Transcript The Electromagnetic Spectrum and Light

 Electromagnetic waves- transverse waves
consisting of changing electric and
magnetic fields
 Carry ENERGY from place to place
 Different from mechanical waves…how?
 An electric field exerts forces on charged
particles

Electric fields are produced by
 charged
particles
 changing magnetic fields
 A magnetic field produces magnetic forces
 Magnetic fields are produced by
 magnets
 changing
electric fields
 vibrating charges
 As the two fields regenerate each other, their E
travels in the form of a wave.
 EM waves DON’T need a medium!!!

Can travel through a vacuum (=empty space) OR
through matter
 EM radiation – the transfer of E by EM waves
traveling through matter or across space.
What is faster…speed of sound
or light?
Light travels much faster than sound. For example:
1) Thunder and lightning start
at the same time, but we
will see the lightning first.
2) When a starting pistol
is fired we see the
smoke first and then
hear the bang.
 First accurate estimations were in 1926 when
Albert Michelson completed his experiment
in California.
 Light (and all EM waves) travel
the same speed in a vacuum…
c = 3.00 x 108 m/s
Or 300,000,000 m/s
(compared to sound @ ~ 340
m/s)
That’s 8 times around the earth
in 1 second!!!!
EM waves vary in l and frequency
c = l * frequency
Since the speed of light (c) is always
3.00 x 108 m/s, you can always
calculate l from frequency and vice
versa.
 EM radiation sometimes behaves like a
wave, and sometimes like a stream of
particles.
 There is evidence for both theories...
 That’s called wave-particle duality
 In some experiments, the wave model works
best.
 In other experiments, the particle model works
best.
 Thus, we use both
Electromagnetic
Radiation
Photons
Light
 Double Slit Experiment

Pass light through two slits and an interference
pattern is observed
 Interference is a property of waves!
 Photoelectric Effect

Light shown on a metal can cause electrons to be
emitted from the metal
 Photons- particles of light
 The greater the frequency of an EM wave,
the more E each of its photons has
Intensity- the rate at which a wave’s
energy flows through a given unit of
area…basically, it is brightness of
light.
As you leave the source of light, does
intensity increase or decrease?
 Farther from the source, the photons spread out
over a larger area, and intensity decreases.
What happens when you put a prism in front of
a window?
 EM spectrum- the full range of frequencies
of EM radiation
 How many different types of EM waves can
you think of?
 EM Spectrum includes:
 Radio waves
 Infrared rays
 Visible light
 UV rays
 X-rays
 Gamma Rays
 Shorter than radio, also
used to carry messages
(pictures & sound) to
our TV sets.
 We can sense the TV
waves around us with
our televisions.
 Emitted by:
Gas clouds collapsing into
stars
 Microwave Ovens
 Radar Stations
 Cell Phones

 Detected by




Microwave Telescopes
Food (heated)
Cell phones
Radar (systems)
 Emitted by




Sun and stars (Near)
TV Remote Controls
Food Warming Lights
(Thermal)
*Everything at room
temperature or
above,=HEAT
 Detected by



Infrared Cameras
TVs, VCRs,
Your skin
 Emitted by

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
The sun and other
astronomical objects
Laser pointers
Light bulbs
 Detected by
Cameras
 Human eyes
 Plants (red light)
 Telescopes

 Emitted by
Tanning booths (A)
 The sun (A)
 Black light bulbs (B)
 UV lamps

 Detected by
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Space based UV
detectors
UV Cameras
Flying insects (flies)
 Emitted by

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Astronomical objects
X-ray machines
CAT scan machines
Radioactive minerals
Airport luggage
scanners
 Detected by
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Space based X-ray
detectors
X-ray film
 Emitted by
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Radioactive materials
Exploding nuclear weapons
Solar flares
 Detected by Geiger counters
Gamma detectors and
astronomical satellites
 Medical imaging detectors

 Cosmic rays come from
deep space and can
pass through the Earth.
 When you are looking at things, anything,
what you are really seeing is light.
 We can’t see through walls because light
doesn’t pass through walls.
We see things because they reflect
light into our eyes:
Homework
 How light behaves when it strikes an
object depends on many factors, including
the material the object is made of
 Materials can be translucent, transparent,
or opaque.
 Transparent objects No scattering
 Color transmitted is color you see
and all other colors are absorbed
 Translucent Light is scattered and transmitted
some
 Opaque Light is either totally reflected or
absorbed
 Color of opaque objects is color it
reflects
 When light strikes a new medium, the light
can be reflected, absorbed, or transmitted.
 When light is transmitted, it can be refracted,
polarized, or scattered.
 Reflection from a mirror:
Normal
Reflected ray
Incident ray
Angle of incidence
Mirror
Angle of
reflection
The Law of Reflection
Angle of incidence = Angle of reflection
The
same !!!
 Smooth, shiny surfaces
have a specular reflection:
 Rough, dull surfaces have
a diffuse reflection
 Diffuse reflection is when
light is scattered in
different directions
 Two examples:
2) A car headlight
1) A periscope
 The bending of light waves as they pass from
one medium to another
 Results in mirages, which are false or
distorted images
Inferior Mirages:
formed when the air near the ground is very
warm compared to the air just above it.
 Polarized light is light that all vibrate in the
same plane (or direction!)
 Light is redirected as it passes
through a medium.
 This is responsible for our red
sunsets!
 White light- not a single color; it is made up of
a mixture of the seven colors of the rainbow
 We can demonstrate
this by splitting white
light with a prism
 This is how rainbows
. are formed: sunlight is
“split up” by raindrops
 Red
 Orange
 Yellow
 Green
 Blue
 Indigo
 Violet
 White light can be split up to make separate colors.
These colours can be added together again
 The primary colors of light are red, blue and green:
Adding blue and red
makes magenta
(purple)
Adding red and
green makes
yellow
Adding blue and green
makes cyan (light
blue)
Adding all three
makes white
again
 The color an object appears depends on the
colors of light it reflects
 For example, a red book only reflects red light:
White
light
Only red light
is reflected
A pair of purple trousers would reflect purple light (and
red and blue, as purple is made up of red and blue):
Purple light
A white hat would reflect all seven colors:
White
light
 Color Blind Tests
If you continue to focus on the sign in the centre of the image
you will notice that the circle of violet circles will
soon disappear completely, and you will see only the green
spot (which is actually violet)
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
 The images formed are upside down
Convex Lenses
Ray Tracing
Focal Point
Object
© 2000 D. L. Power
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.
Ray #1
Convex Lenses
Ray Tracing
© 2000 D. L. Power
 Two rays define an image
 Ray
Ray #2
2: Light ray comes from top of object &
travels through center of lens.
© 2000 D. L. Power
Concave lenses –
 Lens that is thicker at the edges and thinner in
the center.
 Diverges light rays
 All images are upright and reduced
 Vision
 The eye is a convex lens
 Retina
 Lens refracts light to
converge on the retina and
then nerves transmit the
image
 Rods
 Nerve cells in the retina. Very
sensitive to light & dark
 Cones
 Nerve cells help to see
light/color
Rods – responsible for black
and white vision and
detection of motion.
Cones – Seeing in color and
visual acuity. We have three
types of cones: cones that
see red, cones that see blue,
and cones that see green.
 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.
 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.