Transcript The Electromagnetic Spectrum

Light and the Electromagnetic
Spectrum
Chapters 22 & 23
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LIGHT: What Is It?
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• Light Energy
– Atoms
• As atoms absorb energy, electrons jump up to a higher
energy level.
• Electrons release light when falling down to the lower
energy level.
– Photons - bundles/packets of energy released
when the electrons fall.
• Light: Stream of photons
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• An electromagnetic wave consists of electric and
magnetic fields which vibrate thus making waves.
• Energy is perpendicular to direction of motion
• Moving photon creates electric & magnetic field
which are at right angles to each other.
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LIGHT: Particles or Waves?
• Wave Model of Light
– Explains most properties of light
• Particle Theory of Light
– Photoelectric Effect – Photons of light
produce free electrons
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Light Waves
• Properties of light waves include speed,
frequency and wavelength
• Speed (s), frequency (f) and wavelength
(l) are related in the formula l x f = s
• All EM waves travel at a speed of 300,000
km/s in a vacuum
• EM waves will travel at slower speeds
through medium such as air, water and
glass
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Wavelength, Frequency and
Energy
• Light with a short wavelength will have a
high frequency and high energy.
• Light with long wavelengths have low
frequencies and low energy
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Chapter 22 Section 2
The Electromagnetic Spectrum
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Electromagnetic Spectrum
• The electromagnetic spectrum is the complete
spectrum or continuum of light including radio
waves, infrared, visible light, ultraviolet light, Xrays and gamma rays
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Radio waves
• Low energy waves with long wavelengths and
low frequencies
• Includes FM, AM, radar and TV waves
• Used in many devices such as remote control
items, cell phones, wireless devices, etc.
• AM has longer wavelength, is used for audio
portion of TV broadcasts and its long wavelength
can bend around hills.
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Modulating Radio Waves
• Modulation – variation of amplitude or
frequency when waves are broadcast
• AM – amplitude modulation
• FM - frequency modulation
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AM and FM
• AM – carries audio for TV broadcasts and
its longer wavelengths bend around hills
and tall buildings
• FM – carries video for TV broadcasts
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Microwaves
• First used in radar, now
used in communication,
medicine and consumer
use (microwave ovens)
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Infrared waves
• Invisible electromagnetic waves
that are detected as heat
• Can be detected with special
devices such as night goggles
• Used in heat lamps
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Visible Light
• The portion of the electromagnetic spectrum
that human eyes can detect
• ROY G BIV (red, orange, yellow, green, blue,
indigo, violet)
• Red is the lowest frequency and violet is the
highest frequency
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Ultraviolet waves
• Higher energy than light waves
• Can cause skin cancer and blindness in
humans
• Used in tanning beds, food processing and
sterilizing equipment in hospitals, food
processing
• Helps your body absorb Vitamin D
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X-Rays
• High energy waves
• First discovered by
Roentgen
• Used in medicine,
industry and astronomy
• Can cause cancer
• Bones absorb x-rays but
soft tissue does not
• Lead absorbs X-rays
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Gamma rays
•
•
•
•
Highest energy
Shortest wavelength
Come from outer space
Blocked from Earth’s surface
by atmosphere
• Can be used in cancer
treatments
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Time to think…..
•
1. What is the relationship
between frequency and
wavelength?
•
2. What is meant by ‘spectrum’?
•
3. What does ROY G BIV mean?
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Now, let’s really think
• 4. Can you actually see x-rays?
– Support your answer.
• 5. Which color is more energetic, red
or yellow?
• 6. Which type of wave travels faster,
gamma or radio?
• 7. Why are microwaves more
dangerous than radio waves?
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Think about it….
• You have just been involved
in a traffic incident that
leaves you stranded on the
side of the road. Which part
of the electromagnetic
spectrum would be of the
most use to you and why?
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Answers
• 1. Frequency and wavelength are properties of waves and since
speed is constant for EM waves, as frequency increases,
wavelength decreases.
• 2. Spectrum is a continuum of all electromagnetic waves
• 3, ROY G BIV is the difference colors of the visible light in order of
longest wavelength to shortest wavelength.
• 4. X-rays can not be seen, only the waves in the visible light portion
are visible.
• 5. Yellow is higher energy than red because it has a shorter
wavelength and higher frequency.
• 6. Both travel at the same speed, 300,000 km/s (all em waves travel
at the same speed)
• 7. Microwaves have a higher frequency than radio and carry more
energy.
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Chapter 22 Section 3
Interactions of Light
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Refraction of Light
• Refraction – Bending of light due to a change
in speed.
– Index of Refraction – Amount by which a material
refracts light.
– Prisms – Glass that bends light. Different
frequencies are bent different amounts & light is
broken out into different colors.
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Refraction – bending of light
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Diffraction & Refraction
• Diffraction – Bending of waves around the
edge of a barrier. New waves are formed
from the original. breaks images into
bands of light & dark and colors.
• Refraction – Bending of waves due to a
change in speed through an object.
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LIGHT & USES: Diffraction
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• A diffraction grating. Each space between the ruled grooves acts as a
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slit.
light bends around
the edgesSpectrum
and gets refracted.
Reflection
• Reflection – Bouncing back of light
waves
– Regular reflection – mirrors smooth
surfaces scatter light very little. Images
are clear & exact.
