Lenses and the Formation of
Images
Basic Lens Shapes
Converging Lens
-thickest in the middle (thinnest at edge)
-causes parallel light rays to converge (come
together) through a single point after refraction
Diverging Lens
-thinnest in the middle (thickest at edge)
-causes incident parallel rays to spread apart
after refraction
• We will only study the incident ray coming
into the lens and the ray leaving the lens
(in actuality there are two refractions in a lens;
air to glass, glass to air)
Converging Lenses Words
2F’
2F
Converging Lenses Words
Optical Centre (O)- Centre of the Lens
Principal Axis (PA)- The line through the optical centre that
is perpendicular to the central dashed line of the lens
Principal Focus (F)-the point on the PA of a lens where light
rays parallel to the PA converge after refraction
Secondary Principal Focus (F’)- the focus that is on the sam
side of the lens relative to the incident rays
2F- twice the distance from the principal focus
2F’-twice the distance from the secondary principal focus
Diverging Lens Words
2F
2F’
-if you project the diverging rays backward off of a
diverging lens, it looks as if they come from a virtual
focus, this point now becomes the principal focus
Secondary Principal Focus (F’)-now on the other side
of the lens where the rays actually diverge
Converging vs. Diverging Lenses
Converging Lens
-brings parallel light rays
together through a focus
after refraction
Diverging Lens
-spreads parallel light
rays apart after
refraction so that it looks
as though they have
come from a virtual
focus
-the principal focus is on -the principal focus is on
the OPPOSITE side of
the same side of the
the lens as the incident lens as the incident rays
rays
What affects what an image will look
like?
• The type of lens (converging or diverging)
• The location of the object
With lenses, we will use ray diagrams to draw
incident rays and refracted rays.
Emergent rays-the ray that leaves the lens,
refracted as it goes from the lens back to the
air
• In a thin prism, the emergent ray is almost
unaffected by the presence of the prism
• We will work with thin prisms.
Thick Prism
Thin Prism
How to locate the image in a
converging Lens
A ray parallel to the principal axis is refracted
through the principal focus.
• A ray through the F’ is refracted parallel to the
principal axis.
• A ray through O continues straight through
without being refracted
• Where the two refracted lines cross, the
image is produced.
Object beyond 2F’
•
•
•
•
Smaller
Inverted
Between F and 2F
Real
Object at 2F’
•
•
•
•
Same Size
Inverted
At 2F
Real
Object between F’ and 2F’
•
•
•
•
Larger
Inverted
Beyond 2F
Real
Objects beyond 2F’
-the image is smaller than the object and
between 2F and F
-as you move slowly toward the lens, the image
gets larger and larger
-eventually the image and the object are the
same size when the object is located at 2F’ the
image would be at 2F
Object at F
• No clear image formed
Object between F’ and the Lens
•
•
•
•
Larger
Upright
Behind the Lens
Virtual