Transcript 01 notes_em_and_light - McKinney ISD Staff Sites

Welcome Wednesday!
• DUE TODAY: PRACTICE 02 – SIMPLE
HARMONIC MOTION
• Pick up the packet titled “Notes 01- Light and
the Electromagnetic Spectrum”
• Grab a calculator
• Raytheon is coming on Friday….presentations!
• 3rd Quarter test – Next Monday
• Today’s notes are worth a grade
Light The EM Spectrum
2
What is electromagnetic radiation?
• A term used to describe all of the different
types of energies released into space by stars,
like our Sun
• All electromagnetic radiation (EM radiation)
travels in STRAIGHT LINES AND waves
• EM radiation includes visible light, radio
waves, UV rays, microwaves and more
• Speed of light in a vacuum is 3 x 108 m/s
• All electromagnetic waves travel at the speed
of light
Spectrum of Electromagnetic Radiation
Region
Wavelength
(Angstroms)
Wavelength
(centimeters)
Frequency
(Hz)
Energy
(eV)
Radio
> 109
> 10
< 3 x 109
< 10-5
Microwave
109 - 106
10 - 0.01
3 x 109 - 3 x 1012
10-5 - 0.01
Infrared
106 - 7000
0.01 - 7 x 10-5
3 x 1012 - 4.3 x 1014
0.01 - 2
Visible
7000 - 4000
7 x 10-5 - 4 x 10-5
4.3 x 1014 - 7.5 x 1014
2-3
Ultraviolet
4000 - 10
4 x 10-5 - 10-7
7.5 x 1014 - 3 x 1017
3 - 103
X-Rays
10 - 0.1
10-7 - 10-9
3 x 1017 - 3 x 1019
103 - 105
Gamma Rays
< 0.1
< 10-9
> 3 x 1019
> 105
4
Speed of light…
•
•
•
•
•
3 x 108 m/s
or
186,000 miles
Per
second
longer
lower
shorter
higher
6
Remember the following sentence:
• Radioactive Martians invade Venus using x-ray
guns.
• This sentence shows the order of the types of
electromagnetic radiation from lowest
frequency and energy to highest frequency
and energy .
• Radioactive – Radio, Martians – Microwave,
Invade- infrared, Venus – visible, Using –
ultraviolet, x-ray- x-ray, guns- gamma
Radio Waves
8
Radio Waves
Radio waves
bring music to your radio.
carry signals for your television
and cellular phones.
Radio waves are NOT sound
waves
9
Radio Telescopes
• Radio telescopes look at planets and comets,
giant clouds of gas and dust, and stars and
galaxies.
• Radio astronomy has the advantage that
sunlight, clouds, and rain do not affect
observations.
10
Radio Telescopes
In order to make better radio
images, astronomers often
combine several smaller
telescopes, or receiving dishes,
into an array.
Together, the dishes can act as
one large telescope
11
Radio Telescopes
The Very Large Array
(VLA) , located in
New Mexico, is one of
the world's premier
astronomical radio
observatories. The
VLA consists of 27
antennas arranged in
a huge "Y" pattern up
to 36 km (22 miles)
across -- roughly one
and a half times the
size of Washington,
DC.
12
Microwaves
Microwaves have wavelengths that can be measured in
centimeters
13
Microwaves
• Microwaves close to a foot in length are
the waves which heat our food in a
microwave oven.
14
Microwaves
• Microwaves are good for transmitting
information from one place to another
because microwave energy can
penetrate haze, light rain and snow,
clouds, and smoke.
This microwave tower can
transmit information like
telephone calls and computer
data from one city to another.
15
Microwaves
• Shorter microwaves are used for radar
like the doppler radar used in weather
forecasts. Microwaves, used for radar,
are just a few inches long.
16
Microwaves
• Radar is an acronym for "radio
detection and ranging". Radar was
developed to detect objects and
determine their range (or position)
by transmitting short bursts of
microwaves. The strength and
origin of "echoes" received from
objects that were hit by the
microwaves is then recorded.
17
The Infrared
Infrared light lies between the visible and microwave
portions of the electromagnetic spectrum.
30 micrometers = 0.000 03 meters
3 micrometer = 0.000 003 meter
18
The Infrared
Far infrared waves (longer
waves) are thermal.
we experience infrared radiation
as heat.
19
The Infrared
• Shorter, near infrared waves are not hot
at all - in fact you cannot even feel
them.
•These shorter
wavelengths are the
ones used by your
TV's remote control.
20
The Infrared
21
Visible Light Waves
• Visible light waves are the only electromagnetic waves we can
see.
• We see these waves as the colors of the rainbow. Each color
has a different wavelength.
• Red has the longest wavelength and violet has the shortest
wavelength.
• When all the waves are seen together, they make white light.
22
Ultraviolet Light Waves
Though these waves are invisible to the human
eye, some insects, like bumblebees and
butterflies, can see them!
23
Ultraviolet Light Waves
Our Sun emits light at all the different wavelengths
in electromagnetic spectrum, but it is ultraviolet
waves that are responsible for causing our
sunburns.
