Chapter 27
Properties
of Light
Light

The two most commonly used models describe
light either as a wave or as a stream of particles.

Light produces interference patterns like water
waves.

Light can be modeled as a wave.
•
This model describes light as transverse waves that
do not require a medium in which to travel.
•
Light waves are also called electromagnetic waves
because they consist of changing electric and
magnetic fields.
Electromagnetic Waves

Electromagnetic Waves - energy-carrying
waves emitted by vibrating electrons
HowStuffWorks "How Light Works"

Also know as…

 Light
 Radiation
 Photons
These three are the same…

Light
*

Electromagnetic Waves
*

pure energy
energy-carrying waves emitted by vibrating
electrons
Photons
*
particles of light
The Electromagnetic
Spectrum
Radio Waves - communication
 Microwaves - used to cook
 Infrared - “heat waves”
 Visible Light - detected by your eyes
 Ultraviolet - causes sunburns
 X-rays - penetrates tissue
 Gamma Rays - most energetic

Electromagnetic
Wave Velocity

The speed of light is the same for all
forms of light.

It is 300,000 kilometers per second
or 186,000 miles per second.
Speed of Light
Speed of light varies with medium.
 Light slows down in water

 Ex. Straw or spoon appears bent in glass of water
The Electromagnetic
Spectrum

A range of light waves extending in
wavelength from radio waves to
gamma rays
The Visible Spectrum
A range
of light waves extending in
wavelength from about 400 to 700
namometers.
Transparent Materials

Transparent - the term applied to
materials through which light can
pass in straight lines
Opaque Materials


Opaque - the term applied to materials that
absorb light
When the absorption occurs, the vibrations
given to the electrons by the light are often
great enough to last for awhile, during which
the vibratory energy is shared by collisions
with neighboring atoms. The absorbed
energy warms the material.
Reflection
Light may strike a material and bounces
off - this is called reflection.
 Light shines on metal it sets its free
electrons into vibration and the energy is
reemitted as visible light.


When light is incident upon a material it
can behave in three ways:
 It
can bounce off
 It can be transmitted through the material
 It can be absorbed by the material

Usually a combination of all three occurs!
Transparent/Opaque

Is glass transparent or opaque to visible light?


Is glass transparent or opaque to ultraviolet light?



Answer: transparent
Answer: opaque
Atoms give up energy in form of heat as UV light interacts with
atoms in the glass. See Figure 27.7
How do UV light bulbs allow UV light transmission?

Answer: made of quartz glass
Example Questions

Are clouds transparent or opaque to
visible light?


Answer: opaque
Are clouds transparent or opaque to
ultraviolet light?

Answer: transparent
Shadows

Umbra - the darker part of a shadow
where all the light is blocked

Penumbra - a partial shadow

These terms also apply to Solar
Eclipses and Lunar Eclipses.

A solar eclipse occurs when the Moon
passes in front of the Sun.

A lunar eclipse occurs when the Moon
passes into the Earth's shadow.
Questions

Which type of eclipse is dangerous to
view with the unprotected eye?

Why are lunar eclipses more
commonly seen than solar eclipses?
Lab 70 “Shady Business”
SEEING LIGHT – PARTS OF 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 con

Fovea -center of vision, predominantly cones

Blind spot -optic nerve exit, no light sensors
Parts of the Eye

Detectors on the
Fovea
 Rods

light intensity sensitive
 Cones


color sensitive
The blind spot for the
eye is cause by the
optic nerve.
Color Deficiency
Myopia (Near-Sightedness)
People with near-sightedness cannot
see clearly at distance.
Hyperopia (Farsightedness)
People with far-sightedness cannot see
clearly up close.
Doppler Effect
and Expanding Universe
Doppler effect also occurs for light
waves from stars and galaxies.
When a light source approaches
earth there is an increase in its
frequency – called “blue shift”
When a light source goes away from
earth there is a decrease in its
frequency – called “red shift”
Polarized Light and 3-D
Polarizing filters transmit parts of incident
nonpolarized light that are parallel to the
polarization axis, and block components
vibrating at right angles to the polarization
axis. Emergence of polarized light.
 Lab 72 “Shades”
 There are a variety of 3-D viewing
techniques, all of which involve each eye
seeing a different view – binocular vision.
