Download Light can be difficult to study and understand because it behaves in

Light can be difficult to study and understand because it behaves in different
ways in different situations. Over the years, scientists have developed models
of light to explain observations. Some of these models have also changed our
understanding of atoms and molecules.
To understand how images are formed by mirrors and lenses, you can model
light as rays traveling along straight lines. Mirrors and lenses change the
direction of incident light rays—mirrors by reflection and lenses by
refraction. You can use the laws of reflection and refraction to trace the paths
of light rays and predict the location, size, and orientation of images formed
by optical systems.
In other situations, light does not appear to travel in straight lines. It bends
around the edges of obstacles and spreads out when it travels through narrow
slits or apertures. Light produces patterns in these situations. You can
understand these patterns if you model light as a wave. Light waves from the
same source interfere when they travel different distances to arrive at the
same point at the same time. The resulting interference or diffraction pattern
can be used to determine the wavelength of the light. Diffraction limits the
ability of telescopes and microscopes to resolve two objects—or the smallest
detail of one object.
To understand the array of light wavelengths emitted by hydrogen—its
emission spectrum—Niels Bohr developed a model of the atom. Bohr’s
model assumed that the electron in the atom could have only certain discrete,
or quantized, energy levels. When the electron changes levels, it emits (or
absorbs) a discrete bundle of energy, called a photon. This model was
consistent with Einstein’s explanation of the photoelectric effect, in which
light behaves like a particle.
C HAPTER 10
SUMMARY
491
Louis de Broglie hypothesized that, if light has a particlelike nature, particles
such as electrons should have a wavelike nature. This hypothesis led to
development of the quantum model of the atom. The quantum model uses a
wave function as a mathematical description of the atom, which can predict
the energy levels and emission spectra of all the elements and molecules. It
also predicts the existence of metastable energy levels. Laser light is
produced by stimulated emission from metastable states. Because of the way
it is produced, laser light is different from light produced by other sources—
it is monochromatic, highly directional, and coherent. These attributes have
created many applications for lasers.
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C HAPTER 10
LIGHT AND OPTICAL SYSTEMS