http://www.ck12.org Chapter 5. Electrons in Atoms
FIGURE 5.4
At high enough temperatures, a heated
object may glow different colors. The
atoms in this piece of metal are releasing
energy in discrete units called quanta.energy.The energy of a photon of light is quantized according to the equationE=hν. Previously, light had been
described only as a wave, and scientists trained in classical physics found this wave-particle duality of light to be a
difficult idea to accept, particularly because most of the characteristics of light can be fully explained using concepts
that only apply to waves. However, the results of some key experiments could not be explained by the prevailing
theories, but they could be rationalized if the particle nature of light was accepted. Some of these unexplained results
came from experiments on a phenomenon known as the photoelectric effect.
Thephotoelectric effectis a phenomenon that occurs when light shined onto a metal surface causes the ejection
of electrons from that metal.It was observed that only certain frequencies of light are able to cause the ejection of
electrons. If the frequency of the incident light is too low (red light, for example), then no electrons were ejected
even if the intensity of the light was very high or it was shone onto the surface for a long time. If the frequency of
the light was higher (green light, for example), then electrons were able to be ejected from the metal surface even
if the intensity of the light was very low or it was shone for only a short time. This minimum frequency needed to
cause the ejection of electrons is referred to as the threshold frequency.
Classical physics was unable to explain why the photoelectric effect would have a threshold frequency. Based on
a classical view of light, the electron in the metal should eventually be able to collect enough energy to be ejected
from the surface, even if the incoming light had a low frequency. Einstein used the particle theory of light to explain
the photoelectric effect (Figure5.5).
Consider theE=hνequation.Eis the minimum energy that is required in order to eject an electron from the metal
surface. If the frequency (ν) of the incoming light is below the threshold frequency, there will never be enough energy
to cause an electron to be ejected. If the frequency is equal to or higher than the threshold frequency, electrons will
be ejected. As the frequency increases beyond the threshold, the ejected electrons simply move faster. An increase in
the intensity of incoming light that is above the threshold frequency causes the number of electrons that are ejected
to increase, but they do not travel any faster. The photoelectric effect is applied in devices called photoelectric cells,
which are commonly found in everyday items, such as a calculator that uses light to generate electricity (Figure
5.6).
Run a simulation of the photoelectric effect at http://phet.colorado.edu/en/simulation/photoelectric.
Sample Problem 5.2: Quantized Energy
What is the energy of a photon of green light with a frequency of 5.75× 1014 Hz?