Transcript Document 7381508

Chapter 18: Electromagnetic Spectrum & Light
18.1: Electromagnetic Waves
• Question: What do x-ray machines,
microwave ovens, and heat lamps have in
common with police radar, TV, and
radiation therapy???
Electromagnetic Waves
• Answer: They all use WAVES to transport
energy from one location to another!!!
Electromagnetic Waves
• Electromagnetic Waves- (EM) transverse
waves consisting of changing electric fields
and changing magnetic fields
Electromagnetic Waves
• Can carry energy from one place to another
• Produced by constantly changing fields
• Magnetic and electric fields travel at right
angles to each other
EM Waves
• Electromagnetic waves are produced when
an electric charge vibrates or accelerates.
• As fields regenerate, their energy travels in
the form of a wave.
• Unlike mechanical waves, EM waves do not
need a medium to travel through!
– EM waves can travel through a vacuum (or
empty space) or matter.
EM Radiation
• Electromagnetic Radiation- the transfer of
energy by electromagnetic waves traveling
through matter or across space.
THE SPEED OF EM WAVES
• Question: Why do you see lightning before
you hear thunder?
Speed of EM Waves
• Answer: Because light travels faster than
sound!
• But how much faster is light???
How long would it take you to drive
from San Francisco to New York?
Speed of Light Analogy
• Scientists have discovered that light and all
electromagnetic waves travel at the same
speed when in a vacuum  3 x 108 m/s!
• Consider driving non-stop at 60 mph from
NYC to San Francisco.
– This trip would take you ~50 hours
– Light travels this distance in less than 0.02
second!!!
Speed of EM Wave
• Speed of EM wave = wavelength x
frequency
– Wavelength is inversely proportional to
frequency
– As the wavelength increases, the frequency
decreases
Differences between EM Waves
• Even though all EM waves travel at the
same speed, it does not mean they are all
the same!
– EM waves vary in wavelength and frequency
18-2: The EM Spectrum
Prism Experiment
• In 1800, William Herschel used a prism to
separate the wavelengths present in
sunlight. He produced a band of color: red,
orange, yellow, green, blue, indigo and
violet.
EM Spectrum
• The full range of frequencies of
electromagnetic radiation is called the
electromagnetic spectrum
• Which includes the following parts:
– radio waves, infrared rays, visible light,
ultraviolet rays, X-rays, and gamma rays.
EM Spectrum
• Each kind of wave is characterized by a
range of wavelengths and frequencies.
Radio Waves
• Radio waves have the longest wavelengths
in the EM spectrum, from 1mm to 1000’s of
km. They also have the lowest frequencies,
300,000 mHz or less.
– used in radio, TV, microwaves, and radar
Radio Waves
• In a radio studio, music and voices are changed
into electronic signals that are carried by radio
waves.
– AM radio stations broadcast by amplitude
modulation, the amplitude of the wave is varied
– FM radio stations broadcast by frequency
modulation, the frequency of the wave is varied
• A station is lost when its signal becomes too weak
to detect, an FM station is more likely to be lost
because FM signals do not travel as far
Difference between AM & FM
More Applications of Radio Waves
• Radio Waves also include the application of:
– Television
• Radio waves also carry signals for TV, including the
information for pictures
– Microwaves
– Radar (Radio Detection and Ranging)
• Short bursts of radio waves that reflect off objects they
encounter and bounce back, being detected by a radio receiver
Infrared Radiation
• Infrared waves have higher frequencies than radio
and wavelengths that vary from about 1mm to
750nm
– Used as a source of heat and to discover areas of heat
differences
– Invisible to our eye
– Warmer objects give off more infrared than cooler, a
device called a thermograph create thermograms (colorcoded picture) that show temperature variation
– Thermograms can be used to find places where a
building loses heat, search and rescue teams use
infrared cameras to locate victims
Infrared Radiation
Visible Light
• The visible part of the EM spectrum is light the
human eye can see.
– Each color of the visible spectrum corresponds to a
specific frequency and wavelength (ROYGBIV)
Ultraviolet (UV) Rays
• The wavelengths of ultraviolet rays very from
400nm to about 4nm, and higher frequencies than
violet light.
