Transcript 12.1 Refraction and 12.7 Phenomena

12.1 What is Refraction?
What is Refraction?
In a vacuum (air)
light travels in
straight lines.
But what happens
when light travels
from one material
into another?
Like for example,
from air to water?
What Happens When
Light hits a Medium
The straight-line path of light CHANGES!
Light bends (REFRACTS!!)
Your brain THINKS that the
light should travel in a
straight line. But it doesn’t.
Try This! Exploring
With Light - Page 515
1. Place a coin in the middle of a beaker. Fill
the beaker with water.
2. Look at the coin from the edge of the
beaker. Be sure that you are looking ABOVE
the beaker THROUGH the surface of the water
3. Aim a stir stick just inside the outer edge of
the coin so that it looks as though you are
going to just touch the coin. Place the stir stick
in the water and attempt to touch the coin.
4. Now look at the beaker from the side and
notice the position of the stir stick and the
position of the coin.
Why does this happen?!?
REFRACTION
The bending or
change in direction
of light when it
travels from one
medium into
another.
What Causes Refraction?
A light ray bends because light
travels at different speeds in
different mediums.
In a vacuum, light travels at a
speed of 3 x 108 m/sec
But when light travels through a
material, it is absorbed and
re-emitted by each atom or
molecule it hits.
What Causes Refraction?
This absorption and
emission slows down
the light’s rays
The more light is slowed
down the more it bends
Light only refracts at the
boundary
Speed of Light
Light slows down when it travels from air into another medium.
Medium
Speed
Vacuum
3.00 x 108m/s
Water
2.26 x 108m/s
Acrylic
1.76 x 108m/s
Rules of Refraction
Angle of Refraction: the
angle between the refracted ray
and the normal
1. The incident ray, the
refracted ray, and the normal
all lie in the same plane.
The incident ray and refracted
ray are on opposite sides of the
line that separates the two
media
Rules of Refraction
2. Light bends toward the
normal when the speed of light
in the second medium is less
than the speed of light in the
first medium (when light slows
down).
In other words...Light bends
TOWARDS the normal when
entering a more dense medium
Rules of Refraction
2. Continued...
Light bends away from the
normal when the speed of light
in the second medium is
greater (when light speeds up).
In other words...Light bends
AWAY from the normal when
entering a less dense medium
Partial Reflection & Refraction
Light can partially REFLECT and REFRACT at the same time
• Example: You can stand in front of a clear window and
(1) see your faint reflection in the window (partial reflection) and
(2) see through the window at the objects on the other side
(refraction)
•
Coating a glass
surface with a special
film coating creates
two-way mirrors. This
is used in making
mirrored sunglasses
and windows of many
buildings
12.7 Phenomena
Related to Refraction
Apparent Depth:
The depth that an object
appears to be at due to the
refraction of light in a
transparent medium.
•
Refraction causes the pencil to appear
closer to the surface than it actually is.
Objects under water always appear to be nearer to the
surface than they actually are. Apparent depth is an
optical illusion. This is what makes fish in water appear to
be closer to the surface than they actually are.
For the same reason, the legs
of someone standing in water
appear to be shorter
Flattening Sun
When the Sun is close to the horizon, light from the bottom of the
Sun is refracted more than the light from the top of the Sun. The
air is more dense near the Earth’s surface than higher up in the
atmosphere. So the increased density of air closer to Earth results
in a greater bending of the Sun’s rays.
Flattening Sun
The air is more dense
near the Earth’s surface
than higher up in the
atmosphere. So the
increased density of air
closer to Earth results in
a greater bending of the
Sun’s rays.
Mirage
Shimmering on a lake
Occurs when light travels at different speeds
through air layers of different temperatures.
Rainbow Dispersion


Dispersion: the
separation of white
light into its
spectrum.
Dispersion occurs
because each colour
of visible light travels
at slightly different
speeds when it goes
through a glass prism
Rainbow
Th rainbow is caused by a combination of dispersion and
partial internal reflection in water droplets in the
atmosphere.
Millions of raindrops are necessary to produce a rainbow
Page 519 #1-7
1. Refraction is the change in
direction of a light beam that
occurs when light enters a new
medium.
2.a) When light enters a new
medium, its speed changes causing
the light to travel in a new direction.
Light bends toward the normal if its
speed is slower in the new medium
and away from the normal if its speed
is greater in the new medium.
2.b) In order for light to be
refracted, it must travel from one
transparent medium into another,
and light must have a different
speed in each medium.
3. a) Medium A is air and medium
B is ice.
When light passes from one
medium into another, its path will
be closer to the normal in the
medium where lights speed is
slower, so medium B must be ice.
3.b) The diagram does not show
the direction in which the light is
traveling, but this does not matter
because the light would follow the
same path going either way.
4.a) Away form the normal
b) Toward the normal
5. Partial reflection and
refraction are illustrated
in Figure 11.
6. Mirrored sunglasses, energy
saving window coatings, domes
around ceiling-mounted security
cameras, security windows in
stores
7. One application of partial reflection and
refraction is the use of two-way mirrors that are
used in places where people on one side need to
be able to see through the mirror but do not
want people on the other side to be able to see
through.
This is the case for situations such as police
lineups, focus groups or other observations of
behavior in which those being observed might
behave differently if they could see the people
who are observing them.
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