Make a sketch showing a ray coming in from the left and
hitting a slab of glass tilted at some angle. Show the first
transmitted ray, the second (i.e., that transmitted after two
internal reflections), the third.... Now look at a point or line
source through a microscope slide. Hold the slide close to your
eye. Start at normal incidence, gradually tilt the slide. Look
for the "virtual sources" due to multiple reflections. (The effect
is greater near grazing incidence.) Look also for the light that
emerges, not by transmission out of the surface of the slide, but
from the end. This is the "internally trapped" light, which finally
escapes when it reaches the end surface at near-normal
incidence rather than at the near-grazing incidence at which it
encounters the sides of the slide.
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