When a ray of light passes from
the air into glass or plastic—the
most common materials used for
lenses—the light continues at an
infinitesimally slower speed. It’s
like a speed limit enforced by
the laws of nature.3 But if the ray of light is at
an angle to the plane
where air and glass
meet, not all parts of
the ray move at the
same speed as they
cross from air to
glass. The edge of a
light beam that hits the
glass first slows down
while the opposite edge, still
in the air, continues to move at a
faster speed. The difference in speed results in the beam of
light traveling in a different direction. The effect is similar to
a car that has its right wheels in sand and the left wheels
on pavement. The right wheels spin without moving the
car forward much. The left wheels, with more traction, push
the car forward more than the opposite wheels, causing the
car to skid to the right.
4
The process of bending light to
travel in a new direction is
calledrefraction. How much
a substance causes light to
bend, or refract, is called its
index of refraction.
Refraction is the principle that
allows a camera lens to create
a sharp, focused image.5
Air Versus Water
The next time you go spear fishing, remember that air and water
have different indices of refraction. Because of that, a fish that
appears to be 3 feet in front of you might actually be 2 feet away.CHAPTER 3 HOW LENSES WORK^31