Where do
we get
light?
Luminous and
Illuminated Objects
A luminous object is one that produces light.
An illuminated object is one that reflects light.
Luminous Objects
Illuminated Objects
We see things because they
reflect light into our eyes:
Homework
How does light behave?
Reflection of Light
• Reflection occurs when a
wave bounces back after
striking a barrier.
• Example: a reflection in
a mirror.
Three things that affect how
light is reflected:
• Surface of
the Object
• Color of the
Object
• Shape of the
Object
Surface of the Object
Clear vs. Diffuse
Reflection
Smooth, shiny surfaces
have a clear reflection:
Rough, dull surfaces have
a diffuse reflection.
Diffuse reflection is
when light is scattered in
different directions
Color of the Object
Ever notice that an
asphalt driveway seems
hotter on a summer day
than a concrete
sidewalk?
This occurs because light
that is not reflected
from the surface of an
object may be absorbed
by the object and
converted into thermal
energy.
Shape of the Object
Reflection from a mirror:
Normal
Reflected ray
Incident ray
Angle of
incidence
Angle of
reflection
Mirror
The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at
____ _____ angle it hits it.
The
same !!!
Types of Mirrors
Plane (flat), Concave, & Convex
Plane Mirrors
• Flat Surface
• Light is
reflected
straight back,
resulting in an
upright image
that is the same
size as the
original object.
Concave Mirrors
• Curves Inward (like
the inside of a bowl)
• If an object is very
close to the mirror,
light is reflected in a
way that an enlarged,
upright image is
produced.
• If the object is very
far away, the image is
reduced in size and
upside down.
Convex Mirrors
• Curves Outward
• Results in an image
your eyes detect as
upright and
reduced in size.
• The side mirrors on
cars are convex
mirrors. (Objects
in mirror are closer
than they appear.)
Refraction of Light
• Refraction is
the bending of
a wave as it
passes from
one medium to
another.
What causes the light to bend?
What’s different about air and water?
DENSITY – the amount of mass in a certain
volume of a substance (mass/volume)
WATER
AIR
gas
liquid
Example of Refraction
When a straw is placed
in water it looks like
this:
In this case the light rays
are slowed down by the
water and are bent,
causing the straw to look
broken. The two mediums
in this example are air and
water.
Another Example:
Lens
When you use a
magnifying glass, the
object appears larger
because of refraction.
The light waves traveling
in the air change
direction when they
enter the glass of the
lens, and then again
when they move from
the glass back into the
air again.
Diffraction of Light
Diffraction is
the bending,
spreading, and
interference
of waves when
they go
through a
narrow opening.
Diffraction Patterns
• Radio waves can
diffract around hills,
mountains or even
the whole planet.
• Light waves can
diffract through tiny
slits.
• X-rays can diffract
around atoms.
Electromagnetic waves have a
huge range of wavelengths.
If the wavelength is of a similar
size to a gap in a harbor wall, then
the wave will diffract as shown
below.
If the wavelength does not match
the size of the gap, then only a
little diffraction will occur at the
edge of the wave.
Transmission of Light
Transmission is
the passing of
waves through
a medium.
Think about a radio station…
An electromagnetic wave is
produced.
The wave travels from the
stations transmitter out
in all directions at the
speed of light.
Even though you cannot hear
radio waves when your
radio’s turned off, the
waves are still being
transmitted in the room.
What happens when light
hits these objects?
•Glass of water
•School bus window
•Notebook paper
•Waxed paper
•Plastic wrap
•Tissue paper
•Cardboard
•Textbook
•Hand lens…
Many materials are classified by
how well they transmit light.
• Three Types of
Materials:
– Transparent
– Translucent
– Opaque
Transparent
A material
that permits
light to pass
through
Transparent objects:
–
–
–
–
–
–
–
–
The windows on a school bus,
A clear empty glass,
A clear window pane,
The lenses of some eyeglasses,
Clear plastic wrap,
The glass on a clock,
A hand lens,
Colored glass…
•ALL of these are transparent.
•Yes, we can see through them
because light passes through
each of them.
Translucent
A material that
transmits
some light but
also scatter
light in all
directions
Translucent objects:
–
–
–
–
–
Thin tissue paper,
Waxed paper,
Tinted car windows,
Frosted glass,
Clouds,
•All of these
materials are
translucent and allow
some light to pass
but the light cannot
be clearly seen
through.
Opaque
A material
that allows
no light to
pass through
Opaque objects:
–
–
–
–
Heavy weight paper,
Cardboard
Aluminum foil,
Mirror, bricks,
buildings,
– Your eyelids and hands,
– Solid wood door,
•All of these objects
are opaque because
light cannot pass
through them at all.
•They cast a dark
shadow.
Shadows
Shadows are places where light is “blocked”:
Rays of light