Download photocell - BYU Physics

PHOTOCELL
The photocell used in this experiment is a simple silicon semiconductor sheet with two
leads attached commonly known as a solar cell. The doped semiconductor has electron energy
levels such that light photons can excite the charge carriers and give them energy that produces a
voltage. Such a cell is called a voltaic light cell. Thus, when light strikes the sheet, it generates an
EMF, and the cell will produce an electrical current. Both the EMF and the current are related to
the light intensity. You may place a large resistance across the cell to allow current to flow. The
voltages are usually less than 0.4 volt. You can carry out calibration by using a standard meter or
by using a standard light bulb in conjunction with a 1/R2 distance relationship. You will need to
shade the cell from room light. The study of a photocell can have many facets and can involve
much more time than is available in this one laboratory period. Specifically, any given photocell
will have a unique spectral response; that is, it will yield different output voltages for the same
intensity of light of different colors. Furthermore, every light source will emit light of different
spectral distributions, and these spectral distributions for an incandescent light will vary
dramatically with the applied voltage. Since you will not have available any means for separating
specific colors or wavelengths, the measurements you will make using photocells will represent an
average over all wavelengths emitted by your source and detected by the photocell. The standard
meters available for calibrating your photocell have spectral responses purposely selected to
approximate the spectral response of the human eye. The fundamental units of intensity of any
electromagnetic wave would be watts/meter2, but such units have very little meaning when applied
to the visibility of the human eye. In the science of illumination, the unit lux, which represents the
intensity produced at a one-meter distance from a one-candle standard light source, is used.
Spatial variations of light intensity from light sources can vary due to the dimensions of the light
source, distance from the light source, shadow regions, diffraction patterns, etc.