lecdem.physics.umd.edu

The Eye

N3-01: ADDITIVE COLOR MIXING OF LIGHTS

N3-01

Demonstrate color mixing of lights.

This is a commercial color mixing device that has a red light on top, a green light on lower left, and a blue light on lower right (well, actually, a white light with a filter in each position). When the center switch is pushed, the three lights go on, and mixtures can be seen on the little pyramid in the center of the screen: red + blue = magenta on right, and red + green = yellow on left. We added a blue light on the center left, which when turned on in combination with the main three lights produces white at the left by adding blue to the yellow created by the red + green corner lights. Similarly, a green light on the center right adds to magneta of the red + blue combination to produce white light.
N3-02 ADDITIVE COLOR MIXING - PROJECTORS

N3-02

Demonstrates additive color mixing of light

Three slide projectors in a special three-projector mount on a roll-around cart are equipped with color filters. The projectors have been re-wired so that the intensity is adjustable by changing the voltage on the bulb without affecting the fan. The colors are easily seen, and additive color mixing can be nicely shown: R+B=M, R+G=Y, B+G=C, M+G=W, Y+B=W, and C+R=W, where R=red, G=green, B=blue, M=magenta, Y=yellow, C=cyan, and W=white.
N3-04: POSITIVE COLOR MIXING IN COLOR TV

N3-04

Show how color TV produces colors.

A color band test pattern is played onto a color monitor which is viewed by a minicam in the microzoom mode with its picture displayed on a second color monitor. Viewing the various color bands as seen in th photographs below shows how the color TV colors are produced and mixed.
N3-05: COLOR MIXING VIA CHROMATICITY DIAGRAM

N3-05

Detailed demonstration of color mixing.

The three-projector color mixing gizmo from demonstration N3-02 is used to match a variety of colors by adjusting the relative intensities of the primary color components. The colors to be matched are produced by a series of slides on an overhead projector, shown in the three figures below, and displayed on the screen beside the mixture which is being adjusted.
Wratten filters are used to produce these colors and mixtures because they are the purest and most adequately documented color slides. The primary colors are three of these filters that are positioned near the corners of the chromaticity diagram. The colors to be matched lie along the lines between the primary filters, so that they can be readily produced by mixing various levels of the primary colors.

Included with this demonstration is a transparency of the chromaticity diagram, seen at the right above, showing where all of the filters used in this demonstration lie.
N3-06: COLOR WHEEL

N3-06

Demonstrate additive color mixing of lights.

When the color wheel is illuminated by good white light and spun on a fast rotator it appears white - well, almost white! Invite students to discuss why it is the color it is, and what might affect that.

N3
N3-07: COLOR WHEEL - FLUORESCENT

N3-07

Demonstrate color mixing.

This color wheel, painted with fluorescent paint, is illuminated by ultraviolet light (a black light) and spun on a fast rotator. It makes a very nice white mixture, demonstrating color mixing very nicely, as seen in the photograph at the right.
N3-08: STROBOSCOPIC COLOR WHEEL

N3-08

Demonstrate color mixing.

Spinning the color wheel on a fast rotator while illuminating it with a bright stroboscope produces a variety of color mixtures. For example, fourfold symmetry would imply that the color is produced by a mixture of four primaries: G+G+B+B = saturated cyan. Others include 2R+G+G+B = R+G+(R+G+B) = Y+W = unsaturated yellow, and B+B+2R+G =B+R+W = unsaturated magenta. The photograph at the right shows the color wheel rotating while being illuminated by the stroboscope blinking at the rotation rate of the wheel. Challenge students to predict the outcome of various combinations

N3, LS1
N3-11: RGB SNAP LIGHTS AND SPINNER

N3-11

Demonstrate color addition through moving colors.

Snap the red, blue, and green light sticks and attach them to the axle. Dim the lights, turn the crank, and observe a circle of white light!
Assembly instructions:

Unwrap the red, blue, and green light sticks. Arrange the sticks on the bolt so the sticks are resting in the indentations in the positioning plate. Tightly attach the second hexagonal nut. Turn the back of the bolt (on the crank side) clockwise to tighten the front nut. Reposition the light sticks into the indentations if they move. If the assembly does not turn smoothly, with needle-nose pliers, reach behind the large washer and loosen the hexagonal nut (turn counter-clockwise) up to a half-turn. This will allow the assembly to spin as well as further tighten the light stick assembly. Stand the spinner on its base. Turn the crank. If the assembly does not spin smoothly, loosen the hexagonal nut another half-turn. Snap each of the sticks to begin the chemical reaction, and shake the entire assembly to thoroughly mix the chemicals. Hold the base steady with one hand while you turn the crank with the other hand. Viewers should observe a white circle!
N3-23: COLOR PERCEPTION WITH DYES AND FILTERS

N3-23

Compare colors and spectra of dyes and filters.

