Page 28 - Modern Optical Engineering The Design of Optical Systems

Optics Overview 11

Figure 1.11 The passage of a
plane wave front through a re-
fracting prism.

In Fig. 1.11 a wave front from a source so far distant that the cur-
vature of the wave front is negligible is shown approaching a prism,
which has two flat polished faces. As it passes through each face of the
prism, the light is refracted downward so that the direction of propa-
gation is deviated. The angle of deviation of the prism is the angle
between the incident ray and the emergent ray. Note that the wave
front remains plane as it passes through the prism.
If the radiation incident on the prism consisted of more than one
wavelength, the shorter-wavelength radiation would be slowed down
more by the medium composing the prism and thus deviated through
a greater angle. This is one of the methods used to separate different
wavelengths of light and is, of course, the basis for Isaac Newton’s classic
demonstration of the spectrum.

1.5 Interference and Diffraction
If a stone is dropped into still water, a series of concentric ripples, or
waves, is generated and spreads outward over the surface of the water.
If two stones are dropped some distance apart, a careful observer will
notice that where the waves from the two sources meet there are areas
with waves twice as large as the original waves and also areas which
are almost free of waves. This is because the waves can reinforce or
cancel out the action of each other. Thus if the crests (or troughs) of two
waves arrive simultaneously at the same point, the crest (or trough)
generated is the sum of the two wave actions. However, if the crest of
one wave arrives at the same instant as the trough of the other, the
result is a cancellation. A more spectacular display of wave reinforce-
ment can often be seen along a sea wall where an ocean wave which
has struck the wall and been reflected back out to sea will combine
with the next incoming wave to produce an eruption where they meet.
Similar phenomena occur when light waves are made to interfere. In
general, light from the same point on the source must be made to travel
two separate paths and then be recombined, in order to produce
optical interference. The familiar colors seen in soap bubbles or in oil
films on wet pavements are produced by interference.