34.0 - Introduction
Light is a particle. Many of the great scientists of the 17th and 18th centuries who made
fundamental contributions to the study of optics, including Isaac Newton, thought that
light consisted of a stream of “corpuscles,” or particles. In the 20th century, Albert
Einstein explained the photoelectric effect. His explanation, for which he was awarded
the 1921 Nobel Prize, depended on the fact that light acts like a particle. This property
of light led to the coining of the term “photon” for a single particle of light by the chemist
Gilbert Lewis.
Light is a wave. Between the 18th and 20th centuries, physicists discovered many
wave-like properties of light. They found that a number of phenomena they routinely
observed with water waves they could also observe with light.
For instance, the English scientist Thomas Young (1773-1829) showed that light could
produce the same kinds of interference patterns that water waves produce. At the right,
you see examples of interference patterns formed by light and by water waves. The
similarities are striking. In this chapter, you will apply to light some of what you have
studied about the interference of sound waves and traveling waves in strings.
Let there be light. Is light a particle, a wave, or both? Perhaps an Early Authority had it
right. Light is light. It is a combination of electric and magnetic fields. Trying to classify
light as a particle or as a wave may be a fruitless effort í better to revel in its unique
properties. In this chapter, we will revel in its wave-like properties, and discuss the topic
of interference. Your prior study of electromagnetic radiation modeled as a wave
phenomenon will prove useful.
Interference of light waves
Pattern of bright and dark on screen
Interference of water waves
Expanding circular ripples
Pattern of disturbance and calm34.1 - Interference
In Concept 1, you see an interference pattern created
by causing a beam of light to pass through two
parallel slits to illuminate a viewing screen.
Constructive interference of light waves accounts for
the bright regions (called bright fringes) while
destructive interference causes the dark fringes.
In this section, we review some of the fundamentals
of interference, and discuss the conditions necessary
for light to make the pattern you see to the right. You
may have already studied the interference of
mechanical waves; for instance, what occurs when
two waves on a string interact. In this chapter, you will
study what happens when electromagnetic waves
meet. Some of the same principles and terminology are used in discussing both kinds of
interference.
When two light waves meet, the result can be constructive or destructive interference.
In the following discussion, we assume that the waves have equal amplitude.
Constructive interference creates a wave of greater amplitude and more intensity than
either source wave; destructive interference results in a wave of smaller amplitude and
less intensity than either source wave. At any point in a two-slit interference pattern
such as that to the right, light waves from the two sources meet and interfere
constructively, destructively, or partially (exhibiting a degree of interference somewhere
between complete constructive and destructive interference).
To create an interference pattern, a physicist needs light that is:
- Monochromatic. This means light with a specific wavelength. For instance,
experimenters can produce the pattern you see in Concept 1 by using pure red
light.
Grass is green because it selectively absorbs nongreen colors. In contrast, the
shimmering "color" of a peacock's feathers is due to interference effects.Interference pattern
Bright and dark fringes