PHYSICS AND THE VISUAL ARTS
Notes on LESSON 8
Things to remember from Lesson 8.
Light scattering by particles smaller than the wavelength of the
light is strongly dependent on the wavelength. The scattered intensity
goes as one over the wavelength raised to the fourth power. Consequently
blue light is scattered more strongly than red light. Can you use this
observation to explain why the sky is blue (and why it is not violet) and
why sunsets are red? Larger particles scatter light more uniformly. This accounts for
some white smoke.
When light is scattered it is becomes polarized. The direction of
the polarization is the direction of the electric field in the traveling
light wave.
Question: What is unpolarized light? Where is its electric
field directed?
What is (linearly) polarized light?
What are polarizing filters? How do they work? What
happens to the light when two linear polarizers are crossed, that is,
arranged so that their transmission axes (polarization axes) are at right
angles?
Malus's law relates the transmitted intensity I to the incident
intensity I0 through
I = (1/2) I0 (cos θ)2.
Here θ is the angle between the two polarization axes. Note that for I0 incident
intensity, the intensity emerging through a single polarizer is (1/2) I0.
In the video you saw demonstrations of circular polarization. What
did you see? Also, what happened to light passing through the calcite
crystal?
When light is reflected from a dielectric surface it will be partially polarized. There is one angle
of incidence at which the reflected light is completely linearly polarized. That angle is known as
the Brewster angle. The complete polarization occurs then the angle of refraction and the angle of
reflection are at right angles to each other. When that occurs we can rearrange Snell's law to give
the relationship:
tan(θB) = nt/ni,
where nt and ni are the indices of refraction of the transmitting and
incident media and θB is the Brewster angle (of incidence).
Birefringence (or double
refraction) occurs in materials when the index of refraction for one direction of polarization
is different from that for polarization at 90 degrees to that direction.
Light may be Circularly polarized by passing linearly polarized light through a sheet of birefringent
material when the polarization direction of the incident light makes an angle of 45° with the optic
(unique) axis in the sheet. With this orientation, half of the light is parallel to the optic axis and half of
the light is perpendicular to that axis. Because the indices of refraction along the two directions are
different, the two components of the emerging light no longer have the same phase. If the two components differ
in phase by 1/4 wavelength, the resulting electric field vector of the emerging light rotates around the line
of propagation of the light beam. For the 45° orientation, the components are equal and the light is
circularly polarized. (At other angles, the components are unequal and the light is said to be elliptically
polarized.) The birefringent sheet that causes the 1/4 wave shift is known as a quarter wave plate.
Optical activity is the
rotation of the plane of polarization as light propagates through the material.
Karo syrup is optically active because of the sugar molecules. The sugar is called dextrose
because the polarization rotates in a clockwise (right handed) direction as it the light moves
through the syrup.
Stress birefringence (also known as
photoelasticity) is the inducment of changes in index of refraction with stress. Describe examples of
each of these behaviors.
What is an LCD and how does it operate? Where are such things
used?
Here is a link to an interesting site discussing 3-D displays I think you will find it
interesting.
Recent developments in 3-D movies.
In 2007, several of the movie theaters in Columbia (and others around the nation) got rid of their motion picture film projectors and replaced
them with digital projectors that use microelectromechanical devices to direct the light through their lenses.
These DLP projectors actually provide a better image than do the film projectors and completely eliminate the
annoying scratches that so ofter mar the film images. (More on the DLP system in Lesson 14.)
The digital projectors permit the projection of 3-D movies that are much improved over prior techniques.
Because the digital images are not restricted by the frame rate dictated by film, sequential images for the
separate eyes can be so fast that no flicker is observed. The brightness is improved and the crosstalk between
images eliminated through the use of circular polarization. The light from the projector is directed through an
LCD that switches between two polarization states that are 90° apart. That light in turn is passed through
a quarter-wave plate. The result is left or right circular polarized light depending on which state the LCD is
in. The viewer wears circular polarized glasses with left circular polarizer over one eye and right circular
polarizer over the other eye, thus allowing the separation of left and right eye views.
Article on Polarized Light in The Physics Teacher.
Here is an interesting article that appeared in The Physics Teacher recently. You should be able to
find it in your library.
Hasan Fakhruddin, "Some Activities with Polarized Light from a Laptop LCD Screen," Physics Teacher
46, 230-231 (April 2008).
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Last Modified: 08/18/08