13 March 5 Microwaves, Polarization

Today: Quiz Results, Light as a Wave:Microwave Radiation, Polarization

Exam #2 One week from today, BRING A PENCIL!!!

Quiz #5

A blue photon has a higher energy than a green photon

Total internal reflection (TIR) only happens for a wave inside a medium with a slower speed

Air

glass

Air

glass

Total internal reflection with microwaves

Introductory explanation of microwave source and detector

OK, Now a couple clicker questions to lead into next topic

Imagine a slinky stretched across the room in the z-direction, and a longitudinal wave traveling down it.

ZX

y

Along which direction are the displacements of the slinky?

A) X-directionB) Y-directionC) Z-direction

OK, Now a couple clicker questions to lead into next topic

Imagine a slinky stretched across the room in the z-direction, and a longitudinal wave traveling down it.

ZX

y

Along which direction are the displacements of the slinky?

A) X-directionB) Y-directionC) Z-direction Compression / rarefaction wave

OK, Now a couple clicker questions to lead into next topic

Imagine the rubber tube stretched across the room, and a transverse wave traveling down it. It is stretched in the z-direction

ZX

y

Along which direction are the displacements of the rubber tube?

A) X-directionB) Y-directionC) Z-direction

Rubber rope

OK, Now a couple clicker questions to lead into next topic

Imagine the rubber tube stretched across the room, and a transverse wave traveling down it. It is stretched in the z-direction

ZX

y

Along which direction are the displacements of the rubber tube?

A) X-directionB) Y-directionC) Z-direction

Rubber rope

Either X, Y, or any angle in between!

Transverse waves have polarization

Remember: Light is an electromagnetic waveCarries energyHas momentum (oomph) (but does NOT have mass)In a vacuum travels at “light speed”Behaves like particle AND waveOscillating Electric and Magnetic Field: TRANSVERSE

WAVE

So if EM radiation is a transverse wave, can it be polarized?

EM Waves can be “linearly polarized”

Similarly, a transverse wave on a rubber tube has a polarization

hyperphysics.phy-astr.gsu.edu

Special materials can interact preferentially with one polarization of light

vertical polarization is let through

horizontal polarization is absorbed

1. Stretched rubber tube with student barriers

2. Polarized visible light

3. Polarized microwaves

You can find this demo from MIT on youtube: http://www.youtube.com/watch?v=nCAKQQjfOvk

Some applications of polarized light

3-D Movie Projection

Polarized Light Microscopy

Photography (blue sky polarizer)

Polarized sunglasses

How do polarized sunglasses work?

The amount of light reflecting off the surface of water depends on the polarization

One component reflects more than the other,So, reflected sunlight is polarized

Polarized sunglasses can specifically absorb this polarization

You tube: http://www.youtube.com/watch?v=jNXeNdmz92o&feature=related

http://commons.wikimedia.org/wiki/File:Mudflats-polariser.jpg

So far, we have talked about linearly polarized light

A trickier concept is circularly polarized light

This occurs when the X and Y components are out of phase. http://www.ecs.umass.edu/ece/ece334/ece334/JavaApplets/Polarization/Polarization%20Applet.htm

Circular polarization demo with laser?

Specific rotation with sugar

A final application of polarized light

Nerdy physics demo

More cool demo:http://www.austine.com/

OK, so EM Radiation Can be Polarized

Remembering back to sound…What other phenomena can happen with waves?

Interference…Let’s talk a little about microwave EM radiation…

Clicker Question—Microwave Wavelength

Do you remember what the speed of light is? A wireless internet router typically operates at a frequency of 2.4 GHz (2.4 billion Hz, 2.4 * 109 Hz). Which is closest to the wavelength of these microwaves?

A. 100 nanometersB. 100 micronsC. 100 millimetersD. 100 metersE. 100 kilometers

Clicker Question—Microwave Wavelength

Do you remember what the speed of light is? A wireless internet router typically operates at a frequency of 2.4 GHz (2.4 billion Hz, 2.4 * 109 Hz). Which is closest to the wavelength of these microwaves?

A. 100 nanometersB. 100 micronsC. 100 millimeters (about 12 centimeters actually)D. 100 metersE. 100 kilometers

Microwave Radiation

The boundaries between microwave and radio / infrared are somewhat arbitrary.

Microwaves are in the gigahertz range, about centimer wavelengths…This is convenient size for observing interference

Wavelength(meters)

Frequency(Hz)

Temperaturerequired to glow

this color

A microwave oven can produce standing waves

Microwave commonly operates at similar frequency to wireless routers (2.45 GHz)

Why does a microwave oven heat food?

What would size-scale of a standing wave pattern be inside a microwave? (Distance between nodes?)

Can we observe microwave interference in class room?

Marshmallows: http://www.colorado.edu/physics/2000/applets/oven.html