1, the bacterial body size is generally micron level, to E. coli, for example: length of about 1um, width of about 0.5um.

2, resolution is the zui small distance between two object points that can be clearly distinguished.

3, the resolution of the human eye is 0.1mm, that is, in sufficient light, the distance of 1 feet under the conditions of the general human eye can distinguish between two points zui small distance of 0.1mm.

4, in summary: to see the presence of bacteria, then the magnification should be at least: 0.1mm ÷ 0.5um=1000 ÷ 5=200 times. However, at this magnification, the bacteria (E. coli) is only a small point, and the magnification of the microscope must be increased in order to see its more subtle structural information.

The resolution of ordinary optical microscope is limited by the wavelength of visible light (390~770nm,) and generally does not exceed 1000 times, which is the magnification extreme of ordinary optical microscope. More than 1000 times the magnification can not provide more subtle structural information, belong to the range of invalid magnification.

6, the magnification of the electron microscope can reach 800,000 times, the basic principle of the same with the ordinary optical microscope, is through the high-pressure generated by a short wavelength of the electron wave.
Magnified 400 times (10x, 40x) can barely see the bacteria, but only the size of a pinpoint, like a small point. Generally, we still magnify to 1000 times (10x, 100x) to observe, at this time the appearance of the bacteria can still be seen quite clearly, after special staining flagella can also be seen clearly. If we magnify it to 1000x, we need to use an oil lens. The so-called oil lens is a drop of cedar oil in the middle of the objective lens and coverslip. The refractive index of cedar oil is higher than that of air, which makes it possible to use a larger magnification.

10x and 40x indicate that this lens magnifies 10x and 40x. The magnification of the eyepiece is multiplied by the magnification of the objective lens to give the total magnification of the microscope. Secondary school microscopes have interchangeable eyepieces and objective lenses, usually 5x and 10x, and the eyepieces of more modest microscopes are usually only 10x. Have looked at Escherichia coli (E.coli), Staphylococcus aureus, Bacillus subtilis, and Bacillus megaterium (to look at the bacilli).