Biophotonics_Concepts_to_Applications

glass, or crystalline materials is known asspecular reflection. In specular reflection
the surface imperfections are smaller than the wavelength of the incident light, and
the incident light is reflected at a definite angle following Snell’s law. However, two
characteristics of the reflected light need to be explained. First, except at normal
incidence, when light interacts with a medium the intensities of the refracted and
reflected beams do not add up to the intensity of the incident beam. The reason for
this is that the intensity is defined as the power per unit area. Because the
cross-sectional area of the refracted beam is different from the incident and reflected
beams, the intensities of the refracted and reflected beams are not complementary.
Only the total energy in the beams is conserved, if there are no optical power losses
in the reflection and refraction processes.
A second factor related to reflection, as can be seen from Eqs. (2.25) and (2.26),
is that the perpendicular reflectance and the parallel reflectance have different values
and depend on the angle of incidence. Examples of the perpendicular and parallel
reflectances for light traveling in air being reflected at an air-water interface are
shown in Fig.6.3as a function of the incident angle. The reflectance for circularly
polarized or unpolarized light is also shown in Fig.6.3. Here nair= 1.00 and
nwater= 1.33. Although typically tissue surfaces are not smooth, the curves in Fig.
6.3can be used to describe specular reflection from wet tissue surfaces. The per-
pendicular reflectance shown in Fig.6.3increases monotonically as the angle of
incidence increases. On the other hand, the parallel reflectancefirst decreases
gradually until it reaches a value of zero at a specific angle and then it increases
again. The angle for zero parallel reflectance is called theBrewster angle. For an
air-water interface the parallel reflectance reaches a value of zero at a 53° angle of
incidence. Note that Eqs. (2.25)and(2.26) show that the reflectances depend on the
refractive indices of the adjoining materials. Thus the value of the Brewster angle
depends on the value of the refractive index and changes for interfaces between
other materials with different refractive indices. For increasingly larger values of the
angle of incidence beyond the Brewster angle, the parallel reflectance increases

Vitreous humor (n = 1.336)

Cornea

(n = 1.376)

Retina

Incoming light

Lens

(n = 1.406)

Aqueous humor

(n = 1.336)

Normal to

cornea surface

θ 1

Fig. 6.2 The major
components of the human eye
and their refractive indices

6.1 Reflection and Refraction Applications 151