Properties of Light

Properties of Light
Chapter 12

Joseph F. Alward, PhD
Department of Physics
University of the Pacific

Law of Reflection

Angle of Reflection
= Angle of Incidence

Angles are measured with
respect to the normal line
(the perpendicular line).

Reflection Law Example

Example Problem:

90 - 65 = 25
180 - 120 - 25 = 35
90 - 35 = 55

Billiards and Mirrors

A ball bouncing off the
bank of a pool table
behaves like a light
ray reflecting off a
mirror. To bank the
cue ball into another
ball (the object ball),
imagine you are
shooting at the mirror
image of the object
ball.

Billiards and Mirrors
This applet allows
placement and speed
of cue ball.

Specular vs Diffuse Reflection

Diffuse Reflection

Rough surfaces consist of a large number
of different specularly-reflecting planes.
Visible light reflects diffusely from
paper.

Specular and Diffuse Reflection

Rough surfaces seem smooth
for very long-wavelength
electromagnetic waves.

This parabolic dish reflects
long wavelength radio waves
to the detector, but reflects
most of the shorter wavelength
radiation away from the
detector.

Specular vs Diffuse Reflection

The cruiser Aurora, which
played an important role in
the communist revolution
in 1917, is docked on the
River Neva at St. Petersburg,
Russia.
When the water is still,
reflection is specular. The
image blurs when the
water is rough.

(Eugene Hecht, Physics)

Specular versus Diffuse Reflection

What kind of reflections
account for the column
of light reflected off
the water?

What would we see on
the water if it were
perfectly flat, unmoving?

Plane Mirror Geometry

Light from the castle appears to be
coming from a point behind the mirror

Virtual Images in Plane Mirrors

If light energy doesn't flow from the image,
the image is "virtual". Rays seem to come from behind
the mirror, but, of course, they
don't. It is virtually as if the rays
were coming from behind the
mirror.

"Virtually": the same as if

As far as the eye-brain system is
concerned, the effect is the same
as would occur if the mirror were
absent and the chess piece
(castle) were actually located at
the spot labeled "virtual image".

Image vs Object Distance

Flat mirror images are as far
behind the mirror as is the
object.

About 4 percent of light is
reflected off a windowpane.

The Law of Reflection

The girl in Edouard Manet's
painting, The Bar at the
Folies-Bergeres, is
standing in front of a large
plane mirror.

We see reflected in it her back
and the face of a man she
seems to be talking to.

From the law of reflection what
if anything, is wrong with this
painting?

(Eugene Hecht, Physics)

The Law of Reflection

The Toilet of Venus, by
Diego Velasquez.
What does Venus see
in the mirror?

(Eugene Hecht, Physics)

Left-Right Reversal

The woman's image is as
far behind the flat mirror
as she is in front of it.

Note the that "woman" in
the mirror is coming her
hair with her left hand.

Left-Right Reversal

The printing is reversed when viewed in a rear-
view automobile mirror.

Convex and Concave Mirrors

Images in convex mirrors Images in concave mirrors
are always smaller. are always bigger.

Convex Mirrors

Images in convex mirrors are always smaller
than the object.
(M.C. Eshcher)

Convex Mirrors

The Hubble Mirror

Images are always
smaller in a convex
mirror.

What type of mirror
is the Hubble mirror?

What is the person
in the mirror pointing
at?

Concave Make-Up Mirror

Concave Mirror Focuses Colors

White light is formed where red, blue, and green overlap.

Concave Mirror Focal Point

Rays' paths are determined by the angle law.

Applets

1. Concave Mirror Animation
2. Diverging Mirror

Refraction: The Bending of Wavefronts

One side of wave front slows down,
and the entire train of fronts twists.

Understanding Refraction

Refraction Terminology

Angles are measured with
respect to the line which is
perpendicular to the
interface.

When light travels from one
medium--such as air, to
another one of higher
density--such as water,
usually the light ray bends
toward the perpendicular
line.

Apparent Depth in Water

Light exits into
medium (air)
of lower index
of refraction,
and turns left.

More Apparent Depth

Spear-fishing is made more
difficult by the bending of
light.

To spear the fish in the figure,
one must aim at a spot in
front of the apparent location
of the fish.

A FishEye's View of the World
(Excellent applet illustrating refraction)

Refraction at Sunset

The sun actually falls below below the
horizon, i.e., it "sets", a few seconds
before we see it set.

