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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 particlesElectric fields are produced by
charged particles changing magnetic fields
A magnetic field produces magnetic forcesMagnetic 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:
What is faster…speed of sound or light?
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 and frequency
c = * frequency
Since the speed of light (c) is always 3.00 x 108 m/s, you can always calculate 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
LightLightPhotonsPhotons
ElectromagneticElectromagneticRadiationRadiation
Double Slit ExperimentPass light through two slits and an
interference pattern is observed Interference is a property of waves!
Photoelectric EffectLight 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 wavesInfrared raysVisible lightUV raysX-raysGamma Rays
TelevisionTelevision
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.
MicrowaveMicrowave
Emitted by:Gas clouds collapsing
into starsMicrowave OvensRadar StationsCell Phones
Detected byMicrowave
TelescopesFood (heated)Cell phonesRadar (systems)
InfraredInfrared(Heat or Thermal)Are you a source of infrared? YES you are!
Emitted bySun and stars
(Near)TV Remote ControlsFood Warming
Lights (Thermal)*Everything at
room temperature or above,=HEAT
Detected by Infrared CamerasTVs, VCRs,Your skin
VisibleVisibleEach color is a different size wave.Red the longest & violet the shortest
Emitted byThe sun and other
astronomical objects
Laser pointersLight bulbs
Detected byCamerasHuman eyesPlants (red light)Telescopes
UltravioletSunburn / black light
Emitted byTanning booths (A)The sun (A)Black light bulbs (B)UV lamps
Detected bySpace based UV
detectorsUV CamerasFlying insects (flies)
X-rayX-ray Emitted by
Astronomical objects
X-ray machinesCAT scan machinesRadioactive
mineralsAirport luggage
scanners
Detected bySpace based X-ray
detectorsX-ray film
Gamma RayGamma Ray(Short electromagnetic waves but more energetic)
Emitted byRadioactive materialsExploding nuclear
weaponsSolar flares
Detected by Geiger countersGamma detectors and
astronomical satellitesMedical imaging
detectors
COSMIC RaysCOSMIC Rays(The highest energy waves and the deadliest)
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 scatteringColor 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.
ReflectionReflection Reflection from a
mirror:
Incident ray
Normal
Reflected ray
Angle of incidence
Angle of reflection
Mirror
The Law of ReflectionThe Law of Reflection
Angle of incidence = Angle of reflection
The same !
!!
Clear vs. Diffuse Clear vs. Diffuse ReflectionReflection Smooth, shiny
surfaces have a specular reflection:
Rough, dull surfaces have a diffuse reflection
Diffuse reflection is when light is scattered in different directions
Using mirrorsUsing mirrors Two examples:
1) A periscope
2) A car headlight
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 Orang
e Yellow Green Blue
Indigo Violet
Adding ColorsAdding Colors 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 blue and green makes cyan
(light blue)
Adding all three makes white again
Adding red and green makes yellow
Seeing ColorSeeing Color The color an object appears depends on the colors of
light it reflects For example, a red book only reflects red light:
Whitelight
Only red light is
reflected
A white hat would reflect all seven colors:
A pair of purple trousers would reflect purple light (and red and blue, as purple is made
up of red and blue):
Purple light
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)
LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses 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 objectThe images formed are upside down
LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses Convex Lenses Ray Tracing
Two rays usually define an image Ray #1: Light ray comes from top of
object; travels parallel to optic axis; bends thru focal point.
Focal Point
Lens
Object
© 2000 D. L. Power
LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses
Convex Lenses Ray Tracing
Two rays define an image Ray 2: Light ray comes from top of
object & travels through center of lens.
Ray #1
Ray #2
© 2000 D. L. Power
LIGHT & ITS USES: LIGHT & ITS USES: LensesLenses
Concave lenses – Lens that is thicker at the edges and
thinner in the center. Diverges light rays All images are upright and reduced
© 2000 D. L. Power
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.
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.
LIGHT & USES: Optical InstrumentsLIGHT & USES: Optical Instruments
LASERSHolography – Use of Lasers to create 3-
D imagesFiber Optics – Light energy transferred
through long, flexible fibers of glass/plastic
Uses – Communications, medicine, t.v. transmission, data processing.