Lenses
Converging and Diverging Lenses

When a piece of glass has just the right
shape, it bends parallel rays of light so that
they cross and form an image. Such a
piece of glass is called a lens.

A converging lens is a lens that is thicker
in the middle and thinner at the sides. It
converges light to a single point.

A diverging lens is thinner in the middle
and thicker at the edges. It causes light to
diverge, or spread out.

Some key features of a lens are the
principal axis, focal point, and focal length.
◦ The principal axis is the line joining the centers
of the curving surfaces.
◦ The focal point is the point at which the light
converges after traveling through a converging
lens.
◦ The focal length is the distance between the
lens and the focal point.
Image Formation by a Lens

Far away objects are viewed through a
small angle view, while close objects are
viewed through a larger angle view.

Wider angles allow more detail.

A magnifying glass is a converging lens
that increases the angle of view and
allows more details to be seen.

The image appears larger and farther
from the lens than the object actually is.

It created a virtual image.

A real image is formed by a single
converging lens is upside down. These are
used for projecting slides, movies, and
images on camera film.

When a diverging lens is used alone, the
image is always virtual, right side up and
smaller than the object.
Constructing Images through Ray
Diagrams

Ray diagrams show the principal rays that
can be used to determine the size and
location of an image.

To construct a ray diagram, you must know
the size and location of an object, its
distance from the center of the lens, and the
focal length of the lens.

Arrows are often used to represent objects.

The path of one refracted ray will be right
through the center of the lens with no
change in direction.

The path of another refracted ray will
have a ray parallel to the principal axis
and will be refracted by the lens to the
focal point

The third path is found by a ray of light
that passes through the focal point in
front of the lens emerges from the lens
and proceeds parallel to the principal axis.

The image is formed where all 3 rays
intersect.

Most objects will be inverted, and real.
Image Formation Summarized

A converging lens is a simple magnifying
glass when the object is within one focal
length. Over one focal length the image is
real also, but will be inverted.
Some Defects of Lenses

No lens gives a perfect image.

The distortions in an image are called
aberrations.

Because of these problems, glasses and
contacts do not always work out for people.

Today we have techniques to actually
reshape the cornea and fix eyesight without
lenses.