Transcript Chp 18.1 - Electromagnetic Waves

Pg 532-538
Electromagnetic waves
 Waves make it possible for us to:
 Heat up our food in the Microwave oven
 Take X-rays of our bones and teeth
 Watch television and get our favorite music to our
radio
 Carry cell phone conversations
 See different colors
 Without light waves we wouldn’t be able to see
anything at all
Electromagnetic Waves
 Are transverse waves consisting of changing electric
fields and changing magnetic fields
 Carry energy from place to place like mechanical
waves but differ in how they are produced and how
they travel
How are they produced?
 Produced by constantly changing fields
 An electric field in a region of space exerts electric forces on
charged particles.
 Electric fields are produced by
 electrically charged particles
 changing magnetic fields.
 A Magnetic field in a region of space produces magnetic
forces
 Magnetic fields are produced by
 Magnets
 changing electric fields
 vibrating charges.
How are they produced?
 Electromagnetic waves are produced when an electric charge
vibrates or accelerates.
How they travel
 Because changing electric fields produce changing
magnetic fields, and changing magnetic fields produce
changing electric fields, the fields regenerate each
other
 As they regenerate, their energy travels in the form of
a wave
 Electromagnetic waves do not need a medium to
travel through
 Electromagnetic waves can travel through a vacuum,
or empty space, as well as through matter
How they travel
 Electromagnetic Radiation = Transfer of energy by
electromagnetic waves traveling through matter or across
space
Michelson’s Experiment
 In 1926 Albert Michelson measured the speed of light
more accurately than ever before
 Completed his experiment on the top of Mount Wilson in
California
 Shined a bright light and used a stationary mirror and a
rotating mirror 53.4 km away. With his values he
calculated the speed of light quite accurately
Speed of Electromagnetic Waves
 When a thunderstorm is approaching you can see the
sky light up before you hear the thunder rumble
 As the storm comes closer the time between when the
flash occurs and the rumble begins becomes smaller
 Light travels much faster than sound
 Light and all Electromagnetic waves travel at the
same speed in a vacuum
 The speed of light in a vacuum or c is equal to
300,000,000 m/s. Or 3.00 x 108 m/s
 c = 3.00 x 108 m/s
Wavelength and Frequency of
Electromagnetic Waves in a vacuum
 Electromagnetic Waves vary in wavelength and
frequency
 The speed of an electromagnetic wave is the product
of its wavelength and frequency
 Speed = wavelength x frequency
 As the wavelength increases, the frequency decreases
Theories of Electromagnetic Radiation
 EM radiation sometimes behaves like a wave, and
sometimes like a stream of particles.
 Wave Model
 Particle Model
Evidence for Wave Model
 In 1801, Thomas Young showed that light behaves like a
wave
 Interference only occurs when two or more waves overlap
 Double Slit Experiment - pass light through two slits and an
interference pattern is observed
 Constructive Interference – an overlap in waves cause an increase
in amplitude
 Destructive interference – an overlap in waves cause a decrease
in amplitude
Evidence for the Particle Model
 Photoelectric Effect - light striking a metal can cause electrons to
be emitted from the metal
 Photons – are packets of electromagnetic energy
 The greater the frequency of an EM wave, the more energy each of
its photons have
 Blue light has a higher frequency than red light so photons of blue
light have more energy than photons of red light
 Blue light causes emissions of electrons from metal but red light
does not because blue light is higher energy
What is Intensity of light?
 The closer you are to a source of light
the brighter the light appears
 Intensity is the rate at which a wave’s
energy flows through a given unit of
area – (brightness of light)
 Intensity of light decreases as
photons travel farther from the source
Ex. Flashlight’s brightness
Ex. Distance a paint nozzle is to the
piece
The End
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