Joseph F. Alward, PhD
|Applets in this eLecture:|
Ratio of Speed of Light in
Vacuum to Speed in Various
Substances (l = 589 nm )
Index of Refraction
| n = c/v
The larger the index,
the smaller the speed of
Index of Refraction
Reflection and Refraction at an Interface
Refraction: The Bending of Wavefronts
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.
Light twists inward when entering medium
of higher index of refraction
Light twists outward when entering medium
of lower index of refraction
Above: Wheels on axle rolls along a smooth
sidewalk and onto grass. Which picture
path is followed? What happens if the
motion is reversed?
Right: One side of wave front slows down,
and the entire train of fronts twists.
Illustrating Cart Analogy
Right front wheel slows Left front wheel slows
down before left front down before right front
Law of Refraction: Snell's Law
Right front wheel slows down first.
q2 = n1
Snell's Law Example
|n1 = 1.0 (air)
n2 = 1.52 (glass)
q1 = 30 degrees
n2 sin q2 = n1 sin q1
1.52 sin q2 = 1.0 sin 30
sin q2 = 0.33
Put calculator in
q2 = sin-1 (0.33)
= 19.3 degrees
Rear View Mirror
Rear View Mirror Explained
Apparent Depth in Water
Light exits into
of lower index
and turns left.
More Apparent Depth
|Spear-fishing is made more
difficult by the bending of
To spear the fish in the figure,
one must aim at a spot in front
of the fish
Refraction at Sunset
|Why does the sun appear to be
flattened at sunset?
The sun actually falls below below the
horizon, i.e., it "sets", a few seconds
before we see it set.
A Fish-Eye's View
FishEye's View of the World
(Excellent applet illustrating refraction)
Water on Road Mirage
There's no water on the road; why does it appear so?
The Oasis Mirage
Displacement through a Slab of Glass
Entering and exiting rays are displaced
from each other, but parallel.
| All rays reflect internally, but
the top three rays reflect only
a small percentage internally;
most energy leaves the prism.
The fourth and fifth rays are
reflected 100 % internally.
Ray 4 is the first to be 100 % reflected; it's angle of incidence is called the critical angle.
Crictical Angle Calculation
What must be q1 to get q2 = 90 deg ?
q1 = n2 sin
Cone of Light
|Crictical angle for water = 48.8 degrees
Light within the 48.8 degree cone
is detected by fish, while nothing in the
air outside that cone can be seen.
The only light reaching the fish outside
|Internal Reflection Applet|
Critical Angle of Diamond
|n = 2.419
90.00 - 24.42 = 65.58 degrees
Light outside of 65.58 degree
Optical Fibers in Medicine
Arthroscopic Surgery Bronchoscope Colonoscope
Index of Refraction vs Color
|v = c/n
The speed of blue light
The Spectrum of White Light
Note that the shorter wavelengths of light are bent more than the longer; blue more than red.
through Slab of Glass
2. Dispersion through Prism
Refraction in a Raindrop
Forming a Rainbow