Photoelectric Effect

Figure 4 A young Albert Einstein

Figure 4 A young Albert Einstein

The photoelectric effect[xii] is an effect that is widely used in our everyday life and has given us the abilities to have digital cameras, automatic door opening, computer scanners and optical detectors.  The effect that was observed was that light striking certain metal surfaces would emit an electric current.  It was thus believed that a more intense light source would increase the current, this was not observed for all colors for different metals. At this time it was believed that light traveled in waves much like water waves, but only certain colors of light would cause this effect no mater the intensity.  Therefore, a new model, which is called the particle model, was devised to explain this observation. The theory was that light was composed of little particles, photons that had no mass and traveling at 186,000 miles per second. There where other effects that could not be explained by the particle model and only could be explained with waves. This process that particles could only explain some areas and waves other areas is known as the wave-particle duality. Red light is less energetic than blue. It was observed that on certain metals, it did not mater the amount of certain colors of light that struck the metal no electrons were produced, thus the photoelectric effect occurs more often in blue light than in red. There was an energy threshold that must be meet per photon before electrons were emitted. The electrons that are produced are orbiting electrons and thus leave a net charge on the atom in which the electron was ejected from. Not until Einstein paper in 1905 was a true explanation brought forth to explain the observation.  Einstein[xiii] won the Noble Prize in physics for this work.

Inside your digital camera is a devise known as a CCD (charged coupled device) chip it is a black and white device. So that color can be observed red, green and blue filters are used to break up the light.  When light strikes a region (very small) of the chip the photons cause the creation of a current, which is measure to determine the intensity of the light (red, green or blue). These currents are converted in to a numerical value which can be read by a computer interface and recreate the object that was photographed.

Note: The video files listed below are the same video information in two different file formats. The Windows Media Video (.wmv) is in two different size formats, the 720 x 480 size will require a broadband connection, but has higher resolution and more depth. The 320 x 240 wmv file is a smaller file size and can be loaded onto a Microsoft Zune.

Video Files: Atomic, Nuclear and Modern Physics - Photoelectric Effect

.wmv 720 x 480

.wmv 320 x 240