Mirrors and Lenses
Chapter 23
The Law of Reflection
• Light waves are electromagnetic waves.
• Light waves travel from their source in all
directions
• Light is made up of rays that travel in
straight lines.
• An arrow, called a ray, is used to show the
path and direction of light
Mirrors
• Law of Reflection –
states that the angle
of reflection is equal
to the angle of
incidence.
• A ray diagram shows
how rays change
direction when they
strike mirrors and
pass through lenses.
Plane Mirror (Create Image)
• A mirror with a flat
surface is a plane mirror.
• You see a reversed (R-L)
image
• Rays of light strike you
and reflect. They strike
the mirror and are
reflected into your eyes.
Plane Mirror
• The image is virtual, right side up and
reversed.
• Virtual Image- a copy of an object formed
at the location from which the light rays
appear to come. (Rays do not really come
from behind the mirror)
Concave Mirrors
• A concave mirror is
curved inward. They can
produce both a virtual or
a real image.
• Real images are in front
of the mirror
• The point at which light
rays meet is called the
focal point.
Concave Mirror
Can make small objects appear larger
Ex: Make-up mirrors, shaving mirrors
Concave Mirror
• Real images occur when the
object is further away from the
focal point. Reflected rays
meet in front of the mirror.
• Virtual images occur when the
object is closer to the mirror
than the focal point. Reflected
rays spread out and appear to
be coming from behind the
mirror.
How do you locate the image in
a Concave Mirror
Pick a point on the object (usually the one furthest from the principal axis),
and then draw 2 intersecting rays that obey the following rules:
1. Any ray parallel to the principal axis is reflected through the focus.
2. Any ray through the focus is reflected parallel to the principal axis.
3.
Any ray through the center of curvature is reflected back along the
incident ray (back along itself)
Concave Mirrors
C
“OUTSIDE“
the focus
F
Moving towards the focus, the image is REAL, inverted. It could be smaller,
the same size, or larger than the image (depending on the object location)
Concave Mirrors – outside the
focus
Concave Mirrors
C
“INSIDE”
the focus
F
Moving towards the mirror, the image is VIRTUAL, UPRIGHT, and gets
smaller (although the image is ALWAYS larger than the object itself).
Concave Mirrors – inside the focus
Convex Mirror
• A convex mirror is
bent outward. The
object is virtual and
appears smaller and
upright.
• Convex mirrors
spread out light.
Convex Mirrors
Can make large objects appear
smaller (see a WIDE view)
Ex: Security mirrors, driveway
mirrors, car door mirrors
Convex Mirrors
Note: All rays
want to pass
through F, but
none do
F
C’
C
F’
When an object gets closer to the mirror, its image is VIRTUAL, UPRIGHT, and
keeps getting smaller (and the images are always smaller than the object).
Convex Mirrors
Refraction
• The index of refraction
for a material is the ratio
of the speed of light in a
vacuum to the speed of
the light in the material.
• When light enters a new
medium at an angle, the
change in speed causes
the light to bend or
refract.
Concave and Convex Lenses
• Lens- an object made of a transparent material that has
1 or 2 curved surfaces that can refract light.
• The curvature and the thickness affect the way it refracts
light.
Concave Lens
• A concave lens is curved
inward at the center and
the thickest part at the
outside edges.
• The light rays are spread
out.
• Smaller, upright, virtual
images are always
formed. The image is
formed at the point from
which the refracted rays
appear to come
Image Formation in Concave Lenses
F’
F
Concave Lenses
F
Convex Lens
• A convex lens is
curved outward at the
center and is thinnest
at the outer edge.
• Convex lenses form
either real or virtual
images.
• The real image is
upside down.
Convex Lenses
• Real images are produced when an object
is further away from the mirror than the
focal point (The object is also inverted.)
• Virtual images are produced when an
object is closer to the lens than the focal
point. ( The object is upright and larger.)
Image Formation in Convex Lenses
F’
F
Convex Lenses
F
F’
Type
Real / Virtual
Upright/
Upside-down
Smaller/Larger
Plane Mirror
Virtual
Upright
(Reversed)
Same
Concave Mirror
(Behind focal pt)
Real
Upside down
Depends on
location
Concave Mirror
(In front of focal pt)
Virtual
Upright
Depends on
location
Convex Mirror
Virtual
Upright
Smaller
Concave Lens
Virtual
(b/twn focus & lens)
Upright
Smaller
Convex Lens
(Behind focal pt)
Real
Upside down
Depends on
location
Convex Lens
(In front of focal pt)
Virtual
Upright
Larger
Cornea: Protective “Window” of
23.2 Light & Sight
The Human Eye
eye
Iris: Colored part that acts like a
camera shutter.
Pupil: Hole in the middle of the
iris.
Lens: has adjustable focal length.
Retina: Where image is formed.
Optic nerve: Sends image to
brain where it is flipped
upside down.
Muscles that “tense”
the lens
Eye Problems
• Nearsightedness • Farsightedness The eyeball is too
The eyeball is too
thick, causing the
thin, causing the
image to focus in front
image to focus behind
of the retina. A person
the retina. A person
can't see distant
can see distant
objects, but can see
objects clearly, but
near objects well. A
has difficulty with near
concave lens can be
objects. A convex
used to correct this
lens can be used to
problem.
correct this problem.
23.3 Light and Technology
Cameras ……
BOTH use converging
lenses with the object far
beyond the focus and both
form INVERTED images.
Film
….. and the human eye
Magnifying Glasses
2F’
F’
Larger, virtual image
that you see
Object that you are looking
at (under the lens)
F
2F
Film Projectors
White screen