Download Refraction is the


Refraction is the bending of light as it
obliquely passes from one medium into
another.
Snell's law describes the bending of a light
ray as it passes from one medium to another,
for example from air to glass.
n1 sin 1  n2 sin 2
Willebrod van Roijen Snell
1580-1626
n1 sin 1  n2 sin 2

1 = Incident angle
2 = Refracted angle

n = index of refraction, also called the refractive index.

n is a measure of how dense the material is. The higher

the n the slower the light travels in that medium

It For free space, n =1. For any other material n > 1.
Light twists inward when
entering medium of higher index
of refraction and slows down.
Light twists outward when
entering medium of lower index
of refraction and speeds up.
Indices of Refraction (n)
Material
Refractive Index (n)
Air
1.00
Water
1.33
Glycerin
1.47
Immersion Oil
1.515
Glass
1.52
Flint
1.66
Zircon
1.92
Diamond
2.42
Lead Sulfide
3.91
Refraction is the “bending” of a light ray
n1 sin 1  n2 sin 2
Circular wave fronts
Plane wave Fronts
Produced in a Ripple Tank
Wave Fronts and Rays
1. Wave fronts are lines (or surfaces) marking the
location of the crests in a wave.
2. Rays are lines that show the direction of travel.
3. Rays are always perpendicular to wave fronts
spherical
Spherical
Wave fronts
Plane
Wave fronts
One side of wave front slows down, and the entire train
of fronts twists. Analogy: right front tire of vehicle enters
mud, twisting vehicle to the right.
Bend toward the normal and slow down entering the mud.
Wheels on axle rolls along a smooth
sidewalk and onto grass.
Which picture path is followed? What
happens if the motion is reversed?
Speed of light:
Index of
Refraction
In a vacuum 3x108 meters / second:
It’s not just a good idea, it’s the law!
speed of light in
the medium
n  1 so v  c
always!
c
v=
n
speed of light
in a vacuum
index of
refraction

n = index of refraction, also called the refractive index.

n is a measure of how dense the material is. The higher
the n the slower the light travels in that medium It For
free space, n =1. For any other material n > 1.
c
n
v
Speed of light in air
3x108 m / s
Speed of light in that medium
Index of Refraction (n)
c
v
n
1. The frequency of the light wave
does not change when it goes
from one medium to another
2. The wavelength and speed change:
 Low to high index, they both
decrease

High to low index, they both
increases.
Which index is higher
in this picture?
Indicies of Refraction (n)
Material
Refractive Index (n)
Air
1.00
Water
1.33
Glycerin
1.47
Immersion Oil
1.515
Glass
1.52
Flint
1.66
Zircon
1.92
Diamond
2.42
Lead Sulfide
3.91
c
n
v
* We will take the index of air to be one. Light does travel
slower in air than in vacuum, but the difference is so small
that we’ll just the say the speeds are essentially the same.
Light travels from air into an optical fiber with an index of
refraction of 1.44.
1. In which direction does the light bend?
2. If the angle of incidence on the fiber wall is 22o, what is
the angle of refraction inside the fiber?
3. Sketch the path of light as it changes media.
= 220
=1
=1.44
Def: Critical angle
The incident angle at which the refracted angle is 900
At some angle, (the critical angle), the light refracts at 900
and no longer escapes the medium it is traveling in.
The Critical angle only occurs when light goes from a
higher index of refraction to a lower index of refraction.
a. Calculate the critical angle for a ray of light entering air
from water.
n1 sin 1  n2 sin  2
(1.33)sin C  (1)sin 900
C
Water
(1)(1)
sin C 
1.33
C  48.80
b. What will the light do for any angle greater than 48.80?
The critical angle is the incident angle at
which the refracted angle equals 900
Total Internal reflection: When an incident ray is totally
reflected back into the medium. This occurs when the angle
of incidence is greater than the critical angle.
c = sin-1(1.00/2.42)
n = 2.42
= 24.4 degrees
90.0 - 24.4 = 65.6 degrees.
Light outside of 65.6 degree
cone is reflected back inside.
Virtually all light entering the
top face of the diamond is
reflected internally.
Fiber Optic Tubes
Optical Fibers in Medicine
Arthroscopic
Surgery
Bronchoscope
Colonoscopy
A fish can see out in a 960 cone due to total internal reflection,
at angles greater than that he sees a reflection of the bottom of
the water
Fiber optic cables are used extensively in communications,
one cable can carry many signals.
When Crocodile Dundee was young and ignorant of the physics of
refraction he continually was laughed at by villagers as he missed with his
spearing attempts. On one such attempt he was hunting the rare Fishus
Targetus. He tried to spear the fish by throwing the spear into the water a
horizontal distance of 2.6 meters away from him. The fish was actually
under water a horizontal distance of 1.3 meters from the point the spear
entered the water. Dundee aimed for the center of the fish's target
(scientists believe this has contributed to the fact that it is rare) and threw
the spear along the line of his eye sight from his height of 1.6 meters.
Determine the distance above the actual fish that Dundee's spear sailed
over the fish as Dundee aimed directly at the fish's image.
1. When light passes from a more optically dense medium
into a less optically dense medium, it will bend _______
(towards, away from) the normal.
2. When light passes from a medium with a high index of
refraction into a medium with a low index of refraction,
it will bend (towards, away from) the normal.
3. In each diagram, draw the "missing" ray (either
incident or refracted) in order to appropriately show
that the direction of bending is towards or away from
the normal.
SUMMARY:
n1 sin 1  n2 sin 2
c
n
v
 n2 
C  sin  
 n1 
1
Punch a small hole near the bottom of a 1-L soft drink bottle.
Fill the bottle with water. As the laser light exits the hole with
the water, the water will appear red. Secretly depositing a
drop of red food coloring in the pail can be humorous.
A polystyrene cup with a coin at the bottom is placed on a
table. A student is asked to stand far enough away so that the
coin is just out of view. As water is poured into the cup the
coin comes into view.