Biomedical Engineering Reference
In-Depth Information
Figure 8.1
Biomedical imaging technologies.
8.2
Properties of Light
8.2.1 Electromagnetic Spectrum
Some properties of light can be explained with the assumption that light is made
of tiny particles called photons, and other properties of light can be explained with
the assumption that light is some kind of wave. Photons travel at the speed of light
(
V
light
) which is 3.00
10
8
m/s in vacuum (the commonly used notation is
c
—since
c
is used to denote concentration in this topic, a different notation is adapted).
Nothing can travel faster than light in vacuum, and
V
light
is the ultimate speed limit
in the universe. The visible light is a small part of spectrum of energy sources re-
ferred as the electromagnetic (EM) spectrum. The EM spectrum encompasses many
types of light or radiation (Figure 8.2). The EM spectrum includes the microwaves
commonly used in the kitchen to warm food, and the radio waves that are broad-
cast from radio stations.
EM waves are characterized by one of three properties: wavelength (
×
), which
is the distance between two adjacent crests of the wave; frequency (
f
), which speci-
fies how often the fields return to their original configuration of the wave and
measured in hertz; or the energy (
E
) of the individual photons in the wave (dis-
cussed in Section 8.2.2). The wavelength and frequency of the wave are related via
the speed of light:
λ
light
Vf
=
λ
(8.1)
EM waves are grouped into different categories based on their frequency (or
on their wavelength). Visible light, for example, ranges from violet to red. Violet
light has a wavelength of 400 nm, and a frequency of 7.5
×
10
14
Hz. Red light has a
wavelength of 700 nm, and a frequency of 4.3
10
14
Hz. Any EM wave with a fre-
quency (or wavelength) within those two extremes can be seen by the human eye.
×
