Page 37 - Fundamentals of Light Microscopy and Electronic Imaging

20 LIGHT AND COLOR

relative to some fixed reference is called the azimuthal angle of orientation. Com-
monly, the sine waves seen in drawings refer to the average amplitude and phase of
a beam of light (a light train consisting of a stream of photons), not to the proper-
ties of a single electromagnetic wave.
• As vectors, where the vector length represents the amplitude, and the vector angle
represents the advance or retardation of the wave relative to an imaginary reference.
The vector angle is defined with respect to a perpendicular drawn through the focus
of a circle, where 360° of rotation corresponds to one wavelength (2 radians).
• As rays or beams, where the linear path of a ray (a light train or stream of photons)
in a homogeneous medium is shown as a straight line. This representation is com-
monly used in geometrical optics and ray tracing to show the pathways of rays pass-
ing through lenses of an imaging system.

THE QUALITY OF LIGHT

As an analytic probe used in light microscopy, we also describe the kind or quality of
light according to the degree of uniformity of rays comprising an illuminating beam
(Fig. 2-5). The kinds of light most frequently referred to in this text include:

• Monochromatic—waves having the same wavelength or vibrational frequency (the
same color).
• Polarized—waves whose E vectors vibrate in planes that are parallel to one another.
The E vectors of rays of sunlight reflected off a sheet of glass are plane parallel and
are said to be linearly polarized.
• Coherent—waves of a given wavelength that maintain the same phase relationship
while traveling through space and time (laser light is coherent, monochromatic, and
polarized).
• Collimated—waves having coaxial paths of propagation through space—that is,
without convergence or divergence, but not necessarily having the same wave-
length, phase, or state of polarization. The surface wavefront at any point along a
cross-section of a beam of collimated light is planar and perpendicular to the axis
of propagation.

Light interacts with matter in a variety of ways. Light incident on an object might
be absorbed, transmitted, reflected, or diffracted, and such objects are said to be opaque,
transparent, reflective, or scattering. Light may be absorbed and then re-emitted as vis-
ible light or as heat, or it may be transformed into some other kind of energy such as
chemical energy. Objects or molecules that absorb light transiently and quickly re-emit
it as longer wavelength light are described as being phosphorescent or fluorescent
depending on the time required for re-emission. Absorbed light energy might also be re-
radiated slowly at long infrared wavelengths and may be perceived as heat. Light
absorbed by cells may be damaging if the energy is sufficient to break covalent bonds
within molecules or drive adverse chemical reactions including those that form cyto-
toxic free radicals. Finally, a beam of light may be bent or deviated while passing
through a transparent object such as a glass lens having a different refractive index
(refraction), or may be bent uniformly around the edges of large opaque objects (dif-