Download Reflection and Refraction

Refraction:
TIR and
Dispersion
AP Physics: M. Blachly
Light and Optics
Nature of Light
What is light?
• Light is a wave
• But what is the medium?
• Applet
Light is also a particle
• The particle is called a photon.
• The particle nature of light was first proposed
by Einstein to explain the photoelectric effect.
Reflection
A mirror and a piece of white paper both reflect
essentially all light that hits them. What is the
difference?
Reflection
Mirror is example of specular reflection
mirror surface
Reflection
Diffuse reflection from a rough surface
Reflection
Microscopic view of
a sheet of paper
Reflections
The Law of Reflection
Reflection: Headlights
Why do oncoming headlights seem so bright?
Why don’t headlights work in the rain?
Transmission
Light can enter an optically transparent material.
The light slows down as it is absorbed and
emitted by the atoms
Speed of Light
c
n
v
n the called the index of
refraction
it can be thought of as the
“slow down” factor for light
Index of Refraction
Diamond
glass
EtOH
Water
Ice
Air
Vacuum
100%
80%
60%
40%
20%
0%
Index of Refraction
Material
Vacuum
Air
Ice
Water
EtOH
glass
Diamond
Index of Refraction
1.000
1.00029
1.310
1.330
1.360
1.520
2.417
Are you awake?
What is the speed of light in Water ?
Material
Vacuum
Air
Ice
Water
EtOH
glass
Diamond
Index of Refraction
1.000
1.00029
1.310
1.330
1.360
1.520
2.417
Refraction
What happens if a wave of light approached an
interface where it must slow down?
Refraction
Refraction is the bending of a wave when it
travels from one medium to another.
Animation showing how light bends
Refraction Examples
Slice your finger example
The bent straw
Spear fishing
Refraction Examples
Catching fish
n=1
n=1.5
Refraction Terminology
Refraction
Light bends towards the normal when it enters a
material with a slower speed (higher n)
n=1
n=1.5
Refraction
Light bends away the normal when it enters a
material with a higher speed (lower n)
n=1
n=1.5
Refraction: Snell’s Law
The angle of refraction
depends on the indices of
refraction, and is given by
Snell’s law:
Example Problem
n = 1.50
n = 1.33
Light, traveling in glass, strikes an optical
interface at an angle of 50  as measured from
the normal. What is the angle of refraction?
Refraction of Sound
Dispersion
Dispersion is the separation of light into colors by
refraction
Animation
Dispersion
Dispersion
The index of
refraction of a
material varies
somewhat with the
wavelength of the
light.
Dispersion
Most famous
example of
dispersion is the
rainbow
Rainbow
Rainbow
Total Internal Reflection
Simulator
When going from a slower to a faster medium,
light cannot escape if the angle is too big.
Applications
Diamonds
Applications
Light Pipes
Are you conscious still?
n = 1.00
n = 1.78
Light, traveling in plastic, strikes an optical
interface at an angle of 65. as measured from
the normal. What is the angle of refraction?
Total Internal Reflection
If light passes into a medium with a smaller
index of refraction, the angle of refraction is
larger. There is an angle of incidence for which
the angle of refraction will be 90°; this is called
the critical angle:
(23-5)
Total Internal Reflection
If the angle of incidence is larger than this,
no transmission occurs. This is called total
internal reflection.
Example
The index of refraction for glass is 1.33. What is
the critical angle for TIR for a beam of light
traveling in glass and striking an optical boundary
with air ( n = 1 )?
Total Internal Reflection
Binoculars often
use total internal
reflection; this
gives true 100%
reflection, which
even the best
mirror cannot do.
Total Internal Reflection
Total internal reflection is also the principle
behind fiber optics. Light will be transmitted
along the fiber even if it is not straight. An image
can be formed using multiple small fibers.
Dispersion
Wavelengths of visible light: 400 nm to 750 nm
Shorter wavelengths are ultraviolet; longer are
infrared
Dispersion
This variation in refractive index is why a prism
will split visible light into a rainbow of colors.
Dispersion
Actual rainbows are created by dispersion in tiny
drops of water.