Download The polarization of light - along with refraction, diffraction and

Quantum Polarization
The polarization of light - along with refraction, diffraction and interference - provides strong evidence
that light behaves like a wave. The wave model of polarization has allowed us to develop and produce
such applications as polarized sunglasses, flat screen TV`s and 3-D movies. This session assumes that
you are already familiar with polarization and polarizing filters. It also assumes that you are familiar
with diffraction, two-slit interference and photons.
For over a century, physicists have known that there are problems with the wave model of light. In many
situations light clearly acts like a particle. The conflict between the classical wave and particle models
has led to a new model - the quantum model. In this lesson you will explore the behavior of polarized
light and how its behavior is explained by the wave and the quantum models. On the way you will see
examples of four key concepts of quantum physics: intrinsic randomness, wave-particle duality,
measurement-disturbance and entanglement.
1) Put on a pair of the 3-D glasses and close one eye. The polarizing filter in the glasses is reducing the
light that gets through. Suppose 1000 photons head toward the filter. How many will pass through?
a) exactly 500 photons
b) at least 500 photons
c) around 500 photons
Explain:
2) Keep the glasses on and one eye closed. Look at a second pair. One filter should look dark and the
other should look much lighter. If 1000 photons have passed through the lighter filter, how many of
these photons will pass through the one in front of your eye?
a) exactly 1000 photons
b) fewer than1000 photons
c) around 1000 photons
Explain:
3) Using the same set up as in the previous question. Look at the dark filter. If 1000 photons have
already passed through the first filter, how many of these photons would then pass through the filter
in front of your eye?
a) exactly 0 photons
b) over 0 photons
c) around 0 photons
Explain:
4) Look at the very dark filter again. In between the two filters, place a third filter at 45o. If 1000
photons have already passed through the first filter in your hand, how many will pass through the
middle filter and the one in front of your eye?
a) around 500 photons
b) around 250 photons
c) around 125 photons
Explain:
5) A single unpolarized photon is heading toward a polarizing filter. Through the filter you will get
a) a photon
b) half a photon
c) a photon with half the energy
d) no photon
Explain:
6) An unpolarized photon heads toward a filter at 0 and then one at 90o. Through these you will get
a) a photon
b) nothing
c) 50 % chance of a photon
d) 25 % chance of a photon
Explain:
7) A single unpolarized photon is heading toward a filter at 0 and then one at 45 and then one at
another at 90o. Through the three filters you will get
a) nothing
b) 12.5 % chance of a photon
c) 25 % chance of a photon
Explain:
8) Crossed polarizers can be explained by classical physics and by quantum physics. Which is correct?
a) Quantum is always right
b) Classical is always right
c) Neither is always right
d) Each is right in its realm
Explain:
9) Light passes through two slits that are very close together. What will you see?
a)
b)
c)
Explain:
10) Light passes through two slits that are very close together. One slit is covered by a vertical polarizer
and the other is covered by a horizontal polarizer. What will you see?
a)
b)
c)
Explain:
11) The set up is the same as the last question. In addition, a third polarizer is placed in between the slits
and the screen. What will you see?
a)
b)
c)
Explain:
12) If the first experiment was done with only one photon at a time and the image was allowed to build
up slowly you would see
a)
b)
c)
Explain:
13) Why is the third filter called a quantum eraser? What does it erase?
a) polarization
b) light
c) knowledge
Certain crystals can take one photon and produce two photons with half the energy that have identical
but still random polarization. This is called spontaneous parametric down conversion - SPDC. You will
hear lots about this at the Institute for Quantum Computing. These pairs of photons are said to be
entangled. As soon as you measure the polarization of one of them, you immediately know the
polarization of its partner. Measuring one of the pair causes a change to the other one instantaneously –
no matter how far away it is.
14) One of a pair of entangled photons is sent toward double slits covered in opposite polarizing filters.
The other partner is sent in a completely different direction toward a polarizing filter. This is repeated
many times. What will you see after the double slits if you measure the polarization of the photons that
go away from the slit?
a)
b)
c)
Explain:
Reflection: This lesson used a lot of multiple-choice questions. They were being used as a way to
increase learning – not as a way to evaluate students. When used this way they are often referred to as
concept questions or ConcepTests. What distinguishes a good from a bad concept question? How do you
encourage students to have meaningful discussions and not shouting matches? How do you get students
to focus on learning and not on being right? What are the advantages of flip books vs. electronic
clickers vs. voting with your hands?