lecdem.physics.umd.edu - Geometrical Optics

Geometrical Optics

N1-03: PRISMATIC SPECTRUM OF WHITE LIGHT - WATER PRISM

N1-03

Demonstrate continuous spectrum using water as the dispersing medium.

The bright point source is used to provide a continuous white light spectrum. Light from the point source is focused by a condenser lens with iris and a 20 cm focal length cylindrical lens onto a slit. A 20 cm focal length convex lens then images the slit onto the screen through a prism consisting of an equilateral triangular plastic box filled with water. The spectrum created has less dispersion than that of a glass prism due to the smaller index of refraction of water. The spectrum of a quartz glass prism is shown at the right above for comparison.
Before showing the demonstration, invite students to predict how the different index of refraction will change the spectrum seen.

OM1, LS1
N1-05 SPECTRA - VISIBLE AND INVISIBLE

N1-05

Demonstrates continuous spectrum

The carbon arc lamp is used to provide a continuous white light spectrum. Light from the arc lamp is focused by a condenser lens with iris and a 20 cm focal length cylindrical lens onto a slit. A 20 cm focal length convex lens then images the slit onto the screen through an equilateral prism. A fluorescent screen (with fluorescein) is used to show that there is ultraviolet radiation, including a strong UV line, in the carbon arc spectrum. A thermopile is used to sense infrared radiation, where the heat measured by the thermopile causes an audio oscillator to rise in pitch, so a hotter source produces a higher tone. (see I2-06 for more on this apparatus) Aiming the thermopile from the spectrum back toward the prism, it is observed that the hottest part of the spectrum is just off the red color, in the infrared.

N1, OM1, LS1
N1-06: POLYPRISM SPECTRA

N1-06

Demonstrate continuous spectra with several different types of glass.

The bright point source is used to provide a continuous white light spectrum. Light from the point source is collimated by a condenser lens with iris and focused by a 20 cm focal length cylindrical lens onto a slit. A 20 cm focal length convex lens then images the slit onto the screen through an equilateral prism.

N1, OM1, LS1
N1-08: GOETHE'S PUZZLE

N1-08

Show the view of a slide through a prism and argue about color theory.

A 60 degree prism is mounted in front of a slide projector lens such that it can be rotated. The slide used is that of a plain white surface with some marking to show orientation. When projected so that the prism disperses, it adds color only to the edges of the field of the projected slide At another prism angle a mirror image of the slide is seen due to internal reflection in the prism.
Because of the great contrast, the print on the slide cannot be seen in the photograph.

N1
N1-09: COMPLEMENTARY SPECTRUM

N1-09

Display the spectrum of a "negative" slit.

The white light spectrum using a slit as its source is well known. (See adjacent demonstrations.) In this demonstration, the spectrum of a "negative" slit is displayed - that is, the spectrum of a broad uninhibited white light beam with an opaque vertical rectangle in the center. What will the spectrum of this negative slit look like?

To carry out this demonstration, a three-part baffle, photographed below, is inserted in front of the light source and the light is then passed through a prism and focused onto a screen. For comparison, the spectrum of a "normal" slit opening is shown, that of a "negative" slit the same size as the regular slit, inserted onto a wider open area, and the spectrum of a wide open area the same size as that on which the negative slit is positioned. The spectra using these three baffles are shown in the above photo, but they are washed out.

Below the photograph of the baffle used in the demonstration are three spectra, left to right: the standard white light spectrum, the spectrum of the negative slit combined with the regular white light spectrum, and the complete spectrum of the three parts of the baffle shown immediately below. (Note that the parts of the spectra are inverted with respect to the baffle due to the lens that focuses the image of the slit onto the screen.) The negative slit removes the spectral colors from the continuum created by the open baffle, producing a sequence of "negative" colors (right to left): cyan (minus red), blue (minus yellow), magenta (minus green), red (minus cyan), and yellow (minus blue).

OM1
N1-21: COLOR SEPARATION AND RECOMBINATION BY PRISMS

N1-21

Show how a prism can recombine previously dispersed white light.

The prismatic spectrum setup of demonstration N1-01: PRISMATIC SPECTRUM OF WHITE LIGHT - POINT SOURCE is used to create a white light prism spectrum, seen in the photograph in the center. A second identical prism is then positioned in the beam with the base opposite that of the dispersing prism, so that the colors are recombined into a white spot.

