20 PART I Basics of Microscopy
Table 1.2 gives a small impression of the size relations we’re talking about.
Sample Size
Virus 30–50 nm
Bacteria 1 m
Spider web silk thread 1–3 m
Human red blood cell 5–10 m
Human hair (diameter) 20–40 m
Table 1.2: How big is one micrometer?
Let’s Have a Look Through
a Magnifying Glass
To visualize objects that are too small for the human eye, you need to enlarge
them somehow. Let’s try a magnifying glass. A magnifying glass consists of a
curved,
so-called convex piece of glass which works like a lens. When light travels
from the medium air to the medium glass, the light rays are bent. This effect is
called
refraction. Lenses can bend multiple light rays to a specific focal point,
creating a clear image of an object (Figure 1.2). Often, the object appears to be
larger.
The magnifying
glass needs the object to be positioned within its front
focal plane to produce an enlarged image with a defined magnification. The more
convex a lens is, the higher the magnification. Usually, simple lenses cannot
exceed a 15-fold magnification.
The position of the magnifying glass depends on its type. A magnifying
glass used by a dentist is close to the eye and delivers a higher
magnification than a reading glass that is held close to the object.
Build your own little microscope
By the way: assuming, that you’re not a specialist in precision optics, grinding
your own lenses, you can nevertheless build and try out a simple magnifying
glass. A drop of water has a similar refraction index to glass and you can see
letters enlarged through it, for example.