lecdem.physics.umd.edu

The Eye

O2-01: PERSISTENCE OF VISION - ELECTRONIC FLASH

O2-01

Demonstrate persistence of vision.

Look at the flash go off. You see an afterimage for a while due to persistence of vision.

O2
O2-02: ZOETROPE AND PRAXINOSCOPE

O2-02

Individual demonstration of persistence of vision.

The zoetrope and the praxinoscope are moving picture toys dating from the 19th century. The zoetrope is rotated while you view it through the slits on the side, so that a series of still pictures is converted into an apparently moving picture strip; the praxinoscope is viewed from an angle via a rotating mirror.
O2-03: PERSISTENCE OF VISION - MAGIC WAND

O2-03

Demonstrates the persistence of vision

The slide of Einstein (or any of your favorite slides) is focused a few feet from the projector, but unfocused where it strikes any surface on which it might be identifiable. When a white stick pointer is moved rapidly up and down through the focal plane the image of Einstein can easily be identified.
O2-11 PULFRICH PENDULUM

O2-11

Demonstrates visual latency and the Pulfrich phenomenon

Stand back 8-10 feet or more from the pendulum and swing the pendulum perpendicular to your line of sight. Watch the bob with both eyes and hold a dark filter over one eye to see the pendulum appear to move in an elliptical path. Hold the filter over the other eye to make the direction of rotation reverse. This effect is due to a delay in synapses along the optical nervous system between the retina and the brain of dimmer light signals relative to bright signals. Any dark filter or polaroid can be used; most commonly we use either the red or the blue filter from a pair of red/blue stereoscopic goggles. In darker rooms it may help to illuminate the pendulum bob with a goose neck lamp.
O2-12: PULFRICH PENDULUM - BIFILAR SUSPENSION

02-12

Demonstrate visual latency and the Pulfrich phenomenon in a very dramatic way.

Stand back 8-10 feet or more from the pendulum and swing the pendulum perpendicular to your line of sight. Watch the bob with both eyes and hold a dark filter over one eye to see the pendulum appear to move in an elliptical path. Hold the filter over the other eye to make the direction of rotation reverse. This effect is due to a delay in synapses along the optical nervous system between the retina and the brain of dimmer light signals relative to bright signals. Any dark filter or polaroid can be used; we use either the red or the blue filter from a pair of red/blue stereoscopic goggles. In darker rooms it may help to illuminate the pendulum bob with a goose neck lamp.

This is just like Demonstration O2-11: PULFRICH PENDULUM, except that the pendulum has a bifilar suspension. Some people like this one better because the pendulum never changes plane, and some people like the other one better because the bifilar suspension gives additional optical clues to observers at an angle with respect to the two suspension strings.
O2-13: PULFRICH PHENOMENON - ROTATING CRYSTAL MODEL

O2-13

Demonstrate the Pulfrich phenomenon

A three-dimensional crystal model is placed at the center of a 33 1/3 RPM turntable and its shadow is projected on a white screen. An observer views the shadow with both eyes while holding a dark filter over one eye to see apparent rotation of the crystal model. Holding the filter over the other eye, the crystal model appears to rotate in the opposite direction. Polaroids or any other dark color filter can be used; we use one filter from a pair of red/blue stereo goggles.

O2, I7, LS1
O2-14: VISUAL LATENCY - REACTION TIME

O2-14

Demonstrate visual latency.

A meter stick is held by one person directly above the hands of a second person, the victim. When the meter stick is dropped, the victim closes its hands to catch the meter stick. The reaction time T of the victim can be calculated as T=SQRT(2S/g), where g is the acceleration of gravity and S is the distance the meter stick has fallen.

The experiment is then repeated with the room darkened. Typically the meter stick will fall considerably further, due to the longer reaction time in a darkened environment. The increased time, due to visual latency, can be determined by subtracting the reaction time in the light from that in the dark.

This is one reason for increased reaction time in night driving.
O2-21: STROBOSCOPIC CENCO

O2-21

Demonstrate persistence of vision using strobosccope.

A disc contains the letters C E N C O in apparently random order. When the disc is spun rapidly and illuminated by a stroboscope at the right frequency, the word CENCO is clearly formed, accomplishing their advertising goal.

N3, O2, LS1
O2-22: STROBOSCOPE AND FALLING WATER

O2-22

Demonstrate how a stroboscope works, and illustrate persistence of vision.

A water container with a nipple near the bottom is connected to a plastic tube and eye dropper to produce a stream of water which breaks up into a series of water droplets. The water stream is illuminated by a stroboscope and viewed by a TV camera. Adjustment of the strobe frequency can make the water droplets move up or down either slowly or rapidly, or even stand still!

O2, LS1
O2-23: STROBOSCOPIC DISCS

O2-23

Demonstrate stroboscopic effects.

Several stroboscopic discs are available with a high-speed rotator and stroboscope. Look them over to find what interests you. Some are similar to the patterns seen on the obsolete stroboscopic piano tuners. Some are motional things like a swinging pendulum, shown in the photograph above. Another is actually the demonstration O2-21: STROBOSCOPIC CENCO, a shameless advertising ploy.

N3, O2, LS1
O3-01: COLOR PERCEPTION IN SHADOWS - SLIDE PROJECTORS

O3-01

Demonstrate complementary color shadows.

A white (clear) slide and a colored slide are superimposed on a screen with the intensity of the colored slide decreased such that the entire field looks white (or maybe slightly off-white). When an object is held in the light path two shadows are created: the shadow of the white light is the color of the second slide, but the color of the shadow of the colored slide is not white, but rather the complementary color to the colored slide. For example, a red slide makes a cyan shadow, a green slide makes a magenta shadow, and a blue slide makes a yellow shadow.