– In moderation, UV rays help your skin produce vitamin
D
– Excessive exposure can cause sunburn, wrinkles, and
skin cancer
– Used to kill microorganisms
– Plant nurseries use UV to help plants to grow during
winter
UV Radiation
X-Rays
• X-rays have very short wavelengths from about
12nm to 0.005nm , and have higher frequencies
than UV
– Have high energy and can penetrate matter that light
cannot
– Used in medicine (pictures of bones), industry (test
sealed lids), and transportation (contents of truck
trailers)
Gamma Rays
• Gamma rays have the shortest wavelengths about
0.005nm or less, and have the highest frequencies
and therefore the most energy and the greatest
penetrating ability
– Used in the medical field to kill cancer cells, in brain
scans, and in industrial situations such as inspecting
pipelines for sign of damage
– Overexposure can be deadly
Gamma Rays & Radiotherapy
The normal cells receive a lower dose of gamma radiation
than the cancer cells, where all the rays meet. Radiotherapy aims to
kill the cancer cells while doing as little damage as possible to
healthy normal cells.
18.3 Behavior of Light
Question to Ponder…
• What would you see if you were snorkeling
in warm ocean waters over a coral reef?
You might see fish of bright colors, clown
fish, sea stars, etc. Why can you see these
animals SO CLEARLY??? Why can you
see the reef through the water but not
through the bottom of the boat that brought
you to the reef???
Light & Materials
• Without light, nothing is visible!
– When you look at the reef animals, what you
are really seeing is LIGHT
– You can see the reef through the water, because
LIGHT passes through the water between the
reef and your eyes.
– You can’t see the reef through the bottom of the
boat because LIGHT doesn’t pass through the
boat!
Behavior of Light
• How light behaves when it strikes an object
depends on many factors…including the
material it is made of.
• Materials can be:
– Transparent
– Translucent
– Opaque
Transparent
• Transparent: material through which you can see
clearly, transmits light
– Most light is able to pass through
– Examples: water, windows
Translucent
• Translucent: you can see through the
material, but the objects you see through it
does not look clear or distinct.
– Scatters Light
– Examples: some types of jello, certain bars of
soap, frosty windows
Examples of Translucent
Opaque
• Opaque: material either absorbs or reflects
all of the light that strikes it.
– NO light is able to pass through
– Examples: fruit, wooden table, metal desk
Interactions of Light
• When light encounters matter, some or all of the
energy in the light can be transferred to the matter.
And just as light can affect matter, matter can
affect light.
• When light strikes a new medium, the light can be:
– Reflected
– Absorbed
– Transmitted
Reflection
• When you look in a mirror, you see a clear image
of yourself.
– An image is a copy of an object formed by reflected (or
refracted) waves of light.
• Two types of reflection:
– Regular Reflection
– Diffuse Reflection
Regular Reflection
• Regular Reflection: occurs when parallel light
waves strike a surface and reflect all in the same
direction
– Occurs when light hits a smooth, polished surface
– Mirrors or surface of a still body of water (page 547,
figure 18)
Diffuse Reflection
• Diffuse Reflection: occurs when parallel light
waves strike a rough, uneven surface, and reflect
in many different directions
– Paper has a rough surface, (page 547, figure 18)
– Rough surfaces causes diffuse reflection of the
light that shines on it
When Light is TRANSMITTED
• Reflection occurs because there is no
transmission of light (light is not able to
pass through to the new material)
• However, when light is transmitted different
things can happen. Light can be:
– Refracted
– Polarized
– Scattered
Refraction
• Refraction: ability of light to refract, or bend
when it passes at an angle from one medium into
another.
• Two easily observable examples that occur when
light travels from air into water:
– Underwater objects appear closer and larger than they
really are
– Can make an object such as a skewer (or pencil) appear
to break at the surface of the water (page 548, figure
19)
Refraction
Refraction Can Create a Mirage
• Refraction can sometimes cause a mirage.
• Mirage: a false or distorted image.
• Mirages occur because light travels faster in hot
air than in cooler, dense air
– On a sunny day, air tends to be hotter just
above the surface of a road than higher up
– Mirages also form this way above the hot sand
in deserts
Examples of Mirages
What is Polarization?
Polarization
• Light is an EM Wave  EM waves vibrate in
TWO planes
• Light waves that vibrate in only one plane is
called polarized light.
– Polarizing filters transmit light waves that vibrate in
only one direction or plane (page 548, figure 20)
– Unpolarized light vibrates in ALL directions
Polarization
Scattering
• Earth’s atmosphere contains many molecules and
other tiny particles. These particles can scatter
light.
• Scattering: light is redirected as it passes through
a medium (page 549, figure 21)
Scattering explains a red/pink sunset!
• Scattering effect reddens the sun at sunset
and sunrise
– Small particles in the atmosphere scatter shorterwavelength (blue light) more than light of longer
wavelengths
– By the time the sunlight reaches your eyes, most of the
blue and even some of the green and yellow have been
scattered
– Most of what remains for your eyes to detect are the
longer wavelengths of light, orange and red
Scattering of Light by Atmosphere