lecdem.physics.umd.edu - Geometrical Optics

Geometrical Optics

N3-26: COLOR FILTER SETS

N3-26

Several color filter sets are available for use as the instructor desires.

The following color filter sets are available.
(1) Set of 13 2"x2" optically polished chance glass filters from 2.5 to 3.0 mm thick.

(2) Set of 17 4"x4" transluscent glass filters with detailed descriptions of each, including some transmission curves.

Also available on request are the following sets from other demonstrations:

(1) A set of Kodak Wratten filters, used with some of the other demonstrations, including N3-05: COLOR MIXING VIA CHROMATICITY DIAGRAM.

(2) Sets of precision positive and negative lantern slide filters, which are used in demonstrations N3-21 and N3-22.
N3-27: NEGATIVE FILTERS IN SERIES

N3-27

Demonstrate how negative filters combine to give color combinations

Place two negative filters on the overhead projector as seen in the top row of photographs below, so that their colors can be viewed. When they are overlapped, as seen in the lower row of photographs, negative color mixing can be observed. Each negative color slide removes one of the primary colors - its complementary color - from the light. Two series negative filters remove two of the primary colors, leaving the third as the resultant color.
N3-28: HOLLOW PRISM

N3-28

Observe the color spectrum of white light using a prism filled with liquid.

This is a hollow equilateral triangular prism. It can be filled with any liquid -- water, mineral oil, glycerin, colored water, etc. Observe the resulting spectrum.
N3-31: COLOR SEPARATION TRANSPARENCIES

N3-31

Illustrate color mixing of negative color transparencies.

Four negative color transparencies of the subject girl with umbrella are shown: one taken in yellow light, one taken in cyan light, one taken in magenta light, and one taken in white light (left picture, upper left image). When the three individual subtractive color transparencies are superimposed (right picture, right image) the result is the same as the white light transparency (right picture, left image).
This demonstration illustrates how real color films work.

N3
N3-32: COLOR SEPARATION TRANSPARENCIES - LARGE

N3-32

Illustrate color mixing of negative color transparencies.

Four negative color transparencies of the subject shown: one taken in yellow light, one taken in cyan light, one taken in magenta light, and one taken in white light. When the three individual subtractive color transparencies are superimposed the result is the same as the white light transparency.
This demonstration illustrates how real color films work.
N3-33: MAXWELL'S EXPERIMENT - THREE-COLOR MIXING

N3-33

Demonstrate three-color mixing with positive colors.

Three identical slides taken in white light are projected superimposed on the same place by three slide projectors, but each is filtered by one of the three primary color filters: red, green, and blue. Add the three together to get the same effect as the original slide, which can be displayed with white light on a fourth slide projector for comparison.
N3-41: SPLITTING OF SPECTRUM INTO COMPLEMENTARY COLORS

N3-41

Show complementary colors by removing a band from the white light spectrum.

A baffle is inserted into the focal point of the white light spectrum, removing a narrow band of color. The light removed is reflected by the baffle to a mirror which reflects it to the screen, adjacent to the rest of the spectrum, which has been recombined and focused onto the same screen. The two parts of the spectrum are complementary colors.
The baffle can be moved across the spectrum to obtain various complementary color combinations.
N3-42: COLOR MIXING IN PRINTER - CHART

N3-42

Show how modern printer achieves its range of colors.

This is a large chart (over one meter square) which shows all of the 256 colors produced by a modern color printer and specifies the admixtures of the primary colors used for each. It includes 8 scales of gray, 8 primary positive and negative colors (black, white, red, green, blue, yellow, cyan, magenta) and a 10 x 24 grid of colors with identifying admixtures.
This particular chart is an advertising gimmick for the HP Designjet 650C Plotter, and a very good one, I must say.
N3-43: COLORS REFLECTED FROM COLORED OBJECTS

N3-43

Show why colored objects look colored.

A slide projector with Wratten filters is used to illuminate various colored objects such as brilliantly colored fuzzy toy animals. The animals only show a particular color when (1) the fur actually reflects, or will not absorb the color in question, (2) the illumination contains that color of light. For example, a red teddy bear will only appear red if red light is in the spectrum of the illumination. White fur takes on whatever color illuminates it. Pure blue fur appears black (no reflected light) when illuminated by light containing no blue wavelengths. Photographs below show pairs of animals illuminated in white light then red (left and left center) and white then blue (right center and right).
This is the reason why spectral streetlights such as sodium and mercury are often disdained.
N3-44: THE COLOR OF COPPER

N3-44

Show that light reflected from copper is a desaturate orange.

Light from a slide projector reflected off a polished piece of copper plate onto a screen exhibits its classic color, as seen at the bottom of the photo above. The color at the top is a desaturated orange produced as follows: red and green filters are mixed to produce a saturated orange, then dim white light is added by a third projector to desaturate the orange color. Lamentably, in the photograph the white light from the lower projector of the three used in the color mixing, appears off-white; the real demonstration is very clear. The photograph at the right shows the experimental setup.
N3-45: REFLECTION AND TRANSMISSION FROM GOLD FOIL

N3-45

Show the complementary relationship between light reflected from and transmitted by a thin gold foil.

Light from a bright point source with condenser lens and baffle is focused by a 30 cm focal length convex lens through a gold foil onto a distant screen. Some light is reflected off the gold foil, then reflected again by a front surface mirror onto the screen, forming a spot adjacent to the direct transmitted beam. The reflected beam is gold in color, typical of light reflected from gold. The transmitted light is bluish in color, the complement of the color of gold. Rotate the front surface mirror to superimpose the reflected and the transmitted light and get back white!
N3-51: SPECTRUM WITH MINICAM AND VIDEO PROJECTOR

N3-51

Demonstrate how color TV equipment derives its color, and to illustrate a defect in handling of saturated colors such as spectra.

A prism white light spectrum, demonstration N1-01: PRISMATIC SPECTRUM OF WHITE LIGHT - POINT SOURCE, is viewed by a color minicam and displayed by a color video projector adjacent to the original spectrum. The spectrum produced by the projector has three bands of uniform color: red, green, and blue, separated by small gaps.
This effect creates a problem in producing photographs and videos of line spectra or other types of highly saturated colors. Fortunately, most color in the real world contains wide bands or continuous wavelengths, and is therefore handled reasonably accurately by video.

An example of this effect in film is the color slide series THE LIGHTS ON CAMPUS: A STUDY OF ATOMIC SPECTRA, distributed by the AAPT. The hue of the continuous spectra does not change continuously throughout the spectrum, but rather the spectrum on film is formed from three uniformly colored RGB segments separated by small gaps.
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.
O1-23: ASTIGMATISM - ON AXIS

O1-23

Demonstrate on-axis astigmatism.

On-axis astigmatism is the type which occurs in the eye. Light from a bright point source is focused onto a pinhole, which acts as the object for the system. The pinhole object is then focused by an "astigmatic lens" consisting of a 10 cm focal length spherical convex lens and a 30 cm focal length cylindrical convex lens. The vertical focus is a horizontal line and the horizontal focus is a vertical line; between these two foci is the "best" focus circle of confusion. The foci are shown by moving a ground glass screen along the optic axis, and can be seen as follows: Keep the screen perpendicular to the optic axis while moving it along the optic axis to see (moving outward from the astigmatic lens from left to right in the photographs below) (1) the beam immediately following the lens, (2) the horizontal focus (a vertical line), (3) the waist, (4) the vertical focus (a horizontal line), and (5) the expanding beam following the second focal point.