– Diffuse reflection – reflected light is
scattered due to an irregular surface.
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Chapter 22 Section 4
Light and Color
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Color of Light
• Transparent Objects:
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– Light transmitted because of no scattering
– Color transmitted is color you see. All other
colors are absorbed.
• Translucent:
– Light is scattered and transmitted some.
• Opaque:
– Light is either reflected or absorbed.
– Color of opaque objects is color it reflects.
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Color of Light (Cont.)
• Color of Objects
– White light is the presence of ALL the
colors of the visible spectrum.
– Black objects absorb ALL the colors and no
light is reflected back.
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Color of Light (Cont.)
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• Primary Colors of Light
– Three colors that can be mixed to
produce any other colored light
– Red + blue + green = white light
• Complimentary Colors of Light
– Two complimentary colors combine
to make white light-Magenta,Cyan,Yellow
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Paint Pigments
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– Pigments absorb the frequency of
light that you see
– Primary pigments
• Yellow + cyan + magenta = black
• Primary pigments are compliments
of the primary colors of light.
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Complementary
Pigments
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• Green, blue, red
• Complimentary
pigments are
primary colors
for light!
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Chapter 23 Section 1
Mirrors and Lenses
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• Enlarged –
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– Image is larger than actual object.
• Reduced –
– Image is smaller than object.
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• Erect –
– Image is right side up.
• Inverted –
– Image is upside down.
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L
• Real Image –
– Image is made from “real” light rays that
converge at a real focal point so the image
is REAL
– Can be projected onto a screen because
light actually passes through the point
where the image appears
– Always inverted
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• Virtual Image–
– “Not Real” because it cannot be
projected
– Image only seems to be there!
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Mirrors
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– Optical Axis – Base line through the center
of a mirror or lens
– Focal Point – Point where reflected or
refracted rays meet & image is formed
– Focal Length – Distance between center of
mirror/lens and focal point
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Mirrors
• Plane Mirrors – Perfectly flat
– Virtual – Image is “Not Real” because it
cannot be projected
– Erect – Image is right side up
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– Convex Mirror
• Curves outward
• Enlarges images.
– Use: Rear view mirrors, store
security…
CAUTION!
Objects are closer
than theySpectrum
appear!
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Lenses
• Convex Lenses
– Thicker in the center than edges.
– Lens that converges (brings together) light
rays.
– Forms real images and virtual images
depending on position of the object
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LIGHT & ITS USES:
Lenses
• Convex Lenses
• Ray Tracing
Focal Point
Object
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Lens
– Two rays usually define an image
• Ray #1: Light ray comes from top of object;
travels parallel to optic axis; bends thru focal
point.
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LIGHT & ITS USES:
Lenses
• Convex Lenses
• Ray Tracing
Ray #1
© 2000 D. L. Power
– Two rays define an image
Ray #2
• Ray 2: Light ray comes from top of object &
travels through center of lens.
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Concave Lenses
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Lens that is thicker at the edges and
thinner in the center.
– Diverges light rays
– All images are erect and reduced.
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Chapter 23 Section 2
Light and Sight
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How You See
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• Retina –
– Lens refracts light to converge on the retina.
Nerves transmit the image
• Rods –
– Nerve cells in the retina. Very sensitive to
light & dark
• Cones –
– Nerve cells help to see light/color
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How You
See
• Near Sighted –
Eyeball is too long
and image focuses in
front of the retina
• Far Sighted –
Eyeball is too short
so image is focused
behind the retina.
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Eye is a convex lens
• Nearsightedness – Concave lenses expand
focal lengths
• Farsightedness – Convex lenses shortens
the focal length.
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Chapter 23 Section 3
Light and Technology
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LIGHT & ITS USES
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• Sources of Light
– Incandescent light –
light produced by
heating an object
until it glows.
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LIGHT & ITS USES
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• Fluorescent Light –
– Light produced by electron bombardment
of gas molecules
– Phosphors absorb photons that are
created when mercury gas gets zapped
with electrons. The phosphors glow &
produce light.
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LIGHT & ITS USES - Neon
• Neon light – neon
inside glass tubes
makes red light.
Other gases
make other
colors.
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LIGHT & USES: Optical Instrument
• Cameras
• Telescopes
• Microscopes
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Telescopes
• Refracting telescopes – use two convex
lenses
• Reflecting telescopes use a concave
mirror to bounce the light to a plane mirror
which reflects the light to a convex lenses
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LIGHT & USES: Optical Instrument
• LASERS
– Acronym: Light Amplification by
Stimulated Emission of Radiation
– Coherent Light – Waves are in phase so
it is VERY powerful & VERY intense.
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LIGHT & USES: Optical
Instruments
• LASERS
– Holography – Use of Lasers to create 3-D
images
– Fiber Optics – Light energy transferred
through long, flexible fibers of glass/plastic
– Uses – Communications, medicine, t.v.
transmission, data processing.
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Polarized Light
• Light waves vibrate in only one plane or
direction. Can cause glare. Be sure your
sunglasses are polarized to reduce the
glare.
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Communication Technology
• Cordless phone – phone & radio signals
• Cellular phone – mainly uses microwaves
to send signals
• Satellite TV – uses microwaves
• Global Positioning System (GPS) – uses
microwaves and satellite system (24) –
need a signal from at least 4 satellites to
find a location.
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