Flattening of the Sun at Sunset

Bottom of sun appears flattened at sunset.
Rays from top of sun are also
refracted, but not as much because
they enter the atmosphere at a less
oblique angle. Thus, the top of the
sun is also flattened, but not as
much as the bottom.

Water on Road Mirage

There's no water on the road; why does it appear so?

Water on Road Mirage

Explaining the Water Mirage

Differential Color Refraction

Note that the shorter wavelengths of light are bent more than the longer; blue more than red.

Applets

Dispersion through Prism

Refraction in a Raindrop

Violet is bent more than red.

Forming a Rainbow

An observer sees
red light coming
from droplets of
water higher in
the sky, while
droplets of water
lower in the sky
send violet light
to the eye.

The Shape of the Rainbow

All of the droplets of water
along the arc shown in the
figure are equivalent.

A rainbow would form a
complete circle, not just
an arc, if the ground
didn't get in the way.

Understanding Rainbow Geometry

Rainbow

Rainbows always face the
observer. As the observer
moves, the rainbow moves.
One can never get to the
"pot of gold" at the end of
the rainbow.

Double Rainbows

The secondary rainbow
displays colors in the
reverse order.

Looking below about
42 degrees, the sky
is relatively bright, and
acts like a mirror to the
sunlight.

Above 42 degrees, the
water absorbs most of
the sunlight, and the
sky is darker.

Double Rainbows Explained

One reflection in a primary rainbow
droplet.
Two reflections in a secondary rainbow droplet.

Internal Reflection

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.

Internal Reflection and the Critical Angle

Critical angle is 48 degrees. Any ray which strikes the
surface from inside
the water at an angle
greater than 48 degrees will not escape the water.

Internal Reflection Applet

The View from Below the Water

Rays A, B, and C
from the bottom
of the pond are
totally internally
reflected.

Outside the 96-
degree cone, the
fish sees only
light reflected
from the bottom
of the pond.

A FishEye's View of the World
(Excellent applet illustrating refraction)

Internal Reflections in Prisms

Prisms in Binoculars

Magnifying power
of binoculars is
made larger by
increasing the
path length
traveled between
the two lenses.

Total Internal Reflection in Diamond

The critical angle for
diamond in air is
24.5 degrees

Internal Reflections in Diamond

The critical angle for diamond
in air is 24.5 degrees; any ray
which strikes the surface on the
inside at an angle of greater
than 24.5 degrees will not
escape the diamond.

Optical Fibers

Optical Fiber

Optical Fibers in Medicine

Arthroscopic Surgery Bronchoscope Colonoscope

Convex Lenses

The Convex Lens
as a Magnifier

Convex Lenses Used as Magnifiers

Concave Lenses

The De-Magnifier

Convex Lens is Inverse
of Concave Lens

Refraction

Surface of water is like a collection of lenses which change shape. Changing air
densities explain why stars twinkle.

Image Formation

Without angle-selectivity, light from all parts of the object overlap all parts of the wall.
Pinhole in box, or a lense directs rays from one part of the object to only one point
on the screen.

In bright light , the eyes' pupils becomes smaller, allowing sharper images to form.

Image Formation with Lenses

If object is
far from the
lens (beyond)
the focal
point, a real
inverted image
is formed.

Film Projectors

Camera Film Image

Applets

1. Converging Lens Interactive Applet
2. Diverging Lens Interactive Applet

Spherical Aberration

Rays away from the center are not focused at
the focal point.

Chromatic Aberration

Different colors refract by different
amounts.

Photoemission

Important Persons in Early 20th Century Physics

Solvay Conference, 1911 Einstein explained the
photoelectric effect by
assuming that
electromagnetic energy
(light) manifests itself as
quanta of energy--or,
"photons"--of energy hf:

E = hf

f = frequency of light

Duality of Light

Light acts like a wave when
it's moving from one place
to another, but when it
interacts with matter, it acts
like a particle.

Photocells in Garage Door Openers

Light to photocell
is interrupted, and
the corresponding
drop in photocurrent
signals the motor to
reverse.

Photocells in Movie Film

Optical sound track is like
a bar-code, but much more
detailed. Track modulates
the intensity of the light
at a frequency which is the
same as the sound which
was used to produced the
track.