CHARACTERISTICS OF LIGHT
The subject of light as a form of radiant energy has been theorized upon, experimented with, and studied by many physicists and scientists. Until about three centuries ago, no one had developed a reasonable theory of the nature of light. Then Max Planck, a physicist, published a theory in which light was supposed to consist of a stream of high-speed particles. Planck theorized that any light source sent out an untold number of these particles. This then was the quantum theory. The quantum of light is called the photon. The quantum theory is used to explain X ray, radiation, and photoelectricity.
WAVELENGTH, SPEED, AND FREQUENCY
About the same time other physicists, Christiann Huygens and Thomas Young, introduced a theory called the wave motion theory. The wave motion theory is used to explain reflection, refraction, diffraction, and polarization. In wave motion theory, light, wavelength, speed, and frequency are important characteristics, and they are interrelated. The wavelengths of light are so small that they are measured in nanometers (nm). A nanometer is equal to one millionth of a millimeter. Wavelengths of light range from about 400nm to 700nm in length and travel in a straight-line path.
The speed of light varies in different mediums. In air, light travels about 186,000 miles per second. In a denser medium, such as glass, light travels even more slowly. Furthermore, in a denser medium, the speed is different for each color of light. Wavelength is the distance from the crest of one wave to the crest of the next wave (fig. 1-2). Frequency is the number of waves passing a given point in 1 second. The product of the two is the speed of light.
Therefore:
Speed = Wavelength x frequency, or
Wavelength = Speed / Frequency
Since the speed of light in glass is slower than in air, the wavelength must also be shorter (fig. 1-3). Only the wavelength changes; the frequency remains constant. Hence we identify a particular type of radiation (color of light) by its wavelength, bearing in mind that we are speaking of the wavelength in air.
EMISSION OF LIGHT
To the photographer, there are two important characteristics of the way light travels. First, in a given medium, light always travels in a straight line. Second, in a given medium, it travels at a constant speed.
Once light is produced (emitted), it is no longer dependent on its source, and only its speed is affected by the many mediums through which it can travel. Another example of the independence of light is that when light travels from air into a denser but transparent medium, such as glass, its speed is reduced. But when it leaves the glass, it returns to its original speed. This changing speed is important in refraction, a behavior of light that allows lenses to form images.
Unless light is reflected or focused, it travels or radiates in all directions from the source. As light travels from the source, its energy of light spreads out. The greater the distance it travels, the more it spreads out (fig. 1-4). Therefore, the amount of light reaching a given area at a given distance is less than that reaching the same area closer to the source. In other words, the intensity of illumination on a surface varies when the distance between the light source and the surface, or subject, is changed. This becomes important when exposing film with artificial light.
COLOR OF LIGHT
Look at a bright, red apple on a dark, green tree. It is hard to believe that color is not an inherent property of these objects; in fact color is not even inherent to light. What you are seeing is a visual perception stimulated by light. The apple and tree are only visible because they reflect light from the sun, and the apple appears red and the tree appears green because they reflect certain wavelengths of light more than others. In this case, these particular wavelengths are seen by the human eye as red and green. When we see a color, we are simply seeing light of a particular wavelength.
When a beam of light has a relatively even mixture of light of all visible wavelengths, it appears as white light. When this beam of white light is passed through a prism, its different wavelengths are spread apart and form a visible spectrum. This visible spectrum is seen as a band of colors, such as violet, blue, blue-green, green, yellow, and red (fig. 1-5).
COLOR TEMPERATURE
White light is made up of nearly equal intensities of all wavelengths within the visible spectrum. By passing white light through a prism, scientists have found that light sources have many qualities. They are as follows:
Different wavelengths are present in the sources of radiant energy.
The frequency and color of wavelengths vary.
With each wavelength, there is a variation in the amount of energy.
This variation of energy is called spectral energy distribution. The spectral energy of a light source is represented by color temperature. These terms are used in photography to describe and define the sources of light being used. Color temperature describes the color quality of a light source in terms of the amounts of red light and blue light.
Color temperature is based on what is called a Planckian radiator, or simply a black body. As the temperature of the metal of the black body is raised, it goes from a dull black through red and orange to blue and finally to white heat. The quality of the light emitted is a function of the temperature of the metal. When the object is red-hot, the color temperature is low since red is at the low end of the scale; and when it is blue-white, the color temperature is high. However, the temperature at which a light source is burned does not control color temperature; for example, a fluorescent tube burns at a low 125°F, yet it has a high color temperature. Color temperature then is raised or lowered relatively by the amount of visible white light radiated from the source. Be careful not to get confused. Traditionally reddish light is known as warm and bluish light as cold; in actuality, the color temperatures is the other way around.
The most convenient way to describe the color temperature of a light source is by its Kelvin temperature. From a practical point of view, this term refers to the degree of whiteness of the light. Color temperature is measured on the Kelvin scale and is stated as Kelvin temperature. On the temperature scale, 0 K is the same as -273°C. Therefore, degrees Kelvin (K) are always 273 degrees higher than the same temperature on the Celsius scale. Thus a red-hot piece of iron with an approximate temperature of 2000°C has a color temperature of 2273 K. As the Celsius temperature of an object is raised, it emits a whiter light and produces a relatively higher color (Kelvin) temperature.
COLOR RELATIONSHIPS
Many ways have been devised to classify the colors we see. Though terminology may differ, it is generally agreed that color can be defined by three qualities: hue, brightness, and saturation.
Hue-Hue is the actual color or wavelength reflected by an object-red, yellow, green, and so forth. For example, it could be said that the color of an object is blue. Blue identifies the hue. There are seven hues in the visible spectrum. These seven hues are as follows: blue, green, red, cyan, magenta, yellow, and white. Hue, however, is an inadequate description of a cola. To be more specific, we should say that an object is dark blue or light blue. Now we have described the brightness of the color.
Brightness-The brightness of a color is independent of the hue. Two colors may have the same hue but different brightness. Thus, to describe a color or brightness, we say that it is dull, bright, vivid, or brilliant.
Saturation-The saturation of a color is the degree to which the color departs from neutral gray of the same brightness. You can think of it as mixing black, gray, or white paint with a colored paint, thus diluting the color. In other words, saturation is a measure of color purity.