The roots of “no whistling backstage” began in the 17th century. During this time, stage managers (known then as prompters) oversaw a variety of theatrical duties including overseeing rehearsals and prompting, as their title implies, entrances, music, special effects, and set pieces. Well before the advent of electricity, prompters used whistles and bells to signal to stagehands what was needed during the scene. Whistling during the performance was not distracting for the audience–it was an understood convention of the time and promptbooks from the era often include “W” for whistle.

Polarization oflight

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This page titled B25: Polarization is shared under a CC BY-SA 2.5 license and was authored, remixed, and/or curated by Jeffrey W. Schnick via source content that was edited to the style and standards of the LibreTexts platform.

Although theatrical professionals no longer depend on hearing whistles to know when to move set pieces, the ban on whistling has remained lest bad luck be tempted.

In completely unpolarized light, the electric field vectors are oscillating in every direction that is perpendicular to the direction in which the light is traveling. But all the electric field vectors are, as the name implies, vectors. As such, we can break every single one of them up into a component along the direction of polarization of the polarizer and a component that is perpendicular to the polarization direction of the polarizer. A polarizer will let every component that is along the direction of polarization of the polarizer through, and block every component that is perpendicular to the polarization direction. In completely unpolarized light, no matter what the direction of polarization of the polarizer is, if you break up all the electric field vectors into components parallel to and perpendicular to the polarizer’s polarization direction, and add all the parallel components together, and then separately add all the perpendicular components together, the two results will have the same magnitude. This means that we can view completely unpolarized light as being made up of two halves: half polarized parallel to the polarizer’s polarization direction, and half polarized perpendicular to the polarizer’s polarization direction. The half that is polarized parallel to the polarizer’s polarization direction gets through the polarizer, and the other half doesn’t.

Coherentlight

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When unpolarized light (a.k.a. randomly-polarized light) is normally incident on any polarizer, half the light gets through. So, if the intensity of the incoming light is \(I_0\), then the intensity of the light that gets through, call it \(I_1\), is given by:

The parallel component \(E_{\parallel}\) = E cos \theta\) gets through the polarizer, the perpendicular component does not.

The origins of theatrical superstitions can be surprising; they can be reflective of a larger awareness of the supernatural, but they can also speak to the conventions of a bygone way of doing things in the theatre. Last week, we looked at why you should never speak the name of Shakespeare’s Scottish play and the misfortunes that lead to the formation of the curse. This week, we look at two different traditions in the theatre that, while they have their superstitious meanings, they also have an element of practicality behind them.

Jones matrix

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Now the intensity of polarized light is proportional to the square of the amplitude of the oscillations of the electric field. So, we can express the intensity of the incoming light as

The polarization direction of the rectangular polarizer depicted above is vertical. So, it lets vertically-polarized light through and blocks horizontally-polarized light.

Randomly polarized light, a.k.a. unpolarized light, has electric field oscillations in each and every direction perpendicular to the direction in which the light is traveling. Such light is often depicted, as viewed from behind, (where forward is the direction in which the light is traveling) as:

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Light that is traveling horizontally away from you that is polarized at \(30^{\circ}\) with respect to the vertical could be either:

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... the light that gets through is polarized in the polarizer's direction of polarization, and, has an intensity \(I_1=I_0(\cos \theta)^2\).

Circularly polarizedlight

The orientation of the lines is referred to as the polarization direction of the polarizer. The effect of a polarizer is to transmit light that is polarized in the same direction as that of the polarizer, and to block (absorb or reflect) light that is polarized at right angles to the direction of the polarizer.

Light that is traveling horizontally away from you and is polarized, from your point of view, at \(30^{\circ}\) clockwise from the vertical is, however, unambiguously:

P-polarizedlight

where the direction in which the light is traveling is “into the page” and upward is “toward the top of the page.” At a particular position through which the light is traveling, starting at an instant when the electric field vector at that position is upward and maximum, the electric field will decrease to zero, then be downward and increasing, reach a maximum downward, then be downward and decreasing, become zero, then be upward and increasing, then reach a maximum upward, and repeat, continually. The diagram depicting the polarization indicates the directions that the electric field does point, at some time during its oscillations. It in no way is meant to imply that the electric field is pointing in two directions at the same time.

