Today: More wave properties of lightPolarization, Diffraction, Interference

Exam 2 is Thursday, Bring a Pencil!

HW Exeperiment

2/13/04 Calendar note:Looking out airplane window. Very bright cloud floor. See bright (white) point-like flashes. 100's of ms in duration. Seem to appear, then travel in short paths (like 1% of field of vision), sometimes squiggly lines, sometimes curly

orbits. Probably 1000's in field. Ideas: 1. Recreate indoors?2. W/ sunglasses or other optics3. W/ eyes closed and bright?

10/27/06 Colleague visits ABQ:

He calls them “wormy things” and sees them on airplane. He speculates they are due to low pressure, something about your eyeball pressure being too high?

This convinced me to do some poking around

We were screwed by the weather!I don’t think anyone saw what I’m going to talk about

Kyle Dallas: But when it cleared up a little bit I was looking at a angle in which the sun was behind the clouds…noticed that the sky was a darker blue above the clouds where the sun light was easier to see cutting into the blue sky.

Lisa Gillim: The blue from the sky reflects off the clouds making it look like they have a blue outline. (Silver lining?)

Kayla Deresin: …the part of the sky that i did get to see was a violet blue and if i looked really hard I could see a little bit of green and I am not sure why that is!

Any ideas?

Clue #1: What color do you see when you look through your eyelids? Why?

Clue #2: Is your retina composed of living cells?

Clue #3: How do the cells on your retina get oxygen?

Blue Field Entoptic Phenomenon

http://webvision.med.utah.edu/sretina.html

Quiz 6

Some take home messages about polarization

Electromagnetic waves are transverse and can have polarization.

Polarization at an angle can be though of as a mixture of “x” and “y” polarization.

Special materials can absorb EM radiation of one polarization and pass the perpendicular.-> Making non-polarized light polarized.

Polarization can be rotated by special materials.

EM Waves can be “linearly polarized”

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

hyperphysics.phy-astr.gsu.edu

The “rope through a fence” analogy is misleading!This is how EM radiation interacts with matter

vertical polarization is let through

horizontal polarization is absorbed

Let’s confirm this again with the microwave demo and the metal combs

Clicker question

A laser is traveling across the room horizontally. It is perfectly polarized vertically. I place a polarizer in the beam in such a way that 0% of the light is transmitted. If I next rotate the polarizer 45 degrees, how much light will go through?

A. 0% transmittedB. 25% transmittedC. 50% transmittedD. 100% transmitted

Clicker question

A laser is traveling across the room horizontally. It is perfectly polarized vertically. I place a polarizer in the beam in such a way that 0% of the light is transmitted. If I next rotate the polarizer 45 degrees, how much light will go through?

A. 0% transmittedB. 25% transmittedC. 50% transmittedD. 100% transmitted

A) 0% The light started out vertically, so it cannot be made to be horizontal.

B) 25%C) 50%D) 75%E) 100%

Let's shine vertically polarized light on a series of polarized filters.

The 1st filter is vertical, the 2nd filter is at 45 degrees,

the last filter is horizontal. What percentage of light gets through the last filter?

Let’s argue with each other!

A) 0%B) 25% (Half of half = one quarter)C) 50%D) 75%E) 100%

Let's shine vertically polarized light on a series of polarized filters.

The 1st filter is vertical, the 2nd filter is at 45 degrees,

the last filter is horizontal. What percentage of light gets through the last filter?

Key concept:

Polarizers CAN rotate (“twist”) the polarization of light

“Diagonally” polarized light can be thought of as a combination of horizontal and vertical.

If a polarizer is at an angle relative to light polarization,some light will go through…

But polarization of light going through will be rotated!First this applet: http://www.colorado.edu/physics/2000/polarization/blocking_light.html

Confirm with microwaves again.Laser experiment

Huygens’ PrincipleWavelets are a useful way of thinking about wave propagation

First, what do we mean by wave fronts?For water waves, it’s more evident:

Wave fronts are locations of the same phase

Huygens’ Principle: Spherical waves emanate from all points along a wave front.Close examination explainsreflection and refraction

Very nice web applet:http://www.walter-fendt.de/ph11e/huygenspr.htm

Diffraction

For our purposes, “Diffraction” refers to things that happen because EM radiation is wave-like.

Huygens’ principle helps explain diffraction

Single-slit diffraction

Double-slit diffraction

Diffraction and interference from apertures

A small aperture (hole or slit) can turn a plane wave into a “spherical” or spreading wavewater waves applet: http://www.falstad.com/ripple/ http://www.walter-fendt.de/ph14e/singleslit.htm

Laser demo—single slit

Two small apertures can cause a more complicated interference pattern http://www.walter-fendt.de/ph14e/doubleslit.htm

Laser demo – double slit

Water waves – interference pattern (related to double-slit)http://www.youtube.com/watch?v=5PmnaPvAvQY&feature=related