Biomedical Engineering Reference
In-Depth Information
10
3
10
2
10
−
1
10
−
2
10
−
3
10
−
4
10
−
5
10
−
6
10
−
7
10
−
8
10
−
9
10
−
10
10
−
11
10
−
12
10
1
λ
(m)
RADIO WAVES
INFRARED
ULTRAVIOLET
HARD X-RAYS
VISIBLE
MICROWAVES
SOFT X-RAYS
GAMMA RAYS
f
(Hz)
10
6
10
7
10
8
10
9
10
10
10
11
10
12
10
13
10
14
10
15
10
16
10
17
10
18
10
19
10
20
Body is transparent
Most humans are
penetrated in this range
from local radio and TV
stations.
Almost transparent.
Small amounts of
absorption rotates
molecules and
contributes to heating
Stronger
absorption
vibrates
molecules.
Almost transparent as the
energy of photons is so high that
atoms cannot absorb and remain
intact and they ionize.
Strongly absorbed and
causes electrons to jump
to higher levels. However,
does not have enough
energy to ionize.
Strongly absorbed and
causes electrons to jump
to higher levels. Higher
and have enough
energy to ionize.
Figure 8.2
Electromagnetic spectrum.
8.2.2 Energy in an EM Wave
No medium is required for EM waves to travel through, unlike sound waves, which
cannot travel through vacuum. An EM wave carries energy although it carries no
mass. It also has momentum, and can exert pressure (known as radiation pressure).
Tails of comets point away from the Sun primarily due to the radiation pressure
exerted on the tail by sunlight (and other forms of radiation). Plank proposed that
the quantum of energy carried by an EM wave is proportional to the frequency,
f
.
(8.2)
Ehf
=
where
h
is the Plank's constant (6.626
10
−15
eV s). An electron
volt (eV) is defined as the energy that an electron (or a proton) gains when acceler-
ated through a potential difference of 1V. One eV is 1.602
×
10
-34
J.s or 4.136
×
×
10
−19
J.
EXAMPLE 8.1
Calculate the energy of a photon of blue light of wavelength 450 nm.
Solution:
From (8.1),
V
light
f
=
λ
From (8.2),
8
310[m s]
×
−
15
E
4.136
10
[eV.s]
=
×
×
9
450
10
−
[m]
×
E
=
2.76 eV of energy.
Blue light occurs in quantized packets of 2.76 eV, and the energy always occurs in
precisely the same sized energy packets.
