PhotoGroupy

by Nanette Salvaggio

Light is defined as the radiant energy that our visual system, our eyes, is sensitive to and depends upon for the sensation of vision.

Light is fundamental to photography. The function of photographic materials and digital sensors is to capture light and record the patterns it creates. The equipment and we use to create images are lamps to produce light, exposure meters and color temperature meters, to measure and characterize the light and lenses, shutter, apertures and filters to control light. The study of photography must begin with understanding light.

The importance of light is obvious. Light has been the object of an enormous number of experiments and studies over many centuries. Isaac Newton was one of the first persons to make significant headway in understanding the nature of light. In the seventeenth century, Newton performed a series of experiments and proposed that light is emitted from a source in straight lines as a stream of particles.

This theory was called the corpuscular theory.

Any photographer knows that light bends when it passes from one medium to another, and that light passing through a very small aperture tends to spread out. These facts are not easily explained by the corpuscular theory. As a result, Christian Huygens proposed the wavy theory, which stated that light and similar forms of electromagnetic radiation are transmitted as a waveform in some media. In the nineteenth century, Thomas Young performed a number of experiments that also supported the wave theory of light. The wave theory satisfactorily explained with light that the corpuscular theory did not, but it still did not explain all of them.

One of the more notable unexplained effects is the behavior of black-body radiation. Blackbody radiation is radiation produced by a body that absorbs all the radiation that strikes it, and emits radiation by incandescence, depending on its temperature. In 1900, Max Planck suggested the hypothesis of the "quantization of energy" to explain the behavior of blackbody radiation. This theory states that the only possible energies that can be possessed by a ray of light are integral multiples of a quantum of energy.

In 1905, Albert Einstein proposed a return to the corpuscular theory of light with light consisting of photons, each photon containing a quantum of energy. These suggestions along with others, gradually developed into what is know today as Quantum Theory or Quantum Electrodynamics. This theory combines aspects of the corpuscular and wave theories, and satisfactorily explains all of the known behavior of light.

Unfortunately, this theory is difficult to conceptualize, and can be rigorously explained only by the use of sophisticated mathematics. As a result, the corpuscular and wave theories are still used to some extent where simple explanations of the behavior of light are required.

If we accept the idea that light moves as a wave function, it is necessary to determine the nature of the waves and the relationship of light to other forms of radiation. Actually, light is a fractional part of a wide range of radiant energy that exists in the universe, all of which can be thought of as traveling in waves. These forms of enery travel at the tremendous speed of approximately 186,000 miles (3 x 10*8 meters) per second. The differ only in wavelength and frequency of vibration. These waves have been shown to vibrate at right angles to their path of travel. The distance from the crest of one wave to the crest of the next is termed the wavelength represented by the Greek letter lambda (λ).

Figure 1-1 illustrates this concept. The number of waves passing a given point in a second is called the frequency of

vibrator; the symbol f is used to specify it.

The wavelength multiplied by the frequency of vibration equals the speed or velocity (symbol v) of the radiation.

Thus, λ x f = v.

Since the wavelength of radiant energy can be determined with far greater accuracy than the frequency, it is common practice to specify a particular type of radiation by its wavelength. Because of the extreme shortness of the wavelengths encountered with light sources, the most frequently emplobed unit of measure is the nanometer (nm), which is equal to one-billionth of a meter. A somewhat less commonly used measure is the Angstrom (Å), which is equal in distance to 1/10 of a nanometer (e.g., 400 nm equals 4,000 Å).