30 PART I Basics of Microscopy
to the optical properties of light. To study smaller structures, you probably need
other, more powerful methods of image creation: laser beams, electron beams
or X-rays.
Confocal Microscopes Deliver
Optical Sections
Unlike stereo, zoom and compound microscopes, confocal microscopes use
laser
light as a light source. The laser scans the sample using different patterns,
and the image is assembled with a computer. The laser can penetrate the sample
deeper than light from a bulb. The result is a three-dimensional image of controlled
depth of field. You can examine interior structures of cells, model organisms
and tissue by stacking several images from different optical planes.
Electron Microscopes
Put simply, electron microscopes use electrons rather than visible light to create
an image. To use electrons for microscopy, you need to focus the electron beam
and you need a vacuum. The beam is sent onto the sample, bounces off and creates
a three-dimensional image of the surface. The wavelength of the electrons is
much smaller than the wavelength of light from a bulb or laser, resulting in higher
resolution. When you want to use an electron microscope, your sample must
be electrically conductive, so the electrons bounce off. That’s why samples are often
coated in a thin layer of gold or other metal. As you can imagine, this process
doesn’t fit very well with living biological samples.
X-ray Microscopes
X-ray microscopes use electromagnetic radiation (X-rays) to produce images
of tiny objects. Unlike an electron microscope, you can use them to generate an
image of living cells. X-rays penetrate easier than visible light into the depth of a
sample. X-ray microscopes can mirror the inside of samples non-destructively and
create a three-dimensional image of the inner structure of an object with high-
er spatial resolution.