Biomedical Engineering Reference
In-Depth Information
Fig. 8.2
Diffraction by a
grating consisting of a
periodically corrugated
surface. The incident angle is
i
, the diffracted angle is
m
,
and the grating period is
d
d.
sin
i
sin
m
/
. When this optical path difference is an integral number of
wavelengths, i.e., satisfying the following grating equation, diffraction occurs due
to the constructive interference of the scattered light beams, namely,
C
d.
sin
i
C
sin
m
/ D m:
(8.3)
Here,
m
is an integer which can be positive or negative, representing the order
of diffraction. The zero-order diffraction (
m D 0
) corresponds to the specular
reflection. In general, the intensity of nonzero order (
m
ยค
0
) diffraction is much
smaller than that of the specular reflection.
For a fixed wavelength, diffraction occurs at certain diffracted angles that satisfy
the grating equation. For a fixed incident angle, different wavelengths will be
diffracted at differently diffracted angles with each order forming a spectrum,
leading to structural coloration. For 2D diffraction gratings, structural coloration
is also expected due to diffraction, although the grating equation is different from
that for 1D gratings.
8.3.3
Scattering
Light will be scattered when encountering irregularities (scatters), e.g., particles.
Light scattering depends strongly on light wavelength and scatter geometry and
arrangement. In general, light scattering can be classified into incoherent or coherent
scattering.
Rayleigh scattering [
24
] and Tyndall scattering [
25
] are typical incoherent
scattering by single particles. Rayleigh scattering is the light scattering by molecules
or particles whose sizes are much smaller than light wavelengths. In Rayleigh
scattering, the intensity of scattered light varies inversely as the fourth power of
wavelength and shows a strong angle dependence as
.1
cos
2
/
,where
is
the scattering angle. The blue color of sky is caused by Rayleigh scattering from
molecules since short-wavelength light such as violet and blue is scattered much
more than long-wavelength light such as yellow and red.
For particle sizes comparable to light wavelengths, it belongs to Tyndall scat-
tering which is much more intense than Rayleigh scattering. Unlike Rayleigh
C
