It was shown early in the 20th century that light (or, more correctly, electromagnetic radiation) has the peculiar aspect of exhibiting properties characteristic of both particles (called photons) and waves. In fact, both views of light are needed to adequately describe this so-called dual nature of light.
That light has wavelike properties is evidenced by use of common terms such
as the ``wavelength'' or ``frequency'' of light. Our ability to distinguish
colors is due to light being composed of different wavelengths. For example,
a light having a wavelength of 540 nm is perceived as being green, whereas 700
NM is red.
Although the wavelength characteristics of light will be a factor in this project, it is primarily the particle property that we will attempt to model. In this view of light as a stream of particles, we will formulate a model of photon flux, which is defined to be the number of photons striking a unit area per second and is measured in photons / (m2sec). Photon flux can be considered as a measure of light intensity.
To provide an analogy to explain photon flux, imagine a flow of water from a garden hose fitted with a special type of nozzle that releases only tiny individual droplets of water. Let's say that you decide to measure the water droplet flux through a given two-dimensional area. One way to do this would be to aim the garden hose at a plastic ring with an internal area of say, 1 square meter. The water droplet flux would then be the number of water droplets that pass through the plastic ring during a given time period. In this example, the flow of water droplets from the garden hose represents the photons being produced and released from the sun, the principal source of ultraviolet, visible and infrared radiation for the earth. The plastic ring represents some unit area through which we can count the number of photons per unit time.
Incidentally, it is possible to count photons accurately because of a phenomenon known as the photoelectric effect. In a device called a phototube, photons of a certain energy strike the surface of a metal, and the ejection of an electron from the metal can result. This ejected electron (a negatively charged particle present in all atoms) can be accelerated toward a positively-charged wire. The current running through this wire as a result of attracting multitudes of ejected electrons can be measured and is in direct proportion to the number of photons originally striking the metal surface.
