Page 424 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 33

Physical and

Quantum Optics

INTERFERENCE

In examining the reflection and refraction of light, it is sufficient to consider light as though it consisted of rays
that travel in straight lines in a uniform medium. The study of such phenomena, therefore, is called geometrical
optics. Other phenomena, notably interference, diffraction, and polarization, can be understood only in terms of
the wave nature of light, and the study of these phenomena is called physical optics.
Interference occurs when different waves of the same nature meet at the same place and their instantaneous
amplitudes add together. In constructiveinterference the waves are in phase (“in step”) and reinforce each other;
in destructiveinterference the waves are out of phase and partially or completely cancel (Fig. 33-1). All types of
waves exhibit interference under appropriate circumstances. Thus water waves interfere to produce the irregular
surface of the sea, sound waves close in frequency interfere to produce beats, and light waves interfere to produce
the fringes seen around the images formed by optical instruments and the bright colors of soap bubbles and thin
films of oil on water.

Constructive interference + =

Destructive interference + =

Fig. 33-1

SOLVED PROBLEM 33.1
When is it appropriate to think of light as consisting of waves and when as consisting of rays?
When paths or path differences of light are involved whose lengths are comparable with the wavelengths found in
light,thewavenature is significant andmust betaken into account. Thus diffraction andinterference can be understood
only on a wave basis. When paths of light are involved that are many wavelengths long and neither diffraction nor
interference occurs, as in reflection and refraction, it is more convenient to consider light as consisting of rays.
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