Applications and the Future - Codes: The Guide to Secrecy from Ancient to Modern Times - page 366

Photons and a Quantum Experiment
We begin by looking at basic properties of light. The particles that constitute
light are called photons . These photons make up light waves, which are examples
of electromagnetic waves , meaningthat they have an electric field that travels
perpendicular to their associated magnetic field. Photons travelling through
space vibrate (or oscillate) as they move. This vibration can be horizontal,
denoted by → ; vertical, denoted by ↑ ;45 ◦ , denoted by ; or 135 ◦ , denoted

by . The angle of the vibration is known as the polarization of the photon.

This is a simple type of polarization, called linear , meaningthat as the photon
propagates, the electric field stays in the same plane. This linearity assumption
simplifies the situation by allowingonly four possible polarizations, rather than
the infinitely many possibilities (namely all angles in between).
Now we need to understand a little bit about polarization of light. We are
going to look at the effects of a Polaroid filter 9.35 on a light source. We will
assume that the axis of the filter is oriented in one of the aforementioned four
ways. Quantum theory dictates that if α is the angle that the plane of the
electric field of the photon makes with the axis of a Polaroid filter, then there is
a probability of cos 2 α that the photon will emerge with its polarization reset to
that of the filter's axis, and a probability of 1

cos 2 α that it will be absorbed
(to be re-emitted later as heat). For example, if the polarizer axis is vertical,
then light emitted with random polarization means that if α is only slightly off
vertical the photon has a high probability of passing through. If it is 45 ◦ , then
it has a fifty percent chance of getting through, and this decreases to zero at the
horizontally polarized photons. Hence, roughly 50% of the randomly emitted
photons get through and as they pass through the vertical filter, they all emerge
as

−

polarizations. Call that polarization filter V , and the situation is illustrated
in Diagram 9.10.

↑

Diagram 9.10 Polarization with Filter V

Light Source: L

V

polarized
−−−−−−−−−−→

↑

S
−−−−→

50% of S

9.35 Edwin Herbert Land (1909-1991) patented a cellophane-like polarizing filter, the first
to polarize light, a process that reduces light glare. In 1932, Land co-founded the Land-
Wheelwright Laboratories in Boston. By 1937, Land founded the Polaroid Corporation and
began to use his filters in Polaroid sunglasses, and a variety of other applications. However,
Land is best known for his invention and marketing of instant photography, called Polaroid
photography . In 1947 he presented the Polaroid Land Camera , which took1 minute to pro-
duce a finished photograph. After his retirement from Polaroid in 1980, he worked with the
nonprofit Rowland Institute ofScience supported by the Rowland Foundation that he founded
in 1960. Land stands second only to Thomas Edison in the number of patents issued to him,
more than 500. He received a number of awards for his contributions to knowledge about
polarized light, photography, and colour perception. Land died in Cambridge, Massachusetts,
on March 1, 1991.