Transcript Light - PhysicsArts

Properties of Light
Light is a transverse wave.
Light is an electromagnetic
wave. It can travel in a
vacuum.
Light consists of oscillating
electric and magnetic fields.
Light always travels at ONE speed:
300,000,000 meters/second.
Sources of light
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Luminous
 Producing light
 The Sun (luminous) versus the Moon (nonluminous)
Incandescent
 Glowing with visible light from high temperatures
 Examples: flames, incandescent light bulbs
 Produce light via electromagnetic waves
 Electrical charge is accelerated by external force
 Acceleration produces wave consisting of
electrical and magnetic fields
 Wave moves through space; the fields exchange
energy and continue on until the energy is
absorbed by matter
A bright idea
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http://en.wikipedia.org/wiki/Thomas_Ediso
n
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http://rs6.loc.gov/ammem/edhtml/edhome.
html
Tutorials
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http://www.colorado.edu/physics/2000/ind
ex.pl
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http://www.colorado.edu/physics/2000/wav
es_particles/wavpart3.html
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http://www.colorado.edu/physics/2000/sch
roedinger/index.html
The nature of light
Wave-like behavior
Interference
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Young’s two slit experiment
Interference pattern - series of bright and dark zones
Explanation - constructive and destructive interference
Moving electric field creates magnetic field.
Moving magnetic field creates electric field.
The Electromagnetic Spectrum
Transparent Materials
When light hits a transparent material, the light is absorbed
and released.
The time it takes each atom to absorb and re-emit the
light causes a delay.
The apparent speed
of light slows down.
The real speed does NOT slow down.
Fermat’s Law determines the bending. The apparent
speed of light in a “thick” substance is slower than in a
“thinner” one. Light bends to minimize the time through
the thick substance.
The apparent slowing down of light is called the Index of
Refraction. The higher the index of refraction the slower
the light appears to go.
Speed of Light in Vacuum
Index of Refraction (n) 
Speed of Light in Material
If the frequency of light is close or the same as the
resonance frequency of the atoms, the light is absorbed
and held for a long time.
The energy is dissipated as heat as the atom continues
to collide with it’s neighbors.
Opaque Materials
Most materials are opaque. Light
is blocked.
Opaque materials absorb light but
do not re-emit it. The energy is
just what the atom is looking for
so it is absorbed.
Metals: Free electrons reflect light rather than pass it
on. That’s why many metals look shiny.
Earth’s Atmosphere
The atmosphere passes most light. Yellow light is an
example.
The atmosphere blocks other types
of light, infrared is an example.
The human eye is most sensitive to light frequencies
most common in the sun!
Shadows
Light likes to travel in straight lines
(rays). Shadows result when light is
blocked by an opaque object.
Shadows consist of
umbra and penumbra.
Penumbra caused by other light sources “filling in.”
Eclipses
The Human Eye
Rods see B&W
Cones see Color
Colors
Newton discovered that light is made up of various
colors. Each color refracts (bends) a different amount.
Reflection of Light
Your eyes see the color being reflected. All other
colors are being absorbed.
If the color of light is at the
resonance frequency of the
electrons, it is absorbed. If not, it
is reflected.
If the light of some colors is
absorbed, where does the energy
go?
HEAT!!
The absorbed energy raises the average kinetic energy of
the molecules in the object. The temperature goes up.
Selective Transmission
Transmission of color involves letting some frequencies
travel through the object while all others are absorbed.
Transmission of Light
A blue piece of glass looks blue because the dyes and
pigments in the glass absorb all colors except blue.
Red, Green, and Blue are the primary colors. Mixing
these colors in various combinations gives us all the
colors.
Complementary Colors
The primary colors for paints
and pigments are different.
These colors reflect light,
they are not producing light
themselves.
For reflected light such as paint, the primary colors are
Cyan, Magenta, and Yellow. These are called the
Complementary Colors.
You can add a primary color to a complementary color and
get white.
