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Chapter 19
Reflection
 The smooth surface of the lake reflects light rays so
that the observer sees an inverted image of the
landscape.
Law of Reflection
 The law of reflection states that the angle of reflection
is equal to the angle of incidence.
Angle of Incidence & Reflection
The angle of incidence is the angle the incident ray
makes with a line drawn perpendicular to the surface of
the mirror.
The angle of reflection is the angle the reflected ray
makes with the perpendicular line.
Incident ray
 The flat (plane) mirror
and the mirror-like lake
surface both obey the
law of reflection.
Angle of incidence
Angle of refraction
Refracted ray
Plane Mirror
 A plane mirror always produces a virtual image.
 A virtual image is a copy of an object formed at the
location from which the light rays appear to come.
 A mirror with a flat surface is a plane mirror.

When you look into a plane mirror, you see your reversed
reflection—a right-left reversed image of yourself.
Plane Mirrors
To produce your image in a mirror, rays of light
strike you and reflect.
• The reflected rays strike the mirror and are reflected to
your eyes.
• The rays appear to come from behind the mirror.
• Your image appears the same distance behind the mirror
as you are in front, and the image is right side up.
Concave vs Convex
 Concave mirrors can form either real or virtual
images.
 Convex mirrors always cause light rays to spread out
and can only form virtual images.
Concave Mirrors
Concave Mirrors
When the inside surface of a curved mirror is the
reflecting surface, the mirror is a concave mirror.
• The curvature of the reflecting surface causes the rays to
come together.
• The point at which the light rays meet is called the focal
point.
Concave mirrors in automobile headlights and
flashlights direct the illumination from a light bulb
into a beam.
Concave Mirrors
The type of image formed depends upon where the
object is in relation to a concave mirror. When the
object is farther from the mirror than the focal
point, the reflected rays meet in front of the mirror
to form a real image.
• A real image is a copy of an object formed at the point
where light rays actually meet.
• Unlike a virtual image, a real image can be viewed on a
surface such as a screen.
Images
 When the object is closer to the mirror than the focal
point is, the reflected rays spread out and appear to
come from behind the mirror to form a virtual image.
Concave Mirror
 Parallel incoming rays are reflected through the focal
point of a concave mirror.
Focal
point
Concave mirror
Concave Mirror
 Real images form when the reflected light rays
converge.
Focal
point
Concave mirror
Concave Mirror
 Virtual images form when the reflected rays appear to
come from a point behind the mirror.
Focal
point
Concave mirror
Convex Mirrors
Convex Mirrors
When the outside surface of a curved mirror is the
reflecting surface, the mirror is a convex mirror.
• The curvature of the convex mirror causes the reflected
rays to spread out. The image formed by a convex mirror
is always upright and smaller than the object.
• A convex mirror shows a wide angle of view.
• Convex mirrors can only form virtual images.
Convex Mirror
 Parallel incoming rays are reflected away from one
another by a convex mirror.
Focal point
Convex mirror
Convex Mirror
 The virtual images are upright and smaller than the
object.
Focal
point
Convex mirror
Lenses
 When light enters a new medium at an angle, the
change in speed causes the light to bend, or refract.
Light usually travels in straight lines. In a vacuum,
light travels at a speed of 3.00 × 108 meters per
second.
• The speed of light in a different medium depends on the
material of the new medium.
• Air allows light to pass through almost as fast as it would
through a vacuum.
Index
of
Refraction
A light ray bends (refracts) as
it passes through media with
different indices of
refraction.
The ray again travels in its
original direction when it
reenters the air.
Index of Refraction
The amount of refraction depends upon the
difference between the speeds of light in the two
media. 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.
• A low index of refraction (near 1) causes light to slow
and refract very little.
• Diamond (index of refraction = 2.42), causes significant
refraction.
Lenses
 Concave lenses always cause light rays to spread out
and can only form virtual images.
 Convex lenses form either real or virtual images.
Concave Lenses
Concave Lenses
A lens is an object made of transparent material that has
one or two curved surfaces that can refract light.
A concave lens is curved inward at the center and is
thickest at the outside edges.
Concave Lenses
 When parallel incoming rays strike a concave lens,
they are refracted away from one another.
Optical axis
Focal point
Concave lens
Concave Lenses
 As the light rays diverge after passing through the
concave lens, they form a virtual image of the object.
Object
Focal point
Virtual
image
Concave lens
Focal point
Convex Lenses
Convex Lenses
A convex lens is curved outward at the center and is
thinnest at the outer edges.
• As the rays pass through the lens, each one is refracted,
and they emerge at different angles.
• Convex lenses cause incoming parallel rays to come
together, or converge.
Convex Lenses
 Parallel rays are refracted and pass through the focal
point of a convex lens.
Focal
point
Optical axis
Convex lens
Convex Lenses
 When an object is located beyond the focal point, a
real image is formed.
Focal
point
Object
Focal
point
Convex lens
Real
image
Convex Mirrors
 A magnified, virtual image is formed when the object
is located between the focal point and the lens.
Virtual image
Focal
point Object
Convex lens
Focal
point
Focal
point
Total Internal Reflection
The critical angle is the angle of incidence that
produces an angle of refraction of 90 degrees.
• At the critical angle, the light ray bends so much that it
takes a path along the glass-air boundary.
• At angles larger than the critical angle, the light ray
bends so much that it is reflected back into the glass.
Total Internal Reflection
Total internal reflection is the complete reflection
of a light ray back into its original medium. An
important application of total internal refraction is
fiber optics.
• Light rays are generally unable to exit through the sides
of the curving fiber optic strands.
• Fiber optics are able to transmit data in the form of light
pulses over large distances with little loss in signal
strength.
Total
Internal
Reflection
A. For angles less than the critical angle, light is partly
refracted and partly reflected.
B. At the critical angle, the angle of refraction is 90 degrees.
C. For angles greater than the critical angle, all of the light is
reflected.
Eyes
 Light passes through the cornea, pupil, and lens before striking
the retina. Signals from light-sensitive nerves on the retina are
sent through the optic nerve to the brain.
Sclera (white of eye)
Retina Choroid (membrane
containing blood
supply and a dark
pigment that reduces
reflection of light
within the eye)
Cornea
Iris
Pupil
Lens
Aqueous humor
(clear fluid that
maintains shape of
front of eye)
Vitreous humor
(gelatinous filling that
maintains eye shape)
Blind spot
Blood vessels
Optic nerve
Nearsightedness
 When the eyeball is too long, the focused image forms
in front of the retina. By the time the image reaches
the retina, it is no longer in focus.
Problem: Nearsightedness
(Eyeball is too long.)
Image forms in front of retina.
Correction: Eyeglasses
with concave lenses
Concave lens
Image forms on retina.
Farsightedness
 Farsightedness occurs when an image is not focused
before it reaches the retina. Farsightedness can be
corrected by using a converging (convex) lens in front
of the eye.
Problem: Farsightedness
(Eyeball is too short.)
Image forms behind retina.
Correction: Eyeglasses
with convex lenses
Convex lens
Image forms on retina.