Optics
The light ray model
A light ray is a line in the direction that
light energy flows. My laser beam for
example, is really a bundle of many parallel
light rays.
The ray model says
Why does light travel faster in a vacuum?
1. Light travels through a transparent
medium in straight lines, called light rays
at speed v = C/n, where n is the index of
refraction of the medium.
The ray model also says…
2. Light rays do not interact with each
other.
3. A light ray continues forever
unless it has an interaction with
matter that causes the ray to change
directions or to be absorbed.
The ray model also says…
4. Light has four different ways in
which it can interact with matter.
Medium 1
Medium 2
4. Light has four different ways in
which it can interact with matter.
Medium 1
Medium 2
4. Light has four different ways in
which it can interact with matter.
Medium 1
Medium 2
4. Light has four different ways in
which it can interact with matter.
Medium 1
Medium 2
An object is a source of light rays. We
make no distinction between self luminous
objects and reflective objects. Rays originate
from every point of the object, and each point
sends rays out in all directions.
To simplify the picture, we
use a ray diagram that shows
only a few important rays. A
ray diagram does not imply
that these are the only rays.
Luminous objects give off an infinite
number of light rays in all directions.
Our minds locate the object by sensing the
point where the lines diverge from.
Reflective Objects
Reflection
Diffuse Reflection
Reflection off Paper
Regular Reflection
Reflection off a
Mirror
Law of Reflection
i  r
Normal - an imaginary
line drawn perpendicular
to the surface
Reflection
Reflections in a Mirror
Q
An observer observes a light source S in a
mirror. Where does he perceive the mirror
image of S to be located?
●3
S
●4
●1
●2
An observer observes a light source S in a mirror.
Where does he perceive the mirror image of S to
be located?
●3
S
●4
●1
●2
How big does a mirror have to be in
order to see your entire length in it?
Half His Height!
To see more of her head in the mirror, she …
1. should hold the mirror closer
2. should hold the mirror farther away
3. needs a bigger mirror
Another way to
tell when you’re
dealing with
someone who
does not have a
grasp of even
the most
rudimentary
concepts of
physics.
Why does light from the sun or moon
appear as a column when reflected from a
body of water?
How would it appear if the water surface
were perfectly smooth?
If the water were perfectly smooth, a mirror image of the round
sun of moon would be seen in the water. If the water were slightly
rough, the image would be wavy. If the water were a bit more rough,
little glimmers of the sun or moon would be seen above and below the
main image.
This is because the water waves act like an assemblage of small
flat mirrors. For rougher waves, there is a greater variety of mirror
facets properly tilted to reflect the light into your eye. The light
then appears smeared into a long vertical column.
Calm Water
Calmer Water
Very Calm Water
Q
Refraction
Refraction
i
R
As light travels
from one medium
to another, the
speed of light
changes and the
light bends
accordingly.
Must
remember
Snell’s Law
Direction of refraction can
be predicted by thinking about
chariot wheels.
Refraction
i
R
Refraction
i
R
Hits the water
first, and slows
down
Refraction
i
R
Refraction
i
R
Refraction
i
R
Snell’s Law: The Law of Refraction
n1 sin I = n2 sinR
Index of Refraction of
the media in which the
angle of incidence is
Index of Refraction of
the media in which the
angle of refraction is
Index of Refraction
speed of lightin space
indexof refraction
speed of lightin m edia
Cannot be less than 1.00
Air = 1.00
Glass = 1.5
Water = 1.33
Q
Bruno wishes to “spear” a fish with a laser. Should he
aim the laser beam…
1. above
2. below
3. directly at
the observed fish to make a direct hit?
Light is incident on a block of glass as
shown. Which ray is the refracted ray?
1.
2.
3.
4.
Consider a fish with a flashlight
doing experiments….
Total Internal Reflection
•All the light will reflect from the surface.
•None of the light will refract (escape)
X
The angle of incidence where
total internal reflection first
takes place is called the
critical angle.
Example: Calculate the critical
Whenever the
angle for water to air.
n1 sin critical = n2 sin refraction
(1.33) sin critical = (1.00) sin 90o
sin critical = 1/1.33
critical = 48.8o
angle of
incidence is
greater than
48.8o, total
internal
reflection takes
place.
refraction = 90o
critical
Example: O.K. Let’s Calculate the
critical angle for air to water.
n1 sin critical = n2 sin refraction Clearly, this can’t be
(1.00) sin critical = (1.33) sin 90o done. You can only
have total internal
sin critical = 1.33/1.00
reflection going from a
high index of
critical
refraction to a low
refraction = 90o
index of refraction.
Q
Calculate the critical angle θ from glass
(n=1.5) to air.
1. 52.9o
2. 41.8o
4. 22.5o
5. 12.3o
3. 32.2o
θ
Calculate the critical angle θ from glass
(n=1.5) to air.
1. 52.9o
2. 41.8o
4. 22.5o
5. 12.3o
3. 32.2o
θ
n1 sin I = n2 sinR
1.5 sin I = 1.0 sin 90
sin I = .667
I = 41.8
Fiber Optics
Fiber optics uses total
internal reflection
Apparent Depth
Refraction
Should really be…