Using the spectrum setup of demonstration N1-01: PRISMATIC SPECTRUM OF WHITE LIGHT - POINT SOURCE, small tanks of water colored by various colors of food coloring are placed in the spectrum setup just before the slit to see the spectrum of light passing throught the colored water. The color of this water can be seen by holding the tank in the light from an overhead projector. The colors and spectra of water colored by food coloring can be compared with those of positive and negative color filters.
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.
O1-01: EYE MODEL - OPTICS

O1-01

Demonstrates optics of the eye and corrections of optical defects

The eye model is an oval tank, filled with water representing the aqueous humor, with a lens representing the eye lens on one end and a screen representing the retina with three positions: normal, nearsighted, and farsighted.

O1
O1-02: EYE OPTICS MODEL - INDIVIDUAL VIEWING

O1-02

Show inversion of the image on the retina by the eyelens.

In the end of the sphere is a convex lens, which functions as the eyelens, producing a real inverted image of a distant object on the plane at the back of the sphere. This image can be viewed by an individual looking through the cylinder, and observed to be inverted.
O1-03: IMAGE INVERSION ON RETINA

O1-03

Model image inversion on the retina.

A TV camera models the eye to demonstrate that the image on the retina is inverted, and how this arises. A pinhole illuminated by a small light bulb is placed inside the near point of the eye. The head of a pin is inserted into the light path between the eye and the pinhole from below, but appears to be entering from above.

The image of a distant object is inverted on the retina or TV videcon. The head of the pin, however, casts a shadow because it is inside the near point of the TV (or eye) lens. The shadow is on the lower part of the retina, which is the upper part of the image because of the inversion.
O1-04: IMAGE INVERSION ON RETINA - INDIVIDUAL VIEWING

O1-04

Hand out small squares of black paper with pins to the class members to do this experiment.

Hand out small squares of black paper with pins to the class members to do this experiment.

Punch a pinhole in the paper and hold the paper about an inch from your eye (within the near point of the eye) while viewing a white surface through the pinhole. Raise the pinhead from below into the line of sight between the pinhole and your eye. Because the "image" is actually a shadow on the retina, it is not inverted by the eyelens, and appears to be coming from above.

O1
O1-05: EYESCOPE

O1-05

See your retina!

Look into the eyescope to see your retina. The eyescope includes light and appropriate optics.
O1-11: TEST OF NEARSIGHTEDNESS VS. FARSIGHTEDNESS

O1-11

Identify nearsighted and farsighted eyes using laser speckle.

Project the speckled laser pattern on a screen after expanding the beam to about 5 cm diameter. Remove your glasses for the eye test. Watch the speckle pattern as you move your head from side to side. If the speckles move opposite to the direction of your head motion you are nearsighted; if the speckles move in the same direction as your head motion you are farsighted. Put your glasses back on and repeat the experiment to check the adequacy of your lens prescription.

The eye focuses the speckle pattern at its natural focal point. Parallax between the focal plane of the eye lens and the retina cause the apparent motion of the speckle pattern.
O1-21: CHROMATIC ABERRATION IN EYE

O1-21

Observe chromatic aberration in your eye.

A slide having green and cyan segments superposed on a red background, is projected onto a screen. Chromatic aberration in the eye diffuses the boundary between any of the two pairs of colors, causing a blurry white or gray line at the boundary.

These colors were chosen because they are complements as seen by various eyes. One or both of the pairs of colors will mix to form white when viewed by virtually anyone's eyes. The colors are not quite correct in the photograph above.

O1
O1-22: SPHERICAL ABERRATION IN EYE USING PINHOLE

O1-22

Show spherical aberration in eye and its correction.

Hand out to the students a piece of black construction paper with a pin. View a small lighted cross at a large distance from your eye with your glasses removed in a dark room. Because the room is dark, the iris of the eye will open wide, causing a large amount of spherical aberration. Then view the same lighted cross through the pinhole. Because the pinhole limits the useable aperture of the eye, spherical aberration is significantly reduced, and the cross appears clearer. This effect can be used to see distant objects more clearly if you suffer from nearsightedness
O1-31: MACH BANDS

O1-31

Demonstrate Mach bands.

Cast a shadow of a row of lights onto a sheet of light paper or folded cardboard. The photograph at the right shows shadows from a series of goose-neck lamps with the inside of the lamps painted black to provide a more effective point source. Although the illuminantion of the series of gray areas is actually constant for each area, the phenomenon of simultaneous contrast in the eye causes the area to look darker adjacent to a lighter area and lighter adjacent to a darker area.

O1, N3
O1-32: DARK AXLE

O1-32

Demonstrate the dark axle illusion.

A loop of metal rod is illuminated as it is rotated rapidly about a diameter. The vertical diameter of the loop appears to be darker than the rest of the area, which appears as a dimly lighted disc.

O1, D1, LS2
O1-41: FLOATERS

O1-41

Individual observation of floaters in the eye.

Each spool has opaque material across both ends with a pinhole in each center. Viewing a distant light through the pinholes with the spool very close to your eye produces a nearly parallel light beam through your eye onto your retina. Red blood cells located directly in front of your retina diffract this parallel light, causing dark circle diffraction patterns on the retina.

These red blood cells sometimes cause floaters to appear in your eye when you stare into space, for example at the pure blue sky.

O1