##### Characteristics of Light

There are a few fundamental characteristics of light that are useful to be aware of before proceeding with the discussion on how CCD's and Film can function to save a useful, meaningful image. One of these important fundamental qualities is the fact that visible light is electromagnetic radiation.

Electromagnetic Radiation, Photons, and Energy Levels

Electromagnetic radiation has many different classifications. Some such classifications include AM/FM Radio Waves, microwaves, visible light, x-rays, and gamma rays. A key factor in these classifications is that each different type or "level" of electromagnetic radiation contains different energy levels. These energy levels are determined by the speed or rate that charges from a given source move to create an electric field (for instance, moving charges through an antenna or lightbulb) (Serway 1090). Hence, this oscillating electric field has two very important characteristics: it has a frequency and a wavelength. Furthermore, light can also behave as a particle in some instances. This particle of light is called a photon, and is essentially the amount of energy that a light wave has at a certain frequency (the energy of a photon is not dependent on the intensity of the light, but rather only dependent upon its frequency) (Serway 1107). It is this "duality of light" that allows CCD's and film to function as they do, as energy is transferred to materials through light via. photons.

Since the energy of a photon is only related to its frequency, an equation (discovered by Einstein) relates photons to the electrons they produce by:

E = h * f

Where E is the energy of the produced electron, h is Planck's constant (6.63 * 10^-34 J*s), and f is the frequency of the given light source (CS39J Session Seven 1).

To understand the wavelengths associated with the different energy levels of electromagnetic radiation, a plot of the different frequencies can be seen below in figure one.

Figure One (Graphical representation of the Electromagnetic Spectrum)

(Courtesy of "Physics for Scientists and Engineers", Serway, 1094)

In the above figure, the electromagnetic waves with the lowest frequencies (and therefore longer wavelengths) are associated with having lower energies. Also seen from the figure is the fact that the visible spectrum, the one that most film, CCD's, and the human eye is calibrated to, has a wavelength range of 700nm (red) to 400nm (violet) (Serway 1093). A more detailed figure showing the visible spectrum can be seen below in figure two.

Figure two ("Magnified" view of the visible spectrum of light)

(Courtesy of Visible Light Waves - The Electromagnetic Spectrum)

From figure two, it is apparent from the previous discussion about figure one that the lower energy part of the visible spectrum is towards the left end (red at a wavelength of 700nm) and the higher energy region is towards the right end (violet at a wavelength of 400nm). A prime example of energy intensities due to higher frequencies can be introduced here. For instance, the higher energy characteristics of ultraviolet radiation (shorter wavelength), as opposed to the lower energy characteristics of infrared (longer wavelength) is the primary reason as to why ultraviolet radiation is so damaging to your skin and eyes.

Further analysis of figure two yields the primary key to how film and CCD's work. Rather than being sensitive to all the different wavelengths of light, it only needs to be generally sensitive to three - red, green, and blue. From these three colors (and through varying intensities of each), all shades of color can be obtained from simply overlapping them. A figure of this type of red/green/blue color intensity merging can be seen below in figure three.

Figure Three (Animation of merging colors together with different intensities of red, green, and blue)

From figure three, notice the different colors that are appearing in the overlapping regions of red, green, and blue. In the case of a faded red on blue or blue on red, the color is purple. When it's blue on green or green on blue, the color is more turquoise or dark olive green, and when it's red on green or green on red, the color shifts towards more of a brown/orange.

Now that light has been briefly covered, exactly how a picture is "setup" can be further explained in the next section - The Focal Plane.