The practical reason for the ghost light is evident. If someone were to enter a completely dark theatre, it might be a safety concern. Hazards in an unlit house include the edge of the stage, bumps and holes in the stage itself, working equipment for set pieces, and the set itself. Legend has it that the genesis of the ghost light came from an unlucky burglar that fell into the orchestra pit when the house was dark and empty. The story ends with the burglar attempting to sue the theatre. The ghost light was added as a precaution so no one could trip like that again—whether they were there legally or illegally.

Photon polarization

LIGHT COLLIMATION PHANTOM(α+) / RA-09A-2268 / 1 EA.

One reason whistling was also employed at the theatre was that many overhire stagehands were also dockworkers. The theatrical season often coincided with the winter months and stage mechanics of the period included a system of ropes, knots, and pulleys so these workers were not only available but also well-equipped for the task. Whistling was part of the nautical tradition, and it carried over to the theatre. However, it was important that no one else whistle, because an innocent actor whistling backstage could cause a cue to happen at the wrong time leading to unexpected consequences.

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While ghost lights are very practical and utilized in theatres around the world, they also have a spiritual meaning. The lights are said to ward away mischievous spirits. However, the light also serves as a guide for the ghosts that are believed to haunt every theatre. These theatre ghosts are not seen as malicious, but rather they are ghosts that can be kept happy by having their home lit.

Fresnel equation

... the light that gets through is polarized in the polarizer's direction of polarization, and, has an intensity \(I_1=\frac{1}{2} I_0\).

... is normally incident on a polarizer whose polarization direction makes an angle \(\theta\) with the polarization direction of the light...

The polarization direction of the circle-shaped sample of polarizing material depicted above is horizontal. So, it lets horizontally-polarized light through and blocks vertically-polarized light.

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Brewster angle

can be broken up into a component parallel to the polarizer’s polarization direction and a component perpendicular to the polarizer’s polarization direction.

The polarization direction of light refers to the two directions or one of the two directions in which the electric field is oscillating. For the case of completely polarized light there are always two directions that could be called the polarization direction. If a single direction is specified, then that direction, and the exact opposite direction, are both the directions of polarization. Still, specifying one direction completely specifies the direction of polarization. For instance for light that is traveling straight downward near the surface of the earth, if the polarization direction is said to be a compass heading of \(15^{\circ}\), that unambiguously means that the electric field oscillates so that it is at times pointing in the direction with a compass heading of \(15^{\circ}\), and at times pointing in the direction with a compass heading of \(195^{\circ}\) (\(15^{\circ}\) west of south).

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If you encounter such an ambiguous specification of polarization in a problem statement then the answer is the same for either case, so, it doesn’t matter which of the two possible polarization directions you pick. Pick one arbitrarily and work with it.

Now suppose that we have some light that, for whatever reason, is already polarized. When polarized light is normally incident on a polarizer, the intensity of the light that gets through does depend on the direction of polarization of the polarizer (relative to that of the incoming light). Suppose for instance, that the incoming light,

Note how the effect of a polarizer on the intensity of normally-incident unpolarized light does not depend on the orientation of the polarizer. You get the same intensity \(I_1 =\frac{1}{2} I_0\) of light getting through the polarizer, no matter what the direction of polarization of the polarizer is.

A plastic material is manufactured in the form of flat sheets that polarize light that travels through them. A sample of such a flat sheet is called a polarizer. In use, one typically causes light to travel toward a polarizer along a direction that is perpendicular to the polarizer. In other words, one causes the light to be normally incident upon the polarizer.

Schematically, one typically depicts a polarizer by means of a rectangle or a circle filled with parallel line segments.

A ghost light—sometimes just a bare lightbulb on a stand—sits unobtrusively on the stage when the theatre is empty. Ghost lights are often very simple, but their meanings run deep. During the COVID-19 pandemic theatre closures, the ghost light became a symbol of hope to artists during a time of darkness. Although no one was in the theatre for quite some time, the light remained on for their eventual return.

is polarized at an angle \(\theta\) with respect to the polarization direction of a polarizer upon which the light is normally incident:

Much like directly wishing actors “good luck” before they go onstage, the act of whistling backstage is considered to bring on misfortune to a production. Like the ghost light, there was a practical reason for why it became a theatre superstition. Unlike the legend that explained the ghost light, however, there is true historical basis for this superstition.