18.1 Electromagnetic Waves

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18.1 Electromagnetic Waves

http://bldgblog.blogspot.com/2007/10/white-light.html

In the last chapter…

We learned about mechanical waves.Mechanical waves are produced from

a vibrating source.Mechanical waves need

a medium to travel through.Electromagnetic waves are different.

http://www.physicsclassroom.com/Class/sound/u11l1a.cfm



What are electromagnetic waves?

Without electromagnetic waves, we wouldn’t be able to see, talk on cell phones or watch television.

Some electromagnetic waves are visible, some of them pass through us without us knowing it, like radio waves.

Electromagnetic waves are transverse waves consisting of changing electric fields and changing magnetic fields that carry energy from place to place.

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What are electromagnetic waves?

Today, we are going to learn about:

-How electromagnetic waves differ from mechanical waves.

-How are they produced

-How do they travel

-What do electromagnetic waves act like?

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How do electromagnetic waves differ from mechanical waves?There are many ways that electromagnetic

waves differ from mechanical waves.

Where mechanical waves are always visible, some electromagnetic waves are invisible.

Where mechanical waves can be transverse or longitudinal, electromagnetic waves are always transverse.

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How do electromagnetic waves differ from mechanical waves?

Electromagnetic waves differ from mechanical waves in how they are produced.

Electromagnetic waves are produced by constantly changing fields.

http://certificate.ulo.ucl.ac.uk/modules/year_one/NASA_GSFC/goddard/imagine.gsfc.nasa.gov/docs/science/know_l2/gamma_generation.html




An electric field exerts electric forces on charged particles.

A magnetic field exerts magnetic forces.

Electromagnetic waves are produced when an electric charge vibrates or accelerates.

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Electric and Magnetic FieldsElectric fields are produced by electrically

charged particles and by changing magnetic fields.

Magnetic fields are produced by magnets and changing electric fields.

Electric and magnetic fields are related!

Electric and Magnetic Fields

A change in an electric field generates a magnetic field, and a change in a magnetic field produces an electric field.

They regenerate each other. As they regenerate each other, they move in the form of a wave.




Electromagnetic waves also differ from mechanical waves in how they travel.

Unlike mechanical waves, electromagnetic waves do not need a medium.




Electromagnetic waves can travel through a vacuum (empty space), as well as through matter.

The transfer of energy by electromagnetic waves is called electromagnetic radiation.

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The Speed of Electromagnetic Waves

Think about a thunderstorm…

When lightening flashes, it takes a few seconds before you hear the thunder.

This is because light travels faster than sound.

But how much faster is light? What is the speed of light?

http://www.curriculumsupport.education.nsw.gov.au/electricity/assets/images/lightning.gif


Since the beginning of time, people tried to measure the speed of light, but no instrument was accurate enough.

Light moves so fast that people thought its speed was infinite.

http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html


However, in 1926, the American physicist Albert Michelson measured the speed of light more accurately than ever before.

Michelson’s experiment consisted of an 8-sided rotating mirror and another stationary mirror very far away.



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When he lit up the 8-sided rotating mirror, there would be only a small moment when the light reflecting off the rotating mirror would align with the stationary mirror.

Then the light would reflect back to the rotating mirror and be able to be seen and timed.




Michelson’s measurements were extremely close to modern measurements!

Since then, scientists have confirmed that all electromagnetic waves travel at the same speed.

The speed of light has been measured to be 3.00x108 meters per second in a vacuum.

We use the letter c to represent the speed of light.c = 3.00x108 m/s

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In a vacuum, all electromagnetic waves travel at the same speed. But not all electromagnetic waves are the same.

If the speed stays the same, what about a wave can change?




Electromagnetic waves vary in wavelength and frequency.

Speed = wavelength x frequencyAs wavelength increases, frequency decreases.

http://edubuzz.org/blogs/advhigherthings/files/2008/12/spectra.jpg

Calculations with wavelength and frequency

A radio station broadcasts a radio wave with a wavelength of 3.0 meters. What is the frequency of the wave?

Calculations with wavelength and frequency

The radio waves of a particular AM radio station vibrate 680,000 times per second. What is the wavelength of the wave?



Wave or Particle?

We have just learned that electromagnetic waves are created by moving charged particles and can be visible or invisible. They also move at the same speed.

However, electromagnetic waves act strangely.

In the past, scientists wondered what electromagnetic waves (like light) was made out of. Is it a wave or a particle?

http://thesamerowdycrowd.files.wordpress.com/2009/07/sun-light-from-above_1042.jpg



Wave or Particle?

We know that electromagnetic waves can pass through us without us feeling them.

Like radio waves, we don’t feel them as they pass through us.

In this case, electromagnetic waves travel like waves.

http://www.wellbe.info/wp/wp-content/uploads/dict/Foot_x-rays.jpg



Wave or Particle?Also, light can interfere with each other just like

mechanical waves.In 1801, Thomas Young showed that light

behaves like a wave when passed through small slits in a screen.



Wave or Particle?Young observed that light and dark bands were

produced. Light bands were where there was constructive interference, dark bands are deconstructive interference.

Only waves produce interference patterns.



Wave or Particle?

We also know that light can power a solar powered calculator.

In the solar powered calculator, light pushes electrons off some metals!

In this way, light acts like a particle.This is called the photoelectric effect - the

emission of electrons from a metal caused by light striking the metal.



Wave or Particle?

Albert Einstein won a Nobel prize not for E=mc2, but for explaining one of the weirder phenomena in physics.

In 1887, they discovered that blue light would cause electrons to be emitted from a metal.

Scientists thought they could cause electrons to be emitted with red light. However, no matter how hard they tried, red light would not work!

http://www.cartage.org.lb/en/themes/sciences/physics/quantumphysics/ParticlePhysics/photoelectric_effect.gif

Wave or Particle?Albert Einstein proposed that light consisted of

packets of energy (now called photons).Each photon had an energy that was proportional

to the frequency of the light.



http://www.astro.virginia.edu/class/oconnell/astr1230/im/photoelectric-effect-2.jpg



Wave or Particle?The greater the frequency of light, the more energy

the photons have.Blue light has a higher frequency than red light.So even though scientists tried to shine very bright

red light on the metal, the individual red photons did not have enough energy to push an electron off a metal.

http://www.astro.virginia.edu/class/oconnell/astr1230/im/photoelectric-effect-2.jpg



Intensity of Light

Another thing that you can notice, the closer you are to a source of light, the brighter the light appears.

This is because, the closer you are to the light source, the more photons there are in a particular area.

As you get further away, the photons become more spread out.

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Intensity of LightIntensity is the rate at which a wave’s energy flows

through a given area.The intensity of light decreases as photons travel farther

from the source.

18.1 Electromagnetic Waves

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