5.1. Properties of Light http://www.ck12.org
FIGURE 5.3
Photoelectric effect.which modern-day solar cells operate. Whether electrons will be released depends on two factors: the wavelength
of the light source and the material onto which the light is being shined. Here is a video of a simple photoelectric
experiment: http://www.youtube.com/watch?v=WO38qVDGgqw (0:26).
MEDIA
Click image to the left for more content.Light Exhibits Particle Behavior
Further study of the photoelectric effect revealed some perplexing behavior that could not be explained by the
classical view of light as just a wave phenomenon. For example, the intensity of the light did not effect the amount
of energy possessed by the ejected photoelectrons. Photoelectrons emitted with the use of a very bright light had
the same energy as those emitted with the use of a dim light of the same frequency. However, a relationship was
observed between the number of photoelectrons ejected and the intensity of the light source. It was found that the
brighter the light sources, the more photoelectrons were ejected. Another puzzling aspect of the photoelectric effect
was that a minimum frequency of light was required in order to eject any electrons at all, regardless of how intense
the light source was.
Albert Einstein (1879–1955) studied this effect further, and in 1905, he postulated that light can also be thought of
in terms of particles, now calledphotons. Photons of high frequency light have more energy than the photons of low
frequency light (Zukav 1979), which explained why a minimum frequency was required for electrons to be ejected
by a given light source.Figure5.4 illustrates this effect.
Materials that eject electrons when illuminated with light, such as potassium, are called photoemissive. Not all
materials are photoemissive, nor are all light sources capable of initiating electron emission from a given substance.
For example, inFigure5.4, we see that 700 nm light will not initiate electron ejection, while 550 nm light will.