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Chapter 26
Properties of Light
Visible light originates
with accelerated motion
of electrons.
It is an electromagnetic
wave phenomenon.
1. ELECTROMAGNETIC
WAVES

Accelerating charges give off EM waves.

Demo - Tesla Coil, Radio, and Fluorescent Lamp

EM Wave – Next Slide
Electromagnetic Wave Velocity

A certain speed is required to satisfy
conservation of energy requirements.

That speed is c.

c = 300 million m/s = 186,000 mi/s

Large Number of Dots – Next Slide
50, 000 Dots

Maxwell discovered the nature of light.
Hollywood’s View of Light Speed
Jump to Light Speed
Hollywood’s View of Light Speed
Star Trek IV
THE ELECTROMAGNETIC
SPECTRUM
THE ELECTROMAGNETIC
SPECTRUM

Different kinds of sources produce
the different regions of the EM
spectrum.
THE ELECTROMAGNETIC
SPECTRUM
EM Waves
Radio Waves
Microwaves
Infrared
Sources
Vibrating charges
Molecular rotations
Molecular vibrations
Visible
Atomic vibrations
Ultraviolet
Atomic vibrations
X-rays
Atomic vibrations
Gamma rays
Nuclear vibrations

Video - Microwave

URL - X-ray of Hand

URL - X-ray of Lungs

Video - The Atomic Bomb

We live in an “ocean” of waves.

For more information visit URL1.
2. TRANSPARENT MATERIALS

Atom Spring Model – Next Slide

For glass the natural frequency is in
the UV.

Energy of EM wave in UV stays with
atoms longer causing vibrations with
neighboring atoms with loss of
energy.

Thus UV is absorbed.
For visible light the electrons in the
glass are forced into vibration, but at
less amplitude.
 The atom holds the energy for less
time, with less chance of collision
with neighboring atoms, and less
energy transformed to heat.


All visible frequencies get through
clear glass. (Not all their energies
do.)

There is an apparent slow down in the
speed of light.

Light in Transparent Medium – Next Slide

Besides the UV, heat waves (IR)
don't get through either.
TRANSLUCENT MATERIALS

Frosted glasses are good examples of
translucent materials.

They scatter light on transmission.

You can’t read or identify objects from
light passing through translucent materials.

Demo – Wax Paper
3. OPAQUE MATERIALS
Most things around us are opaque.
 Metals are shiny and reflect.
Because of free electrons - they
don’t pass on energy to atoms.
 Our atmosphere is transparent in
visible and part of IR.
 It's good that UV is absorbed in our
atmosphere. UV causes sunburns.

Demo - Radiometer and Filters
Penumbra
Umbra
SHADOWS






When light is blocked, shadows are created.
The kind of shadow (umbra or penumbra) depends
on the source of light and the relative positions of
the light source, object, and shadow of the object.
Extended light sources cause shadows of objects
to be partial shadows (penumbras) that may or
may not have umbras.
A point source of light will create a shadow that
only has an umbra.
A penumbra will be small if the light source is far
from the object and its shadow.
Demo – Shadows using Projector then Penlight
Sun
Umbra
Full
Shadow
Earth
Moon
Partial
Penumbra
Shadow
Solar Eclipse
Looking at the sun while the
moon passes in front of it.
Annular Eclipse
Again looking at the sun while the
moon passes in front of it.
Lunar Eclipse
New Moon Phase Full Moon Phase
Earth’s shadow
Looking at the moon while it
passes through the earth’s shadow.
4. SEEING LIGHT - THE EYE

Cornea - does most of the focusing

Iris - has the eye color and controls
light intensity

Pupil - the hole in the eye

Lens - does remainder of focusing

Retina - location of light sensors, has rods and cones

Fovea - center of vision, predominantly cones

Blind spot - optic nerve exit, no light sensors



Demo - Blind Spot
Primates and a species of ground
squirrel are the only mammals that
experience full color vision.
At the periphery of our vision we see
Motions first as objects come from
behind
Colors second
Rods are more sensitive to low light
levels than cones are.
 Examples: stars and twilight



Demo - Turn off Room Lights
Rods see blue better. Blue will appear
much brighter than red in dim light,
though the red might be much
brighter than the blue in bright light.
Red Eye Photography
Lateral Inhibition
We don't perceive the actual differences
in brightness in fields of light and dark.
We even out our visual field.
This allows us to discern detail in very
bright areas and in dark areas at the same
time.
We accentuate differences rather than
similarities.
Lateral Inhibition – Next Slide
Chapter 26 Review Questions
The speed of light
(a) has never been measured
(b) is about the same as that of sound
(c) is infinitely fast
(d) is very fast, but not infinite
You will observe a total eclipse
of the sun when
(a) you stand in the penumbra of the moon’s
shadow
(b) you stand in the umbra of the moon’s
shadow
(c) sunlight diffracts around the moon
(d) sunlight reflects from the moon to the
earth
Which of the following will
cast a shadow that has an
umbra but no penumbra?
(a) the sun
(b) an incandescent lamp
(c) a fluorescent lamp
(d) a point source of light
What is the purpose of the
pupil in an eye?
(a) to adjust the focal length of the
lens
(b) to adjust the amount of light
passing through the lens
(c) to focus the image
(d) to change the width of the field of
view