FS1, E2
O3-02: COLOR PERCEPTION IN SHADOWS - DOUBLE POINT SOURCE

O3-02

Demonstrate complementary color shadows.

A double bright point source is used with colored filter in one side and the other side with no filter, so the light on a nearby screen appears either off-white or slightly colored. When an object is held in the light path two shadows are created. The shadow of the white light is the color of the second slide. The color of the shadow of the colored slide is not white, but, rather, the complementary color to the colored slide. For example, a red slide makes a cyan shadow, a green slide makes a magenta shadow, and a blue slide makes a yellow shadow. This demonstration is like Demonstration O3-01: COLOR PERCEPTION IN SHADOWS - SLIDE PROJECTORS, except that it is a little easier to move but not as adjustable.
O3-03: LAND EFFECT

O3-03

Demonstrate the Land effect using red and white light.

This experiment must be done in the dark. Two slides of the same object which were taken in white and red light, are lit by white and red light respectively and superimposed on a screen using slide projectors with zoom lenses and horizontal and vertical adjusters. The intensity of the red light is then adjusted so that the full spectrum of colors is seen in the picture. The two component images are shown below.
O3-04: GREENER THAN GREEN

O3-04

Demonstrate negative color afterimage due to saturation.

Here's the question: What is greener than green? To find the answer, you must carry out the following experiment. Two slides are prepared, the first with the right half covered by a green and the left half by a magenta filter, the second with the right half covered by a green filter but with the left half open so it is white; both have a black dot in the center to stare at.

Everyone stares at the dot in the middle of the first slide for about 20 to 30 seconds. That slide is then quickly replaced by the second slide, with everyone continuing to stare at the black dot in the center. On the second slide the green half on the right is the same as the first slide, but the magenta left side of the first slide is replaced on the second slide by a white field. The white field viewed on the left side of the second slide is actually greener than the actual green field at the right.

Why do you see green on the right side of the second slide, and why is it greener than the actual green side? Staring at the first slide saturates your green receptors on the right side, but saturates your red and blue (magenta) receptors on the left side. When the second slide comes up, the green receptors on the right remain saturated (even more), causing the color to be washed out. However, the magenta side has saturated your red and blue receptors, but left your green receptors totally unused. Therefore, when the white light is seen, red and blue are washed out but the green receptors are strongly excited, leading to a beautiful green field.

To perform the experiment using your video monitor, click on the photograph above to get the first slide, then click on that slide when you have stared at it for 20-30 seconds to saturate your eyes. Prepare your computer to do the transitions rapidly by cycling through the photographs to place them in your cache memory or use the PowerPoint program linked below. However, this only works if your monitor is at the proper pixel size.
O3-11: COLOR BLINDNESS TESTS

O3-11

Test your students for color blindness.

This test manual contains a sequence of 24 color plates with numbers, arranged to determine if the viewer has color blindness to certain colors or combinations of colors. The instruction manual is included.
O3-12: COLOR BLINDNESS TESTS WITH VIDEO PROJECTOR

O3-12

Test class for color blindness, and demonstrate how color blind people with certain defects see the color blindness tests.

Clicking on any of the images above will bring up a full-screen version. In a class the color blindness test patterns are viewed using a video projector using a computer and this web page. If you click your mouse on one of the three patterns pictured above you will see a larger version of the pattern. Clicking on that pattern, you will see another copy of the same pattern with the red removed, that should look similar to the way it would appear to a person with red-green color blindness. By switching out one of the three (red, blue, or green) projector colors using the included switch box we can see how that particular color blindness test looks to an individual with that type of color blindness. In the first image, the number 12 is visible to all people, including those with any type of color blindness. The middle image appears as a number "15" to those with normal vision, a "17" to those with red-green deficiencies, and no number is visible to those with total color blindness. The rightmost image appears as a number "74" to those with normal vision, a number "21" to those with red-green deficiencies, and no number is visible to those with total color blindness.
O3-21: BENHAM TOP

O3-21

Demonstrate perception of color due to periodic excitation of the eye.

A disc contains half black and half white fields with various sets of azimuthal black lines on the white field. Rotating the disc a few times per second creates circles which appear to have various mild coloration, from greens to browns to blues. Rotating in the opposite direction changes the colors.
O3-22 BIDWELL'S DISC

O3-22

Demonstrates positive and negative color afterimages

A half white and half black disc with a small cutout is rotated a few revolutions per second with a red light bulb visible through the cutout. When the black segment follows the cutout (counterclockwise rotation) a red afterimage is seen, due to saturation of the red cones while the bulb is visible. When the white segment follows the cutout (clockwise rotation) a cyan afterimage is seen, the complementary color to red. Because the red cones are saturated by direct viewing of the bulb, the white field activates the green and blue cones more strongly, producing a negative afterimage.
O4-01: SCIENCE

O4-01

Trick the unwary student

A phony "Maine license plate" has been rigged so that if only the tops of the letters are seen it appears to say SCIENCE. Cover the letters with a white cardboard baffle, and lower the baffle until everyone recognizes that the sign says SCIENCE.

Point out that in science we try to see enough of the total situation that we avoid dumb conclusions like the one they just made.

OS2
O4-02: PARIS IN THE SPRING - EXPECTATIONS

O4-02

Show how your mind affects what you see.

Hold up the sign above for a few seconds, then ask everyone what it says. Most of the class will agree that it says "Paris in the Spring." If you agree, then read the sign again - very carefully. In fact it says PARIS IN THE THE SPRING - two THEs!!