Red + Cyan = White
Green + Magenta = White
Blue + Yellow = White
Reflection
Light bouncing off of an object is called reflection.
Fermat’s Principle of Least
Time: Light always takes the
fastest path.
There are many ways for light to get from
Point A to Point B.
Which path is the fastest?
Law of Reflection: The Angle of Incidence
equals the Angle of Reflection.
Reflections are all around us.
Light bouncing off of a smooth surface is called a specular reflection.
Light bouncing off of a rough surface is called a diffuse reflection.
Specular Reflection
Specular reflection is a reflection from
a highly polished surface. Reflections
from a mirror or shiny metal are
specular.
A plane, or flat
mirror does not
distort the image.
Remember, you only
see light that hits your
eyes!
An image in a mirror appears
an equal distance behind the
mirror as it exists in front of
the mirror.
The image in the mirror
is not real, it is called a
virtual image.
Diffuse
Reflection
What is a “rough” surface? It
depends on the wavelength. A
radio telescope is specular for
long radio waves but extremely
diffuse for very short light waves.
Is the moon a specular or diffuse reflection?
How do we know the answer just by looking?
A curved mirror can focus light energy at a the
mirror’s focal point.
The same mirror can take rays that originate from a point and turn
them into plane waves.
Archimedes
invented
a device that
used a
curved mirror to
focus
the sun on
enemy
ships, setting
them
on fire.
Curved mirrors can create special effects.
Look at a spoon, it is concave on one side, convex on the other.
Refraction
Refraction is the bending of light as it travels between
two different substances.
Refraction as Illusion
Optical Illusions
Bending of
light
analogy.
Light rays
bending
like the
lawn mower
wheels.
Refraction and Dispersion
In real life, different frequencies (colors) of light bend
different amounts. Blue bends more than red. This leads to
dispersion.
Rainbows
Rainbows are caused by the dispersion of light in drops of
water. Rainbows form complete circles, the ground blocks
our view.
Dispersion from a Single Raindrop
A rainbow is the cumulative effect of dispersion
from many raindrops.
The rainbow you see is the edge of a three
dimensional cone.
The ground interrupts the circle turning it into an arc.
From the air, the full circular rainbow can be seen.
Lenses
Lenses are based on refraction (bending) of light.
Convex (Converging) lenses:
Thick in the middle.
Concave (Diverging) lenses:
Thin in the middle.
Eyeglasses correct
the distortions of
the eye’s lenses.
Why is the Sky Blue?
Molecules in the sky like to scatter light (Rayleigh
scattering). Light is absorbed and then re-emitted in all
directions.
The air is mostly Nitrogen and Oxygen. These
molecules like to scatter blue and violet light the
most.
Nitrogen
Oxygen
Why are the Sunsets Red?
The setting sun sends it’s light through a thicker
chunk of air. That air, closer to the ground, is
denser and dirtier. Most of the blue and violet light
are scattered away before the light gets to you.
Why are Clouds White?
A cloud contains water droplets and particles of many
different sizes. Each size likes to scatter light of different
colors. The end result is that all colors are scattered and
the result is white light.
Why is Water Blue-Green?
Although the water can appear blue by reflecting the sky,
that is not the real reason for water’s color.
Water absorbs infrared and reds and Rayleigh scatters
blue and green.
That blue-green scattering acts a lot like the color
of the sky.
Leaves in the spring and summer are green thanks to
chlorophyll used for photosynthesis.
Chlorophyll reflects green light but absorbs the
other colors.
In the fall, the chlorophyll breaks down and allows
carotenoids, reflecting red, yellow, and orange to
dominate.
Polarization
Polarization: Transverse waves vibrating in one
direction only.
A single vibrating electron creates a
plane-polarized wave
Herapathite is a crystal that can polarize light.
This crystal is used to create polarizing plastics.
Glare from reflections are often polarized
← Without Polarization
With Polarization →