The microscope is a kind of precision optical instrument with a history of more than 300 years.  Since the advent of microscopes, people have been able to see many tiny organisms and the cell, the basic building block of living things, that were never seen before.  Not only there are optical microscopes that can magnify more than 1000 times, but also there are electron microscopes that can magnify hundreds of thousands of times so that we have a further understanding of the law of life activities of organisms.  Most of the experiments stipulated in the general middle school biology teaching syllabus are completed by microscope. Therefore, the performance of the microscope is the key to doing a good job in the observation experiment.  

An optical microscope (OM) is an optical instrument that uses the principle of optics to enlarge images of tiny objects that can not be resolved by human eyes so that people can extract information about fine structures.  

The light microscope has been a standard tool in life science and materials science for more than half a century.  In order to use this tool cost-effectively, it is especially helpful to understand the basics of optics, especially for each microscope component.  Let’s take a look at the light microscope.  

1. Lenses and mirrors  

Optical instruments such as microscopes, telescopes, and binoculars use optical elements to produce images of objects.  The two most common elements used to image objects are convex and concave lenses.  

Lenses are more common in light microscopes.  Therefore, in the following exploration of basic microscope functions, we will focus on the lens.  Concave lenses are used for imaging reflective telescopes.  Concave lenses are also commonly used for lighting, such as headlights in automotive applications.  

2. Image through a lens  

Before we can discuss how the lens works, we must understand the key terms and definitions of the lens.  Everyone who has ever (misused) a magnifying glass as burning glass has discovered that “hot spots” are created when the lens is pointed at the sun.  This is the point of contact.  The distance from the center of the lens to that focus is called the focal length.  

When the experiment is reproduced with different types of convex lenses, it is found that the focal length depends mainly on the curvature of the lens.  In practice, a smaller radius of curvature results in a shorter focal length.  Will discover another fact: large-diameter lenses are more “effective” than small-diameter lenses.  Based on this conclusion, we have defined the two most important reference data for a lens: focal length and aperture (diameter).  

To simplify the processing of the lens diameter, it is usually expressed relative to the focal length.  In the field of microscopy, this parameter is called aperture (also known as numerical aperture NA).  The numerical aperture is defined as NA = n sinα, where n is the refractive index of the medium filling the space between the object and the lens, and α is the half Angle of the maximum light cone that can enter the lens.  The photographer defines the aperture of the objective lens by its f value.  This is defined as the ratio of the focal length to the lens diameter (N = f/D).  In contrast to the NA value, a smaller f value indicates a larger aperture.  

3. Optical principles of microscopes  

The microscope is the use of the convex lens magnification imaging principle, the human eye can not distinguish small objects to the size the human eye can distinguish, which is mainly to increase the Angle of small objects near the eye (objects with a large Angle of view on the retina of large imaging), with the Angle magnification M to represent their magnification power.  Because the same object to the eye Angle and the object from the eye distance, so the general provisions of the distance from the eye is 25 cm (clear distance) at the magnification of the instrument.  Microscopes usually view objects from a very small Angle of view, so the ratio of the Angle of view can be replaced by the ratio of its tangent.  

4. How to take microscope images?  

Since the normal output of an optical microscope is parallel rays, a realistic image must first be generated.  Fortunately, standard compact digital cameras include a lens (called an objective lens) just like our eyes.  The lens can handle objects at great distances.  The photographer called the distance “infinite.”  In other words: light from these objects reaches us in parallel.  

With the compact camera behind the microscope eyepiece, we can take pictures through the microscope.  To avoid frustration: this combination yields very limited results.  That’s because the optical design of compact cameras doesn’t take microscopes into account.  Several dimensions (diameter, distance) limit practical use.  Therefore, digital cameras specially designed for the special conditions of optical microscopes can be used for different applications.