| Re: SEM vs Optical Microscopy
This is a great question, and the answer has to do with the properties of light. I'm not sure of your level, so I'll try to explain it in some detail.
Not only does an SEM have a greater depth of field, but it also has a greater resolution than a traditional optical microscope. This greater resolving power is usually more importaint than the depth of field when viewing an object (although I'll get into that later as well). Optical microscopes use normal light in combination with lenses to resolve (or distinguish between two seperate points) the object on your slide. The minimum resolving distance of a microscope is limited by the wavelength of the light being bounced off the object, so the theoretical resolving distance of an optical microscope is in the range of 10E-9 meters. The SEM uses electrons instead of visible light which have a wavelength on the order of 10E-12 meters (all objects have something often called the Compton Wavelength, even non-photonic particles such as electrons so that is where that number comes from).
As you can see, this helps see smaller objects, but still doesn't completely answer the depth of field question. The human eye cannot distinguish very small degrees of unsharpness, some subjects that are in front of and behind a sharply focused subjects can still appear sharp. The zone of acceptable sharpness is referred to as the depth of field. Thus, increasing the depth of field increases the sharpness of an image. So, increasing the resolution often 'tricks' the eye into thinking the depth of field has been increased as well. But that is not the whole story, as the lenses present in the microscopes are what control this effect. The following may be a little confusing if you haven't had some optics backround, but I'll try to explain as best I can.
Lenses do not focus light beams parallel to the optical axis into an infinitely small 'point' (as would be needed for a perfectly sharp image), rather they tend to smear out the incomming light into what is called the 'circle of confusion'. Plus, so called 'thick lenses' don't focus all light comming from different hights on the optical axis at the same points. AND All of this is affected by the wavelength of the light being used, because different wavelengths of light don't focus at the same place either. Since white light is made of many different wavelengths, it will smear out the image as well! So, due to the optical properties of the glass used in the microscopes, there is some limit as to the minimization of the depth of field.
So the SEM has a better depth of field because:
The electrons used are focused through the use of an EM field, which does not have some of the same problems as a glass lens;
The electrons have a smaller difference in wavelengths (delta lambda), which is somewhere less than 10E-12 meters as compared to less than 10E-9 meters;
A greater resolving power can trick the eye as to a greater depth of field.
Hope this somewhat late explaination still helped.
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