Transcript light - Cloudfront.net

Physics
Mrs. Tobler
What is Light?
• Debated by scientists
– Dual “Personality”
• Acts like transverse waves
• Acts like it is made of
particles
–Photons (packets of
energy)
• Light is a form of energy
Light as a Wave
• Light travels as a transverse wave
– Made up of Electric and Magnetic Fields moving
perpendicular to one another
• Light is a range of frequencies of
electromagnetic waves
– A.K.A. Electromagnetic Radiation
Properties of
Electromagnetic Waves
• Travel at 3.0 x 108 m/s in a vacuum (empty
space)
• Light year = distance light travels in one year
(about 6 trillion miles)
– The light from a star 4 light years away traveled
for 4 years before reaching your eyes
– Light from our own sun takes 8 minutes to reach
Earth
– When you look at the stars you are looking into
the past
• Transmit energy via Radiation
– Don’t need a medium
• Slow down when it enters a medium.
– Diamond slows it down to 124 m/s
Visible Light
• Remember “ROY G BIV”
• Which has a longer wavelength Orange or
Blue?
– Orange
Complete the Spectrum
Uses for Electromagnetic
Waves
• Radio Waves
– Transmit TV, AM &
FM
– Satellite radio
– Cell phones
– RADAR for
communication and
police
• Microwaves
– Cook food by
exciting water
molecules to produce
heat
Uses Continued
• Infrared
– Keep food warm (like at McDonald’s or Luby’s)
– Night vision
• Ultraviolet
– Suntans, skin cancer
– Sterilize goggles and other items
– Fluorescence (Black Light)
• X-rays
– For looking at bones such as teeth or broken
arm
• Gamma
– To kill off cancer cells
– Can also damage healthy cells
Why can’t we see all
electromagnetic waves?
• Human eyes contain rods and cones
in the retina that are sensitive to
only a small range of frequencies
• We see only reflected or emitted
light
– Reflected = waves bounce off of a
surface back to our eyes
– Emitted = object produces light at
frequencies our eyes can detect
What color is white light?
• Isaac Newton
– Discovered white light
consists of various
colors of light in 1666
using prisms
• Made of the 3 Primary
Colors of Light
– Red
– Blue
– Green
How we See Color
• Our eyes detect the color
(frequency) of light that is being
Reflected off of a surface.
– If an object appears blue that means
blue light is bouncing off of the object
and the other colors of light are
absorbed
• What happens to the light if the
object appears white?
• What if it appears black?
More Fun with Color
• Primary colors of Pigments
• Magenta
• Cyan
• Yellow
– Color printers use these same colors
• Color Filters
– If an object reflects red light when
exposed to white light, then when
exposed to green light the object will
appear almost black because there is
NO red light to reflect.
Behaviors of Light
• All electromagnetic waves
experience the following wave
behaviors similar to mechanical
waves
–
–
–
–
Reflection
Refraction
Diffraction
Interference
• Constructive
• Destructive
– Polarization
Words to Know
• Ray = beam of electromagnetic radiation
(light)
• Normal = imaginary line that is perpendicular
to the surface of an object
• Incident Ray = emitted or incoming light ray
• Reflected Ray = light ray that bounces off a
surface; outgoing wave
• Angle of Incidence = angle at which the
incoming light ray hits a surface
• Angle of Reflection = angle at which
reflected light ray leaves the surface
Reflection of Light
• Occurs when a wave reaches a boundary
between 2 different mediums which
causes
– The wave to bounce back into the 1st
medium (totally reflected)
– Or some of the wave bounces back and some
is absorbed into the 2nd boundary (partially
reflected).
• Depends on how rigid the boundary is
– Very rigid then most of the wave is
reflected
– Less rigid then only some of the wave is
reflected
Law of Reflection
• When a wave hits
a boundary the
angle of incidence
equals the angle
of reflection.
