Transcript LIGHT

· From the Fenway Park bleachers,
why do you see Jacoby Elsbury hit the
baseball before you hear it?
LIGHT
What is Light?
Early theories of light varied. One was that your
eyes emitted beams that would illuminate things for
you. That was why you could only see when your
eyes were pointed at something(~500 BC).
Eventually people came to believe that light was either a
wave or a bunch of moving particles.
Einstein coined the term “photon” which represents the
tiny massless particle that light may exist as.
What is light?
The fact of the matter is that light behaves in certain ways so
that currently it can only be described as both a wave and a
particle.
Therefore, it is appropriate to discuss light as if it were a wave.
Electromagnetic radiation (electromagnetic waves):
Measuring the speed of light
Can you think of a way that a person might measure the
speed of light?
During early attempts to measure the speed of light,
scientists would up just measuring their own reaction time.
Measuring the speed of light
Using the relative motion of Earth toward and away from Io
(the innermost moon of Jupiter), Roemer was able to calculate
the speed of light to an amazingly accurate approximation
(1675).
Measuring the speed of light
Michelson
•Accurately measured the speed of
light on earth in 1880
•first American to win the Nobel prize
in 1907
•reflected light from a mirror 35 km
away
•spinning octagonal mirror allowed
him to measure the time it took
•299,920 km/s 300,000 km/s
Measuring the speed of light
http://www.shep.net/physics/
http://fmp.shep.net/physics/Physics_20
/UNIT_4/Animation/michelson.html
Measuring the speed of light
Just how fast is light?
• Speed of light in a vacuum is constant in
universe
• 7.5 round trips around the earth in one
second
• 8 minutes from the sun to the earth
• 4 years from the nearest star, Alpha Centauri
• 100,000 years to cross our galaxy
• some galaxies are 10 billion light years away
Measuring the speed of light
A “light year” is a measurement of distance based on how far
light can travel in a single Earth year. How far is a light year?
Distance = velocity x time
Light year = 300,000 km/s x 1 year
= 300,000 km/s x 1 yr x 365 day/yr x 24 hr/day x 3600 s/hr
Light year = 9.5 x 1012 km
Electromagnetic Spectrum
• Light is energy that is emitted by
vibrating electric charges
• called an electromagnetic wave
• radio waves, microwaves, infrared,
ultraviolet, x-rays, gamma rays are also
electromagnetic waves
• lowest frequency we see is red
• highest frequency we see is violet (more
energetic)
ROY-G-BIV
R
ed
O
range
Y
ellow
G
reen
B
lue
I
ndigo
V
iolet
The Doppler Effect and Light
What is the Doppler Effect?
How do we perceive the Doppler Effect for light waves?
In what ways can scientists use this knowledge?
Red Shift
Blue Shift
When we see
•Light has a very high
frequency
–100 trillion times/second
–1014 hz
When light waves hit a material a number of things can
happen depending on the resonant frequency of the
particles of the material and the type of light that hits it.
Light hitting an object causes its electrons to vibrate
Transparent to Light
• Transparent: lets light pass through in a
straight line
• Glass and water are transparent to light
• Spring and ball model of light
transmission
– light causes springs and balls to vibrate
– vibration transfers within the material
– energy is released on the other side
http://www.physicsclassroom.com/mmedia/waves/em.html
Transparent to Light
Glass Example:
Glass has a natural frequency similar to that of UV rays.
When UV rays hit glass it resonates and the amplitude of
vibration increases.
BUT! Glass doesn’t transmit UV rays (in other words you are
safe from sunburn due to UV radiation if you sit by a closed
window). Glass likes to hold onto UV radiation. But what
happens to that energy then?
Results in heat
Transparent to Light
Particles in glass hold onto frequencies lower than UV rays for
less time than UV rays. As the light gets transmitted from atom to
atom through the glass, the relative speed of light reduces to
lower than c.
So, glass does transmit visible light but doesn’t transmit UV rays
and IR rays.
The speed of light (c)
•Speed of light = c = 300,000 km/s
•speed of light in glass = 0.69 c
•speed of light in water = 0.75 c
•speed of light in diamond = 0.40 c
Just like all other waves, the speed of light in different materials
has different speeds, so as light goes from on material to
another, the speed changes!
Opaque Materials
An opaque material absorbs certain frequencies of visible
light but reflects other frequencies (it does not transmit light).
We perceive the color of something based on the colors that
get reflected off an opaque material.
Rays
• A thin beam of light is a ray
• Any beam of light can be considered a
bundle of rays
• Rays travel in straight lines
Example: Lasers are like “ray” guns
Shadows
• A shadow is formed when a light ray
cannot reach a surface
• sharp shadows
– produced by small source close by
– large source far away
• total shadow: umbra
• partial shadow: penumbra
– light from another source fills in
– large source only partially blocked
solar eclipse and lunar eclipses are
examples of shadows on a large scale
Polarization
• Light is a transverse wave
• Light from most sources vibrates in all
planes
• Each light ray can be considered to
have horizontal and vertical
components
• Separating vertical and horizontal
components is called polarization
Polarization
• Polarizing filters are
like sewer gratings
that look like slits.
• Light waves vibrating
in the plane of the slit
can make it through
• Light waves that
vibrate perpendicular
to the grates cannot
make it through
Polarization
• A single polarizing filter will let about
one half of the light through
• Two polarizing filters aligned in the
same direction will still let about one half
of the light through
• Two polarizing filters aligned
perpendicular to one another will let
almost no light through
Applications of Polarizing
Filters
• Sun Glasses
– reduce glare
– block out half of the light
• 3-D movies
Why is the sky blue?
Where do rainbows come from?
Diffraction
• When waves bend
around an object
• The smaller the object
compared to the
wavelength, the more
the bending.
• Wavelength and
velocity stay the
same.
Diffraction of Light
Radio waves can be broadcast through cities or over far
distances due to diffraction.
What type of wave gets diffracted more?
· Why is it hard to pick a penny off of the
bottom of a swimming pool?
Why is it difficult to catch a fish with a spear? (think of why it
your hand looks all squishified when you look at it in a sink full
of water?)
Refraction
• Refraction is the bending of the wave when it
goes into a different medium.
http://images.google.com/imgres?imgurl=http://
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Light can be bent as it passes past barriers. This is how a
mirage occurs. It is the bending of light as it passes through
different temperatures of air.
And it is why some roads look wet on a hot day
Reflection - how a mirror
works
• Smooth surface
• angle of incidence of ray equals angle
of reflection - no matter the shape of the
mirror (law of reflection)
• All light is reflected, none is absorbed
Plane mirror
Bill looks at Emily’s reflection in the mirror.
Draw the position of Emily’s image. Draw the
path of the light rays that travel from Emily to
Bill.
Emily
Bill
Concave Mirrors: (like a cave!)
Satellite dishes are like concave mirrors
shaped like a parabola
Image from a concave mirror depends on the placement
of the object in relation to the mirror
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=48
As you begin to learn to drive you notice your mirrors say
“objects in mirror are closer than they appear.”
What the heck?
Convex Mirrors
• A convex mirror is
shaped like the outside
bottom of a spoon.
• Convex mirrors are
used as rearview
mirrors in cars and in
stores to observe
shoppers.
http://micro.magnet.fsu.edu/primer/java/mirrors/convexmirrors/index.html
Both of these types of mirrors still obey the law of reflection