Page 34 - Fundamentals of Light Microscopy and Electronic Imaging

LIGHT AS A PROBE OF MATTER 17

y
x
z

B

x
E

Figure 2-2
Light as an electromagnetic wave. The wave exhibits electric (E) and magnetic (B) fields
whose amplitudes oscillate as a sine function over dimensions of space or time. The
amplitudes of the electric and magnetic components at a particular instant or location are
described as vectors that vibrate in two planes perpendicular to each other and
perpendicular to the direction of propagation. However, at any given time or distance the E
and B vectors are equal in amplitude and phase. For convenience it is common to show only
the electric field vector (E vector) of a wave in graphs and diagrams and not specify it as
such.

related through the following equations, which can be used to determine the amount of
energy associated with a photon of a specific wavelength:

c
,

E h
,

and combining,

E hc/ ,

10
where c is the speed of light (3 10 cm/s),
is the frequency (cycles/s), is the wave-
length (cm), E is energy (ergs), and h is Plank’s constant (6.62 10 27 erg-seconds).
The first equation defines the velocity of light as the product of its frequency and wave-
length. We will encounter conditions where velocity and wavelength vary, such as when
photons enter a glass lens. The second equation relates frequency and energy, which
becomes important when we must choose a wavelength for examining live cells. The
third equation relates the energy of a photon to its wavelength. Since E 1/ , 400 nm
blue wavelengths are twice as energetic as 800 nm infrared wavelengths.