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 arebrought 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.

F – Focal Point (Principal Focus).

Convex lens

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

FOCAL POINT OF A CONCAVE LENS

Principal axis.

Parallel rays

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 Refractionfor 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 Refractionfor 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.

• 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.

Diverging Lenses – Ray Diagrams

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)

• image will be

an inverted image

• the image is

reduced in size

Object – Image Relationship

Case 1: The object is located beyond 2F:

• the image will be

inverted

• the image

dimensions are

equal to the object

dimensions

Object – Image Relationship

Case 2: The object is located at 2F:

• the image will

be located

beyond the 2F

point

• the image will

be inverted

• the image is

larger in size

Object – Image Relationship

Case 3: The object is located between 2F and F:

• no image is formed

Object – Image Relationship

Case 4: The object is located at F:

• somewhere on

the same side of

the lens as the

object

• an upright image

• the image is

enlarged

Object – Image Relationship

Case 5: The object is located in front of F:

Object – Image Relationship

Converging Lens Summary

Object – Image Relationship

Diverging Lens Summary