Transcript File - Science with Ms. Tantri

Refraction
•
•
Changing of speed of a wave (light) when it travels from
one medium to another.
When light travels from one medium into another at an
angle, the difference in speed causes a change in
direction of the light.
When light travels from air into this glass block, it bent towards the
normal. What do you think will happen when light travels from the
block back into air?
Refraction
Incident Ray
Angle of
Incidence
For media of different optical
densities,
θi
Medium 1
θ(r) ≠ θ(i); for θ(r) ≠ 0 º
Medium 2
Angle of
Refraction
θr
Refracted Beam
Example:
Water is optically denser than air. This means that light travels more
slowly in water than in air.
Light travelling from air into water with an incident angle of 56º will
bend, towards the normal, with an angle of refraction of 38º.
Refraction
• How can you predict whether light will bend
towards or away from the normal?
Light traveling from a less
dense medium to a more
dense medium, light bends
TOWARDS the normal
Less Dense
More Dense
Light traveling from a more
dense medium to a less
dense medium, light bends
AWAY from the normal
More Dense
Less Dense
Index of Refraction (n)

Ratio of speed of light in a vacuum to the speed of
light in a medium.

The higher the index of refraction:
◦ The higher the medium’s optical density
◦ The slower light travels in the medium
Speed of light in a vacuum (c)
Speed of light in medium (v)

Example:
The speed of light in a solid is 1.24 × 108 m/s.
a) Determine the solid’s index of refraction.
b) Identify the material, using Table 1 on pg. 524 of your
textbook.
Refraction Effects: Apparent Depth

Objects that are under water appear to be
closer to the surface than they actually are.
• This is due to the
refraction of light
that travels up
through the water,
and out into the
air towards your
eyes.
Refraction Effects: Lateral Displacement

Refraction can also shift light rays over to
one side.
• When light passes from air into water
then back into air, the emergent ray
is parallel to the incident ray, but no
longer moves in the same path.
This stick appears “broken”:
can you explain why?
Applications of Refraction
Inferior Mirages

Occur when cooler layers of air
lie above warmer air (close to ground!)
◦ The index of refraction of air decreases from sky to ground
◦ As light travels downwards, it refracts away from the normal
◦ Light is eventually totally internally reflected upward from one
of the layers
◦ We perceive a reflective (shiny) surface, which is actually an
image of the sky reflected upward
“from the road”.
Why does the road look wet in this
photograph?
Applications of Refraction
Superior Mirages

Occur when denser layers of air lie below less dense
air
◦ The index of refraction of air increases from sky to ground
◦ As light travels downwards, it refracts towards the normal
◦ We perceive a virtual image
A photo of a superior
mirage taken in
Finland.
Applications of Refraction
Sunsets

The index of refraction of the atmosphere
increases moving from the outermost layer to
the ground
◦ Light travelling at a large angle to the layers of the
atmosphere causes greater refraction than when
the sun is directly overhead
◦ Refraction causes the sun to appear
flattened when it is close to the horizon
◦ Refraction also prolongs sunsets
Applications of Refraction

More on sunsets…
Images of the rising sun near Colorado Springs, Colorado.
Earth Science Picture of the Day, http://epod.usra.edu/
Snell’s Law
SNELL’s LAW
n1 sin θ1 = n2 sin θ2
Snell’s Law

Predicts the amount that light bends due to a change
in speed
n1
θ1
n1 sin θ1 = n2 sin θ2
n2
θ2

Example:
Light travels from air into an unknown solid at an
angle of incidence of 45˚. The angle of refraction is
27˚.
 Determine the index of refraction of the solid.
 Identify the unknown solid.