Biophotonics_Concepts_to_Applications

Example 1.1If a biological tissue sample has a refractive index n = 1.36,

what is the speed of light in this medium?

Solution: From Eq. (1.1) the speed of light in this tissue sample is

s¼

c

n

¼

3  108 m=s

1 : 36

¼ 2 : 21  108 m=s¼ 22 :1cm=ns

Figure1.5 shows the spectral range covered by optical wavelengths. The
ultraviolet (UV) region ranges from 10 to 400 nm, the visible spectrum runs from
the 400 nm violet to the 700 nm red wavelengths, and the infrared (IR) region
ranges from 700 nm to 300μm. Biophotonics disciplines are concerned mainly
with wavelengths falling into the spectrum ranging from the mid-UV (about
190 nm) to the mid-IR (about 10μm).
Also shown in Fig.1.5is the relationship between the energy of a photon and its
frequency (or wavelength), which is determined by the equation known asPlanck’s
Law

E¼hm¼

hc

k

ð 1 : 2 Þ

where the parameter h = 6.63× 10 −^34 Js = 4.14× 10 −^15 eVsisPlanck’s con-
stant. The unit J meansjoulesand the unit eV stands forelectron volts. In terms of
wavelength (measured in units ofμm), the energy in electron volts is given by

EeVðÞ¼

1 : 2406

klðÞm

ð 1 : 3 Þ

Table1.5lists the correspondence between the wavelength range and the range
of photon energies for various biophotonics spectral bands. The spectral band

Table 1.4 Indices of
refraction for various
substances

Material Refractive index

Air 1.000

Water 1.333

Cornea 1.376

Cytoplasm 1.350–1.375

Epidermis 1.34–1.43

Extracellularfluids 1.35–1.36

Human liver 1.367–1.380

Melanin 1.60–1.70

Mitochondria 1.38–1.41

Tooth enamel 1.62–1.73

Whole blood 1.355–1.398

1.3 Biophotonics Spectral Windows 9