Chapter 30: Lenses
Types of Lenses
Piece of glass or
transparent material
that bends parallel
rays of light so they
cross and form an
image
• Two types:
– Converging
– Diverging
Converging Lenses
Parallel rays are
brought to a
focus by a
converging lens
(one that is
thicker in the
center than it is
at the edge).
Diverging Lenses
A diverging lens (thicker at the edge than
in the center) make parallel light diverge;
the focal point is that point where the
diverging rays would converge if projected
back.
CENTERS OF CURVATURE, PRINCIPAL AXIS,
OPTIC CENTER.
The centers of the two spheres of which the
spherical surfaces of the lens form parts are called
CENTERS OF CURVATURE.
CENTERS OF CURVATURE, PRINCIPAL AXIS,
OPTIC CENTER.
The straight line joining the centers of curvature of
a lens is called its PRINCIPAL AXIS.
CENTERS OF CURVATURE, PRINCIPAL AXIS,
OPTIC CENTER.
The point inside the lens through which a ray of light
passes without any deviation is known as its OPTIC
CENTER.
FOCAL POINT
Convex lens
F – Focal Point (Principal Focus).
Rays parallel to the principal axis of a convex lens, after
refraction through the lens, get converged to a fixed point
on the principal axis of the lens. This point is called the
Focal Point of the lens.
FOCAL POINT OF A CONCAVE LENS
Parallel rays
Principal axis.
F – Focal Point
Rays parallel to the principal axis of a concave lens,
after refraction appear to diverge from a fixed point
on the principal axis. This point is called the FOCAL
POINT of the lens.
Focal Length
The focal point F
and focal length f
of a positive
(convex) lens,
a negative
(concave) lens,
a concave mirror,
and a convex
mirror.
Focal Plane
Incident parallel
beams, that are
not parallel to
the principal
axis, focus at
points above or
below the focal
point, making up
the focal plane
Constructing Images
Through Ray Diagrams
Three Rules of Refraction
for a double convex lens
• Any incident ray traveling parallel to the principal
axis of a converging lens will refract through the
lens and travel through the focal point on the
opposite side of the lens.
• Any incident ray traveling through the focal point on
the way to the lens will refract through the lens and
travel parallel to the principal axis.
• An incident ray which passes through the center of
the lens will in effect continue in the same direction
that it had when it entered the lens.
Three Rules of Refraction
for a double convex lens
Diverging Lenses –
Ray Diagrams
• Any incident ray traveling parallel to the principal
axis of a diverging lens will refract through the
lens and travel in line with the focal point (i.e., in
a direction such that its extension will pass
through the focal point).
• Any incident ray traveling towards the focal point
on the way to the lens will refract through the
lens and travel parallel to the principal axis.
• An incident ray which passes through the center
of the lens will in affect continue in the same
direction that it had when it entered the lens.
Diverging Lenses – Ray Diagrams
• 1. Pick a point on the top of the object
and draw three incident rays traveling
towards the lens.
Diverging Lenses – Ray Diagrams
• 2. Once these incident rays strike the
lens, refract them according to the three
rules of refraction for double concave
lenses.
Diverging Lenses – Ray Diagrams
3. Locate and mark the image of the top of
the object.
Object – Image Relationship
Converging Lens
• Case 1: the object is located
beyond the 2F point
• Case 2: the object is located
at the 2F point
• Case 3: the object is located
between the 2F point and the
focal point (F)
• Case 4: the object is located
at the focal point (F)
• Case 5: the object is located
in front of the focal point (F)
Object – Image Relationship
Case 1: The object is
located beyond 2F:
• image will be
an inverted
image
• the image is
reduced in
size
Object – Image Relationship
Case 2: The object is
located at 2F:
• the image will be
inverted
• the image
dimensions are
equal to the object
dimensions
Object – Image Relationship
Case 3: The object is
located between 2F
and F:
• the image will
be located
beyond the 2F
point
• the image will
be inverted
• the image is
larger in size
Object – Image Relationship
Case 4: The object is
located at F:
• no image is formed
Object – Image Relationship
Case 5: The object is
located in front of F:
• somewhere on
the same side of
the lens as the
object
• an upright
image
• the image is
enlarged
Object – Image Relationship
Converging Lens Summary
Object – Image Relationship
Diverging Lens Summary