The Nature of Light

James Richard Fromm

The area of chemistry and physics associated with the emission and absorption of light by atoms and molecules is called spectroscopy. The light emitted and absorbed by atoms and molecules has characteristic wavelengths or frequencies, and spectroscopic measurement of these can tell us a great deal about the energies associated with a particular atom or a particular chemical bond in a molecule since these frequencies are directly related to energy changes that take place with in the molecule or in its interactions with other molecules.

The wavelength l and the frequency fof electromagnetic radiation, or light, are simply two different ways of measuring the same property. All light travels at the speed of light, c, which is about 300 million meters per second; the best value available is 2.997925 +/- 0.000003 x 10+8 m/s, in vacuum. Light travels only slightly slower in other media. As a consequence, wavelength l (usually written as the Greek lambda) and frequency f (usually written as the Greek lower-case letter nu) are related:

l x f = c

The values of frequencies f are often given in hertz (Hz), which is the same as cycles per second and has the units s-1. Wavelengths are given in meters, m, or more commonly in nanometers (1 nm is 1 x 10-9 m). Visible light has wavelength of about 10-6 m or 1000 nm; the frequency of visible light is then about 3 x 10+14 Hz.

The visible region, which is the region of the electromagnetic spectrum visible to the average human eye, extends from about 200 nm to 900 nm. Other living organisms can detect light over slightly different regions. These are the extreme limits which the human eye can detect. Most of the sunlight available at ground level lies between 400 nm and 700 nm, because our atmosphere absorbs strongly at both longer and shorter wavelengths. The human eye is most sensitive in this region.

The different types of receptors within the human eye make it possible for us to determine by direct observation the wavelengths which are associated with many of the interactions between light and matter. The light of the sun is often called white light because it contains wavelengths for the entire visible region. Light containing only the wavelengths of parts of the visible spectrum is called colored light or colors. The colors which are used to describe parts of the visible spectrum range from violet (near 400 nm) through indigo, blue, green, yellow, and orange to red (near 700 nm).

An object appears to us to be the color of the light that it reflects and does not absorb rather than the color of the light which it absorbs. For example, the chlorophyll of leaves appears to be green because it does not absorb green light; chlorophyll absorbs both red light (absorption maximum 655 nm) and blue-violet light (absorption maximum 430 nm) while reflecting the green light toward an observer.

Table: Electromagnetic Radiation and Physical Objects
Wavelength Mode Transition (s) Involved
10+2 m Radio Waves Electron Spins
10 m Microwave Radiation Spins, Rotations
10-2 m Far Infrared Radiation Molecular Rotations
10-4 m Infrared Radiation Rotations, Vibrations
10-6 m Visible Light Molecular Vibrations
10-8 m Ultraviolet Light Electronic Excitations
10-10 m X-Rays Nuclear Transitions
10-12 m Gamma Radiation Nuclear Transitions
Sizes of Physical Objects
10+28 m Diameter of Observable Universe
10+21 m Diameter of Milky Way Galaxy
10+16 m Distance to Nearest Star
10+9  m Diameter of Sun
10+7  m Diameter of Earth
10+3  m Diameter of City
10+0  m Diameter of (overfed) human; 1 m
10-5  m Diameter of a Blood Cell
10-8  m Diameter of a Large Molecule
10-9  m Diameter of a Small Molecule
10-10 m Diameter of an Atom
10-14 m Diameter of Atomic Nucleus

The region of the electromagnetic spectrum between 1 nm and 400 nm, adjacent to the visible region, is called the ultraviolet region because it is just beyond violet, the color at the high-energy, short-wavelength, and high-frequency end of the visible region. The region of the electromagnetic spectrum with wavelengths greater than 900 nm adjacent to the visible region is called the infrared region because it lies just beyond red, the color at the low-energy, long-wavelength, and low-frequency end of the visible region. The infrared region is often divided into the near-infrared and far-infrared sections even though there is no clear wavelength at which they are divided.

Chemists make use of the entire electromagnetic spectrum to obtain information about atoms and molecules. The very low-energy radio waves are used in nuclear magnetic resonance spectrometers and the very high-energy gamma radiation is used in tracer studies.

Copyright 1997 James R. Fromm