– Also true when
playing wall ball,
basketball and
tennis
Complete the Reflection Diagram
Diffraction
• Bending of a wave around a barrier,
such as an obstacle or the edges of
an opening.
– Amount depends on the size of the
wavelength compared with the size of
the obstruction or opening.
• Wide opening = less bending
• Narrow openings = more bending
• Thomas Young’s Double Slit
Experiment
Diffraction Gratings
• A series of closely spaced parallel
slits or grooves
• Used to separate colors of light by
interference.
– Used instead of a prism
– Found in objects used to reflect
various colors of light
• Back of CDs & DVDs
• Bird feathers
Total internal Reflection
• At a certain angle (critical angle)—
no ray will emerge into the air
above the surface.
– For water it is 48 which does
not allow the ray to enter the
air, it is only reflected inside the
water.
– Other Examples
• Glass (43)
• Mirrors (90-95)
• Diamonds (24.6)
Review Refraction
• Bending of a wave as it enters a
new medium due to a change in
velocity.
– Angle of Incidence is NOT EQUAL to
the Angle of Refraction
• Refraction of Sound
• Even though speed of sound is greater in
warm air; sound waves bend away from
the ground which doesn’t allow it to carry
as well.
• In cold air sound bends toward the ground
and sound carries well.
Refraction of Light
• Less dense to More dense
medium
– Speed of light slows down
– Refracted Ray bends toward
the normal
• air to glass
• More dense to Less dense
medium
– Speed increases
– Ray bends away from the
normal.
• water to air
• As it leaves the new
medium it goes back to the
original angle.
Complete the Refraction Diagram
Observing objects in Water
Refraction of Light
Causes white light
to separate into the
primary colors as
it passes through
the water droplets
How are Rainbows Produced?
• Water droplets act like prisms
• Each color of light is refracted
at a different angle due to its
frequency
– Violet has the highest
frequency and is refracted
the most.
– Red has the lowest frequency
and is refracted the least.
• Sun has to be behind the
observer
• Circular rainbows can be seen
from airplanes
Atmospheric Refraction
• Mirage – caused
by change in air
temperature
resulting in light
refraction
– Road appears to
be wet
• Actually
reflection of
the sky
Why is the Sky Blue?
• Light from the sun
gets scattered as it
collides/passes
through molecules in
the atmosphere
– Blue light is
scattered the
most because of
its short
wavelength
Index of Refraction
• Unique to the substance
– Based on density
• Ratio between the speed of light in a
vacuum to the speed in a particular
material
c
n
v
n = index of refraction
c = speed of light in vacuum
(3x108 m/s)
v = velocity of light in
material
Example Index of Refractions
Material
Air
Water
Glycerin
Immersion Oil
Crown Glass
Flint Glass
Zircon
Diamond
Lead Sulfide
Refractive Index
1.0003
1.333
1.473
1.515
1.520
1.656
1.920
2.417
3.910
Polarization of Light
• The aligning of vibrations in an
electromagnetic wave, usually by
filtering out waves of other
directions.
• Polarized Filter
– Only allows waves through that are
parallel to the slits
• Occurs only with transverse waves
– Not all light is polarized
How Polarization Works
Fiber Optics
• A.K.A. Optical fibers or light pipes
• Thin glass fibers
– Use total internal reflection
• Developed for communication and
data transmission
– Replacing electric circuits and
microwave links
• Thousands of miles of it in our
phone lines
Polarized Sunglasses.
• Only allow light that is in the same
direction as its polarized axis to pass.
– If polarization axis of 2 filters are at right
angles to each other then the light can’t pass
through.
Traditional 3-D Glasses
• The red and blue
lenses filter the
two projected
images allowing
only one image to
enter each eye.
• Brain combines to
images to look
3-D
Modern 3-D Glasses
• Uses polarized
filters instead of
color
• Allow only one of
the images into
each eye because
each lens has a
different
polarization.