c. Part A of Figure 153 shows the curved wave fronts of light rays
striking a convex glass lens. Notice that the light waves coming out of the
lens are straight and parallel.
The reason is that light travels more
slowly in glass than in air. So, the center of the wave fronts are slowed
down more than their edges because there is less glass for them to travel
through at the edges.
Figure 153. Comparison of Action of Glass
Lens to That of Metallic Lens.
d. In the case of short radar waves, however, the antenna lens must be
concave rather than convex.
Part B of Figure 153 shows the curved
wavefronts of the RF energy radiating from the horn feed and striking the
concave lens. The radar waves are straight and parallel as they come out of
the lens just like light waves coming through the glass lens. The reason is
that the parallel plates of the antenna lens act like waveguides to increase
the phase velocity of the RF energy. The longer the waveguide, the greater
is the increase in phase velocity. So, the phase velocity of the RF energy
is increased more at the edges of the lens than at the center. This results
in parallel wavefronts as the RF energy comes out of the lens.