The hologram, a clear glass plate, is illuminated from a distance of less than a meter by a laser beam that has been diverged by scattering through a piece of wax paper. The laser beam strikes the hologram perpendicular to the plate. The proper view is seen by observing transmitted light at an angle on one side of the laser axis. However, if you view reflected light or look at the laser at a different angle (opposite the axis of the laser beam) the hologram appears backwards or upside down! Go figure that one out.

The hologram is mounted at an angle and illuminated from above by a bright point source. It includes three successive views of a really ugly man turning into a really scary werewolf. Don't let your young children see this one.

The photographs above show the display setup with three white light holograms and close-ups of two scenes from the werewolf hologram.

Two crossed polaroids, oriented vertically and horizontally, are placed in front of a goose-neck lamp, thereby preventing light from passing to the viewers. When a third polaroid is inserted between the two crossed polaroids at an angle of 45 degrees with respect to the original axes, light can be seen passing through the system.

This demonstrates that the electromagnetic field of which the light consists is a vector. The diagonal polaroid passes a component at 45 degrees with respect to the original light, and the second polaroid passes a component at 45 degrees with respect to the diagonal polaroid. The component of a component is actually perpendicular to the axis of the second original polaroid.

The real paradox involving this system involves an analysis of single photons. How can a single photon originally polarized parallel to the first polaroid have its angle of polarization rotated 90 degrees and exit the final polaroid polarized perpendicular to its original plane of polarization?

Compare M7-03, a simpler demonstration using only two polarizing filters.

When sunlight reflects off a horizontal dielectric surface such as water in a lake, wet roads, or even dry smooth roads, the reflected light is largely horizontally polarized. Polaroid sunglasses are oriented vertically so they remove "glare," which is horizontally polarized specular reflection from such surfaces.

Position the point source so that it reflects from the lecture table onto the front white screen at about the Brewster angle. Rotating a polaroid in the light from the point source before reflection shows clearly that the reflected light is polarized. Individually viewing the reflected light directly using a polarizing filter demonstrates the value of polaroid sunglasses in removing glare. Placing a piece of aluminum foil where the light hits the table demonstrates that reflection from a conducting surface is not polarized.

The photographs above show the polarization axis (a) parallel and (b) perpendicular to a dielectric surface, and (c) parallel and (d) perpendicular to a conducting surface (a sheet of aluminum foil).

Hand out, for the students to keep, 1"x2" pieces of replica diffraction grating material. Look at the bulb with the diffraction grating about one inch in front of your eye as the bulb warms up to see the following: (1) Several relatively weak lines are initially seen, both from sodium and from mercury, which is used as a seed to get the lamp started (top spectrum). (2) As the bulb warms up collision broadening of the lines occurs, so the weak lines become much brighter and spread out to form a nearly continuous spectrum (second and third spectra). (3) When the lamp is operating at full temperature, "cooler" sodium vapor around the periphery of the bulb absorbs light at the frequency of the sodium doublet, producing a nice dark absorption line (bottom spectrum). In the bottom spectrum the exposure has been reduced using a neutral density filter so that the bright areas and the absorption line are not washed out by overexposure.

The photo above shows the second-order spectrum, so some of the red from the adjacent first-order spectrum causes the blue and violet region of the spectrum shown to be slightly magenta (R+B) colored in some of the individual photographs where the intensity has been adjusted.

A low-pressure sodium lamp is used to project the line spectrum of sodium. This one is not real bright, but can be seen if the screen isn't too far away. The advantage is that because it operates at a lower temperature it shows that the yellow light from a sodium lamp has a single yellow component.

The light is defined by a slit, which is imaged on a distant screen after the light passes through the diffraction grating. Gratings of 2000 and 7500 lines per inch are furnished.

Light from the superpressure mercury lamp with a condenser lens and iris passes through a slit, which is imaged on a distant screen by a 20 cm focal length convex lens. A grating is placed in the light after the slit and the focusing lens. For this one we have been using a triple grating with 2400, 7500, and 15,000 lines per inch. Several orders are visible.

The spectrum is a continuous spectrum with some brighter lines superimposed on the continuum. The photograph shows the first order spectrum in some detail. The blue line at the right is actually ultraviolet, but is rendered visible due to fluorescence of a whitener in the white paper used as a photography backdrop. The photographs of the spectra seen above is overexposed so that more of the background and weaker lines can be seen. This leads to distortion of the colors of the lines, introducing some red into the blue lines and causing them to appear magenta in color. Your eye sees these colors correctly, but cannot see as far into the ends of the spectrum.

Food coloring produces colors by a negative process, quite like that of negative color filters. Virtually every type of coloration except light, including food color, ink, dye, paints and other pigments produce negative or subtractive colors. The spectra of red, green and blue food color are shown below along with light shining through a sample of water containing that color placed on a baffle on the overhead projector. The blue one leaves something to be desired, but the others are reasonably true.

Engagement Suggestion:

Background:

If the frame is at rest, the projected balls fail to even go through the first set of holes because they are deflected by gravity. As they travel, they are pulled downwards, following parabolic paths with respect to the frame. If the frame is raised, held in place by an electromagnet and released, it falls with the acceleration of gravity and becomes a "local inertial frame of reference." The balls are automatically fired by a gravity switch when the frame begins to fall. The balls will travel along straight lines in the local inertial frame of reference and end up in the sacks before the frame stops on the shock absorber.

Note that this is a fairly large demonstration and requires some time to set up. P1-11 is recommended for smaller spaces.

In this experiment the arc lamp acts both as a source of ultraviolet radiation for discharging the zinc plate and as a bright light to shadow project the apparatus.

A zinc plate connected to an electroscope and charged positive will not discharge under the influence of ultraviolet radiation. When the plate is charged negative, however, light from the arc lamp, which contains much UV, will discharge the plate, as indicated by the electrometer. A 1/8" glass plate inserted into the light from the arc lamp prevents passage of UV and the discharge ceases. Removing the glass plate allows the discharge to continue.

Collimated light from a laser diode is incident on a double slit, creating interference. A photomultiplier tube sensitive to single photons is attached to the rotating telescope of the spectroscope opposite the light source. The signals from the tube are seen on the oscilloscope and heard using an audio amplifier and loudspeaker.

As the photon counter is slowly rotated the intensity of photons traces out the interference pattern for the double slit used. An identical slide on the Laser Cart (Demonstration M1-11) can be used to display the double slit interference pattern.

The double slit used has slit width of 0.04mm and slit spacing of 0.250mm. With the single photon counter you can hear about nine interference maxima in the main diffraction maximum, pass through the diffraction minima on either side, and hear a few interference maxima in the second diffraction maximum, as seen in the photograph above.