The Pharmaceutics and Compounding Laboratory
Spectrophotometry as an
Analytical Tool

The Nature of Light

Light is a form of energy. In scientific terms, it can be described in two ways: (1) electromagnetic radiation, or light waves, and (2) particles of energy, known as photons. The light wave is comprised of an electric field (E) and a magnetic field (H), which travel through space at right angles to each other. The wave is created by the acceleration of some charged particle, such as an electron. The acceleration of the particle induces the electric and magnet fields which travel away from the charged particle as a light wave.

A light wave is described by a number of characteristics: wavelength, frequency, velocity, and amplitude. The wavelength (λ) is the distance between two consecutive wave peaks. The frequency (v) is the number of complete wave forms (wavelengths) that pass a point in space in a period of time. The velocity of light through any homogeneous medium is constant. Consequently, the frequency and wavelength of a light wave can be related by the following equation:

    velocity = λv

The velocity of light in a vacuum (c) (3.00 x 108 m/s) is commonly used in scientific calculations of frequency and wavelength, giving:

    c = λv

Finally, the amplitude (a) is the height of the wave and is related to the intensity of a light beam.

When light is described as "particles of energy," the smallest definable particle is the photon. The energy of a photon (E) is dependent on the frequency of the light and can be qualified by the following formulas:

    E = hv or E = hc/λ

where h is Planck's constant (6.62 x 10-34 J-sec). This relationship illustrates two important points: that light of a particular wavelength (or frequency) is quantized - that is, it has a single energy value at that wavelength; and that light can be used as a source of quantized energy in the study of chemical systems. It is important to note that energy and wavelength are inversely proportional: short wavelength light has higher energy than longer wavelength light.

If you could physically "grab" a light wave and stretch it to make its wavelength slightly longer, you would also change the energy level of that light, as related by Planck's equation. Every light wave has a specific wavelength, and each wavelength has a corresponding energy level. There is a relatively small region called "visible light" that includes the range of energies with which the molecules in the retina of our eyes interact in order to transmit visual images to our brain. Our eyes are capable of sensing only a very small portion of the entire electromagnetic spectrum! Similarly, a radio receiver cannot sense visible light, although it can interpret much longer wavelength (lower energy) radio waves.