Download Light Handout

Chemistry Exploration
What is the Nature of Light? – keep for reference
Exploration 1: How does light behave as a wave?
We have seen that light can be described by electromagnetic waves that have the properties of wavelength and
frequency. In addition to these properties, waves will bend around corners and will simultaneously pass through
two slits or small openings in a barrier. In this exploration we will consider these two properties of waves and
begin to deepen our understanding of the nature of light by answering the question, How does light behave as a wave?
As an example of wave behavior you are likely to be familiar with, let’s consider the interaction of water waves
with solid objects. When water waves encounter an edge or a corner such as a rock or a concrete pier, the
wavefront will appear to bend around the corner as part of the wavefront is cut off by the obstacle. This wave
behavior is called diffraction. Light will also diffract when it encounters an edge or corner. Figure 1 demonstrates
the diffraction or bending of light from a single slit, and the resulting diffraction pattern that is produced on a screen
far away. The alternating light and dark bands are called fringes, where light and dark fringes correspond to
maximum and minimum wave intensity, respectively.
When there are two or more barriers, such as in the case of a double slit, interference can occur where the
wavefronts overlap. Figure 2 shows examples of interference between two waves in the same region of space.
Constructive interference (A) occurs when equivalent parts (crests, troughs, etc.) of waves 1 and 2 coincide,
producing a wave with increased amplitude. However, when the equivalent parts of waves 1 and 2 do not coincide;
destructive interference (B) produces a wave with decreased amplitude. Figure 2-B shows an example of a
destructive interference where the crests of one wave coincide with the troughs of another, resulting in a wave with
zero amplitude.
Figure 2: Constructive and destructive interference in waves.
Exploration: How Does Light Behave as a Particle?
Thus far, we have described light as a wave. In class you saw that diffraction and interference patterns are
produced when light passes through one or more small openings in a barrier. That particular experiment supports
a wave theory of light. Can we be sure, however, that light doesn’t have particle-like characteristics?
What is the Nature of Light?
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By the end of the nineteenth century, most scientists supported a wave theory of light. According to this theory,
the energy of light is determined by the amplitude or intensity of electromagnetic waves. However, in 1900, a
German physicist named Max Planck proposed the existence of “particles” or packets of radiation, later called
photons, in which the energy of a packet of radiation is proportional to frequency. Many scientists found it
difficult to believe a particle theory of light; they needed experimental evidence to support the theory. In this
Exploration, we will examine one such experiment and answer the question, How does light behave as a particle?
Astronomers who are interested in analyzing starlight often use instruments called detectors. We can think of a
detector as any device in which some measurable property changes in response to incident electromagnetic
radiation. One common example is sunglasses that darken in response to sunlight. Historically, astronomers used
the human eye and photographic plates to detect light gathered by a telescope. These detectors now have largely
been replaced by electronic devices such as CCDs (charge coupled devices) in large telescopes. CCDs can also be
found in night-vision goggles and binoculars. They are often made of silicon, but other elements and mixtures of
elements can be used (for an example of a CCD, see http://ccd.ifa.hawaii.edu/images/ger/8kphoto2.gif). In order
to understand how detectors measure the light from stars or other objects, we need to learn more about the nature
of light, and whether it behaves as a particle, a wave, or both.
The Photoelectric Effect: Using Light to Remove Electrons
Some modern detectors such as CCDs rely upon the discovery, made in 1887 by Henrich Hertz, of the photoelectric
effect. The results of this experiment greatly puzzled scientists until they were explained by Albert Einstein in
1905.
Hertz demonstrated that shining radiation on metals can generate an electric current. With the apparatus
diagrammed in Figure 3 below, referred to as a photocell, Hertz observed that when radiation shines on a metal
plate in a circuit, the radiant energy is converted into the energy of motion, or kinetic energy, of electrons. The
electrons are ejected from the metal with a variety of kinetic energies and move toward another metal plate. The
flow of electrons in the photoelectric effect is measured in units called Amperes (A). The amount of current is
proportional to the number of electrons produced by the radiation.
Radiation
source
-
-
+
+
voltage source
Figure 3: Current flows in a photocell when
radiation strikes a metal surface, causing electrons
to be ejected from the metal and into the circuit.
Now continue on and answer questions associated with this handout as well as information presented in class.
What is the Nature of Light?
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