32.4 - Interactive checkpoint: Snell’s law
What is the index of refraction of the
second material?
Answer:n 2 =
32.5 - Everyday effects of refraction
The refraction of light can cause interesting and sometimes confusing results as the
brain interprets the position of objects it sees via refracted light. For instance, in the
upper illustration to the right, you see an ancient Egyptian fisherman trying to spear a
fish. The solid line indicates the refracted path of the light traveling from the fish to his
eyes. Since his brain expects light to travel in a straight line, he projects the fish to be in
the position indicated at the end of the dashed line, which causes him to think the fish is
nearer to the surface than it is. Experienced spear fishermen know how to compensate
for this effect.
The fisherman is not the only one to experience the effects of refraction. The fish does,
too. The light from the world above seen by the fish also refracts. Light coming straight
down will pass through the water’s surface unchanged, but light at any angle will refract,
in the process giving the fish a wider field of view of the world above than it would have
if there were no refraction. In essence, the fish sees a compressed wide-angle view of
the scene above. Certain camera lenses, appropriately called fisheye lenses, can
create the same effect, as illustrated in Concept 2.
Another interesting consequence of Snell’s law concerns light passing through a
window. The light refracts as it travels from air to glass. After passing through the glass,
it refracts again at the second interface. The ratio of the indices of refraction is now
reversed, so the initial change in angle is cancelled out. The light ray that emerges is
parallel to the initial ray, but displaced a small amount. The amount of displacement is
small enough that we ordinarily do not notice “window shift.” However, if you place a
newspaper page on a tabletop and cover half of it with a flat pane of glass, you will be
able to observe the displacement effect.Refraction confuses fisherman
He misjudges position of fish
A fisheye view
Wide-angle view compresses images
Light through a windowpane
Light refracts twice
(^600) Copyright 2007 Kinetic Books Co. Chapter 32