1. Chp 18.1 - Electromagnetic WavesPg

2. 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

3. 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

4. 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.

5. How are they produced?
Electromagnetic waves are produced when an electric charge vibrates or accelerates.

6. 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

7. How they travel
Electromagnetic Radiation = Transfer of energy by electromagnetic waves traveling through matter or across space

8. 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

9. 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

10. 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

11. Theories of Electromagnetic Radiation
EM radiation sometimes behaves like a wave, and sometimes like a stream of particles.
Wave Model
Particle Model 11

12. 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 12

13. 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 13

14. 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 14

15. The End
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