Black lights emit longer wavelengths of UV light
24
X Rays
•As the wavelengths of light decrease,
they increase in energy.
•X-ray light tends to act more like a
particle than a wave. X-ray detectors
collect actual photons of X-ray light
25
X Rays
X-rays were first
observed and
documented in 1895 by
Wilhelm Conrad
Roentgen.
26
Gamma Waves
Gamma-rays have the smallest
wavelengths and the most energy of
any other wave in the electromagnetic
spectrum. These waves are generated
by radioactive atoms and in nuclear
explosions
27
Sun in radio
waves
sun
Sun 2
Sun 3
otherl
pulsars
Sun in microwaves
Sun in infrared
Sun in visible light
Sun in
Ultraviolet
(UV) light
Sun in xrays
Sun in gamma
rays
Happy Thursday!
• Get notes out from last time
• Pick up calculator
• Get out sheet of paper to take a few extra
notes
• Due today:
• SHM worksheet
• Light and EM Notes/worksheet
• Raytheon tomorrow
• Doppler effect packet due Monday
• 3rd quarter review
Speed of Light
cf
C = speed of light (3.0 x 108 m/s)
λ = wavelength (m)
f = frequency (Hz)
A reminder: λ and f
• The relationship between λ and f
is
an inversely proportional relationship.
• This means that as λ gets bigger f gets
smaller, and as λ gets smaller f gets
bigger.
Example problem
• If a wave has a wavelength of 5.4 x 10-7 m,
what is its frequency in Hz?
Example problem
• If a wave has a wavelength of 5.4 x 10-7 m,
what is its frequency in Hz?
C=λ∙f
Example problem
• If a wave has a wavelength of 5.4 x 10-7 m,
what is its frequency in Hz?
C = λ ∙ f
3.00 x 108 = 5.4 x 10-7
∙
f
Example problem
• If a wave has a wavelength of 5.4 x 10-7 m,
what is its frequency in Hz?
C = λ ∙ f
3.00 x 108 = 5.4 x 10-7
5.4 x 10-7 5.4 x 10-7
∙
f
Example problem
• If a wave has a wavelength of 5.4 x 10-7 m,
what is its frequency in Hz?
C = λ ∙ f
3.00 x 108 = 5.4 x 10-7
5.4 x 10-7 5.4 x 10-7
f = 5.5 x 1014 Hz
∙
f
Diffraction of light
This is how the waves of light look when
Moving through a diffraction grating
Diffraction divides
light into different wavelengths
• Calculate the frequency of microwave
radiation if it has a wavelength of 12.3 cm.
(Don’t forget to convert cm into m)
Color
• The spectrum is what Newton called the
arrangement of visible light colors
• red light + blue light + green light = white
light
• White light is a combination of the spectrum of
colors, each having different wavelengths
Why does the light do this?
Why does
the pencil
look like
this?
Answers: Because of “refraction”.
Refraction is:
“the change in a wave’s direction and
speed that is caused by a change in the
medium.”
remember:
1. The “medium” is the material that the wave
travels through.
2. The only thing that can affect the speed of a
wave is if there is a change in the medium.
Characteristics of Refraction:
All types of waves refract:
Light waves, sound waves, water waves, etc.
Longer wavelengths change direction less.
Shorter wavelengths change direction more.
The speed of a water wave depends
on the depth of the water.
This means that water waves will refract (change
speed and/or direction) near a shoreline.
(This is why near the beach, the top of the wave “falls
forward” – it is moving faster than the bottom of the
wave!)
See how the waves aren’t parallel?
Parts of each wave have changed
direction due to refraction.
These waves changed direction a lot!
March 11, 2011
earthquake in Japan
 = 700
nm
Q: How does a prism cause the colors
to separate?
A: The longer the wavelength, the less it
is affected by refraction. The shorter
the wavelength, the more it is affected
by refraction.
 = 400
nm
Road Mirages are caused by refraction
(the air directly above the road is hotter than the
air that is a little higher up, so the light bends as
it travels through the layers of air)
http://www.youtube.com/watch?v=_M0FcpQWh5E
Slow down, Light!
• Light moving from less dense medium into a
more dense medium slows down.
– Example: when light goes from Air  Water
– Think of a truck on a smooth road suddenly hitting
a patch of sand at an angle
* First tire to hit slows down
* Truck turns because the tires Air
have different amounts of
Water
traction.
Θi
Θr
Speed up, Light!
• Light moving from more dense medium into
less dense medium speeds up.
– Examples: when light goes from Water  Air
– Think of a truck on sand suddenly hitting a a
smooth road at an angle
– First tire to hit speeds up
– Truck turns away from Water
normal
Θi
Air
Θr
Index of Refraction
• Ratio of speed of light in a vacuum (3 x 108 m/s) to
speed of light in a transparent substance is the index
of refraction (n).
