Define the Basic Concepts behind the Spectral Techniques?
Energy is transmitted via electromagnetic waves that are characterized by their frequency and wavelength. The physical definition of wavelength is the distance that a periodic wave propagates in one period or the distance between wave crests. Analytically the term wavelength describes a posi-tion within a spectrum. Electromagnetic radiation includes radiant energy that extends from cosmic rays with wave-lengths as short as 10-9 nm up to radio waves longer than 1000 km. However, in this unit the term light is used to describe radiant energy from the visible and ultraviolet portions of the spectrum (290 to 800 nm). In addition to possessing wavelength characteristics, light also behaves as it is composed of discrete energy packets called photons. The relationship between the energy of photons and their frequency is illustrated as:
E = hν (1)
where:
E = Energy in ergs
ν = Frequency of the light given in cycles per second
λ = Planck's constant (6.62 × 10-27 erg seconds)
The frequency of light (ν) is related to the wavelength by
ν = c/ λ (2)
where:
ν = Frequency of light in cycles per second
c = Speed of light in a vacuum (3 × 1010 cm/sec)
λ = Wavelength in centimeters.
By combining equations (1) and (2), the product is
E = hc/λ
This equation shows that the energy of light is inversely proportional to the wavelength. For example, ultraviolet (UV) radiation at 200 nm possesses greater energy than infrared (IR) radiation at 750 nm. The human eye responds to radiant energy with wavelengths between about 380 and 750 nm, but modern instrumentation permits measurements at both shorter wavelength (UV) and longer wavelength (IR) portions of the spectrum. Sunlight, or light emitted from a tungsten filament, is a mixture, or spectrum, of radiant energy of different wavelengths that the eye recognizes as "white."