound lens uses two lenses. Using an objective lens, a real, inverted and an enlarged image of the specimen is produced inside the microscope and then using a second lens called the eyepiece, the image formed by the objective lens is magnified still further.
Image of a moss leaf (Rhizomnium punctatum) under a light microscope (x400). Compare the size of these chloroplasts (green blobs) with a more detailed version (from a different specimen) taken from an electron microscope below.
What is an Electron Microscope
Electron microscopes illuminate their specimen using a beam of electrons. Magnetic fields are used to bend beams of electrons, in much the same way as optical lenses are used to bend beams of light in light microscopes. Two types of electron microscopes are widely in use: transmission electron microscope (TEM) and scanning electron microscope (SEM). In transmission electron microscopes, the electron beam passes through the specimen. An objective “lens” (which is really a magnet) is used to first produce an image and using a projection “lens” a magnified image can be produced on a fluorescent screen. In scanning electron microscopes, a beam of electrons is fired at the specimen, which causes secondary electrons to be released from the surface of the specimen. Using an anode, these surface electrons can be collected and the surface could be “mapped”.
Typically, the resolution of SEM images are not as high as those from TEM. However, since electrons are not required to pass through the sample in SEM, they can be used to investigate thicker specimen. Furthermore, images produced by SEM reveal more depth details of the surface.
TEM Image of a chloroplast (x12000)
An SEM image of pollen from different plants (x500). Note the depth detail.
Resolution
The resolution of an image describes the ability to distinguish between two different points in an image. An image with a higher resolution is sharper and more detailed. Since light waves undergo diffraction, the ability to distinguish between two points on an object is intimately related to the wavelength of light used to view the object. This is explained in the Rayleigh criterion. A wave also cannot reveal details with a spatial separation smaller than its wavelength. This means that the smaller the wavelength used to view an object, the sharper is the image.
Electron microscopes make use of the wave nature of electrons. The deBroglie wavelength (i.e. the wavelength associated with an electron) for electrons accelerated to typical voltages used in TEMs is about 0.01 nm whereas visible light has wavelengths between 400-700 nm. Clearly, then, electron beams are able to reveal much more detail than beams of visible light. In reality, the resolutions of TEMs tend to be of the order of 0.1 nm rather than 0.01 nm due to effects of the magnetic field, but the resolution is still about a 100 times better than the resolution of a light microscope. Resolutions of SEMs are a little lower, of the order of 10 nm.
Difference Between Light Microscope and Electron Microscope
Source of Illumination
Light microscope uses beams of visible light (wavelength 400-700 nm) to illuminate the specimen.
Electron microscope uses electron beams (wavelength ~0.01 nm) to illuminate the specimen.
Magnifying Technique
Light microscope uses optical lenses to bend rays of light and magnify images.
Electron microscope uses magnets to bend rays of electrons and magnify images.
Resolution
Light microscope has lower resolutions compared to electron microscopes, about 200 nm.
Electron microscope can have resolutions of the order 0.1 nm.
Magnification
Light microscopes could have magnifications of around ~×1000.
Electron microscopes can have magnifications of up to~×500000 (SEM).
Operation
Light microscope does not necessarily need a source of electricity to operate.
Electron microscope requires electricity to accelerate electrons. It also requires the samples to be placed in vacuums (otherwise electrons may scatter off air molecules), unlike light microscopes.
Price
Light microscope is much cheaper compared to electron microscopes.
Electron microscope is comparatively more expensive.
Size
Light microscope is small and could be used on a desktop.
Electron microscope is quite large, and could be as tall as a person.
References
Young, H. D., & Freedman, R. A. (2012). Sears and Zemansky’s university physics: with modern physics. Addison-Wesley.
Image Courtesy
“Punktiertes Wurzelsternmoos (Rhizomnium punctatum), Laminazellen, 400x vergrößert” by Kristian Peters — Fabelfroh (photographed by Kristian Peters) [CC BY-SA 3.0], via Wikimedia Commons
“A cross-sectional, simplified diagram of a transmission electron microscope.” by GrahamColm (Wikipedia, from GrahamColm) [Public Domain], via Wikimedia Commons
“Chloroplast 12000x” by Bela Hausmann (Own work) [CC BY-SA 2.0], via flickr
“Pollen from a variety of common plants…” by Dartmouth College Electron Microscope Facility (Source and public domain notice at Dartmouth College Electron Microscope Facility) [Public Domain], via Wikimedia Commons