• The larger the n, the slower light travels
• nair = 1 (memorize this!)
c
n
v
n = index of refraction (no
units)
c = speed of light (3 x 108
m/s)
v = velocity of light in the
transparent substance (m/s)
Sample Problem
• Ex: The hardest mineral is diamond, with an index of
refraction of 2.42. At what speed does light travel through a
diamond? What percentage is this of the speed of light in a
vacuum?
n = 2.42
c = 3 x 108
m/s
v=?
c
n
v
1.24 x 108 m
3 x 108 m
3 x 10 m
8
2.42 
s
s
v
v  1.24 x 108 m
s  41%
s
In a diamond,
light only travels
41% of the “speed
of light”
Index of Refraction
for some common substances
Vacuum
Air
Ethyl alcohol (ethanol)
Water
Diamond
Cubic zirconia (fake diamonds!)
Cornea (human)
Corn Oil
Crown Glass (pure)
Flint Glass (pure)
Pyrex (borosilicate glass)
Salt
Sapphire
1 (by definition)
1.000277
1.36
1.33
2.42
2.15 to 2.18
1.37 to 1.40
1.47
1.50 to 1.54
1.60 to 1.62
1.470
1.52
1.77
Critical Angle
• Critical Angle – the angle of incidence that
produces a 90°angle of refraction
Air n = 1
Θr = 90°
Water n = 1.33
Θc
Total internal reflection can be seen at
the air-water boundary.
Total Internal Reflection
• Occurs when a light ray tries to escape
for a dense medium into a less dense
medium. If the light ray strikes the
surface at an angle GREATER THAN the
critical angle then it can’t escape – it is
trapped inside the denser material.
• This is how fiber optics transfer info
along glass fiber
Total internal reflection:
The light waves can’t get out!
“Fiber Optic” cables use total internal
reflection to transmit data long
distances. (Cable TV & Internet)
Fiber Optic cables
–
the light finally
escapes
at the end
of the line.
Using Refraction to Create Invisibility
http://www.youtube.com/watch?v=BS5BPB4l3Eo
http://www.youtube.com/watch?v=KyWgnFm3ebc
Mirrors reflect, but lenses refract.
The exact shape of a lens, combined with
the refraction, is how light can be focused:
This type of lens can be used as a magnifying class, or
to start fires. The focal point gets very hot because all
of the light converges to that point.
(In Latin, the word “focus” means “fire place”.)
Refraction is how lenses are able to
focus images:
Light refracts in your eye at two different locations:
1st at the cornea and 2nd at the lens.
Inverse Square Law
• Light illumination follows the inverse-square
law (like gravity)
1
I 2
r
Reflection of Light
• Law of Reflection for smooth surfaces, is
called specular reflection
• Diffuse reflection for rough surfaces
Specular and Diffuse Reflection
• The chemical process of coating a glass
surface with metallic silver was discovered by
Justus von Liebig in 1835, before that they
coated the back of glass with tin and mercury,
before that they had shiny silver plates, or
very still water
• Mirrors became most popular with the French
3 types of mirrors
• Plane
• Concave
• Convex
Plain Mirror
Plain or Flat Mirror
• optical illusion of plane mirror animation
Refraction of light
• Bending of light as light
moves from one media
to another at an angle
• Draw diagram:
• Normal
• Angle of incidence
• Angle of refraction
Mediums
• Optically less dense – allows light to travel
faster
• Optically more dense – slows down the speed
of light,
Snell’s Law
• 1621, Dutch Scientist
ni Sini  nr Sinr
Fish in the water problem:
Periscope:
Index of refraction
• Every substance has its own index of
refraction
• It is the ratio of the speed of light in a vacuum
to the speed of light in a given transparent
medium
c
n
v
Demonstration!
• “n” is the index of refraction for that
substance
• “c” is the speed of light in a vacuum
• “v” is the speed of light in that substance
c
n
v
Facts you need to know
• Light travels at different speeds in different
substances
• Light bends as it changes media
• If c is slower in one medium than another, then the
medium is optically dense
• When light passes from a more dense to a less dense
medium, light will bend away from the normal
• When light passes from less dense to more dense
medium, light will bend toward the normal.
For example:
• a light beam in air hits a sheet of crown glass
at an angle of 30.0°. At what angle is it
refracted?
• Homework: p. 400 #1-4
Total Internal Reflection
 If the angle is smaller that the Critical Angle,
the light will refract
• If the angle of incidence is at the Critical
Angle, the light will move along the surface of
the medium
• If the angle of incidence is greater than the
Critical Angle, the light ray will reflect
• Total internal reflection occurs only when light
moves from from a medium of high index of
refraction to a medium of low index of
refraction
• For example: when light moves from water to
air
Critical Angle
• When a light ray at a boundary does not
refract, but moves parallel to the boundary
nr
Sin c 
ni
Practical Application
• Fiber Optics
• Total internal reflection only occurs when light
moves along a path from a high index of
refraction to a lower index of refraction
No Reflection and No Refraction
Refraction Causes:
• A sunset is a mirage
• Double moon is a mirage
• Optical illusions on hot days
Dispersion of Light
• White light being separated into the colors of
the visible spectra
• Prism
• Dispersion of light in water droplets cause a
rainbow
• rainbow animation
Homework
• P.411 #31-58