Ch. 29
Reflection and Refraction
Reflection:
When a wave reaches a
boundary between 2 media,
some or the entire wave
bounces back into the first
medium.
Example:
is a slinky that is held stationary
at one end and is wiggled at the
other end. The wave goes
down the slinky and then
reflects off the stationary end
and comes back down the
slinky. This is total reflection.
Partial reflection happens when
a wave is not totally reflected
back into the first medium.
Examples of this are in your
book on page 443.
You have a small slinky hooked
up to a large slinky. The wave
starts down the small slinky and
when it hits the large slinky part
of the wave travels into the
large slinky and some of it
reflects back down the small
slinky.
Reflection in one and two
dimensions.
If I bounce a ball straight down
then the ball will hit the ground
and bounce straight back up
into the air.
This is one dimension reflection.
If I throw the ball at an angle the
ball will hit the floor and bounce
away at the same angle that it
came in at.
This is two dimensional
reflection.
Light does the same thing.
In chapter 27 we hopefully
learned that a RAY is a thin
beam of light.
There are different types of
rays.
Incident Ray: (in blue)
a ray of light coming toward a
surface.
Reflected Ray: (in red)
A ray of light reflecting away
from a surface.
So the manner in which light
travels to your eye as you view
the image of an object in a
mirror can be summarized as
follows.
To view the image of an object in a
mirror, you must sight along a line
at the image. One of the many rays
of light from the object will approach
the mirror and reflect along your
line of sight to your eye.
Normal:
A line that is perpendicular to the
surface.
Angle of Incident:
the angle between the incident
ray and the Normal.
Angle of Reflection:
the angle between the reflection
ray and the Normal.
Angle of Incident and angle of
Reflection should be identical.
Law of Reflection:
The angle of incidence for a
wave strikes a surface is equal
to the angle of reflection.
Check your understanding;
Consider the diagram. Which one
of the angles (A, B, C, or D) is the
angle of incidence? Which one of
the angles is the angle of
reflection?
A ray of light is incident towards
a plane mirror at an angle of 30degrees with the mirror surface.
What will be the angle of
reflection?
When you look into a mirror you
see yourself in the mirror
because you image is reflected
off the mirror
Virtual image:
An object formed through
reflection or refraction that can
be seen by an observer but
cannot be projected on a screen
because light from the object
does not actually come to a
focus.
Your body can not tell if it is
seeing a real image or a virtual
image.
The distance the image appears
behind the mirror is equal to the
distance as the object is in front
of the mirror.
Diffuse Reflection:
When light is incident on a
rough surface, it is reflected in
many directions.
Most of the reflected light we
see is diffuse reflected light.
Most objects that light hits will
be rough, or not polished.
Sound reflects also.
If you hear an echo you are
hearing the reflection of sound.
Acoustics:
The study of the reflective
properties of surfaces
In concert halls you will normally
see curved areas on the walls
and ceiling. This allows for the
sound to be partially absorbed
and partially reflected.
If the sound is not partially
absorbed you would get
reverberations which are
multiple reflections.
Reverberations make sound
garbled and mix all together.
People who design concert halls
have to make sure they reflect
enough of the sound so the
audience can hear but not
reflect too much as to garble out
the sound.
Refraction:
The change in direction of a
wave as it crosses the boundary
between 2 media in which the
wave travels at different speeds.
Sound is a good example of
refraction.
Speed of sound is dependent
on the type of medium and the
temperature of the medium.
The air around us is not all the
same temperature. You can
have several different changes
in air temperature. These
changes in temperature cause
sound to curve away from
traveling in a straight line.
Sound always curves toward
the cooler air.
At night, when the air is cooler over
the surface of the lake, sound is
refracted toward the ground and
carries unusually well.
Light is also refracted
As light travels through a given
medium, it travels in a straight
line. However, when light
passes from one medium into a
second medium, the light path
bends; refraction takes place.
Refraction occurs when light
changes speed as it passes
from one medium to another.
Light waves slow down when
passing from air to water.
Soft drinks are often served in a
thick mug because refraction
through the thick glass creates
the illusion of having more
beverage than is actually
present.
Dispersion:
separation of light into colors
according to their frequency.
Read about rainbows on pages
455 - 457