OM1, LS1
N1-22: OPTICAL BOARD - PRISMS - SEPARATE AND RECOMBINE

N1-22

Separate white light into its component colors with a prism and use an identical prism to recombine the light.

Use the optical board with a single slit baffle and a convex lens to limit the size of the ray. The single ray is passed through a prism which separates the light into its component colors, which can be cast onto a white surface. Placing a second identical prism in the beam in the opposite orientation recombines the colors into a white light spot and directs the light in a ray parallel to the original ray.
Use a mirror to reflect the original dispersed light onto the screen, then move the support to the left to intercept the beam with the second prism, which will direct the beam onto the same spot after recombining. The optical element configuration is shown at the left (one prism) and at the right (two prisms) above The resulting "spectrum" for each configuration is shown below.
N1-23: FINE RESOLUTION OF SPECTRUM USING TWO PRISMS

N1-23

Increase the dispersion in a white light spectrum by adding a second prism.

Use the prismatic spectrum setup of demonstration N1-01: PRISMATIC SPECTRUM OF WHITE LIGHT - POINT SOURCE to obtain a standard white light spectrum, shown in the photograph in the center above. A second identical prism is inserted into the beam in the same sense as the first prism, so that the dispersion is increased, as seen in the photograph at the right with the same scale for comparison.

OM1, LS1
N1-32: ICE CRYSTALS - PAPER MODELS

N1-32

Illustrates how two types of ice crystals formed high in the atmosphere float downward through the air with preferred orientations.

Ice crystals may be formed in the upper atmosphere and float downward through the air. Pencil shaped hexagonal crystals fall with their axis of symmetry horizontal, while flat crystals float with their axes vertical (flat area horizontal). This can be seen by throwing the crystals in the air and observing how they fall.
Pencil shaped and flat hexagonal ice crystals made of paper and foam are provided for this demonstration.

Scattering and refraction of light by these crystals creates several important natural atmospheric phenomena: (1) The sun pillar is formed by reflections of the sunlight by the flat surfaces of flat crystals when the sun is near the horizon, (2) The 22 degree halo around the moon is formed by refraction with the minimum angle of deviation from pencil ice crystals when the moon is very high in the sky, (3) The sun dog is formed to the side of the sun by refraction from flat ice crystals when the sun is low in the sky.
N1-41: RAINDROP RAY MODEL

N1-41

Illustrate formation of a rainbow.

This is a large roughly scaled model showing the refraction of light rays in a raindrop leading to the formation of a rainbow.

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N1-42: RAINDROP MECHANICAL MODEL

N1-42

Illustrate rays in formation of a rainbow.

A styrofoam ball represents a raindrop, with a plastic dowel representing the incoming white light ray and painted wooden dowels representing the exiting red and blue rays for the first order rainbow. This shows why the red and blue rays from the same raindrop cannot be seen by the same person.

N1
N1-43: RAINBOW - GLASS BEAD MODEL

N1-43

Observe an almost complete circular rainbow formed by glass beads.

Glass beads are glued to a black screen to simulate water drops. When the screen is viewed from close up, with the light source (or sun) coming from behind the observer, the primary rainbow can be seen at an angle of about 22 degrees. The secondary rainbow is at an angle of about 88.5 degrees, and must be viewed by looking nearly parallel to the surface of the card, so it cannot be seen while viewing the first order rainbow. This is best observed on an individual basis. The dark region in the center of the photograph is a shadow of the camera taking the photograph, where the observer would position his or her head.
Several features of this rainbow are similar to features of real rainbows: the colors are in the correct order (red outside and blue inside, a bit washed out in the central region in the photo) and are reasonably realistic, the area outside the rainbow is very dark, compared to the area inside the rainbow, and several supernumerary bows (white circles) can easily be seen inside the rainbow.

The real primary rainbow is at about 41 degrees, and the secondary rainbow is at about 52 degrees. The higher index of refraction of the glass beads shifts the primary rainbow to 22 degrees and the secondary rainbow to 88.5 degrees.
N1-44: OPTICAL BOARD - RAINBOW

N1-44

Demonstrates internal reflection and dispersion of light

A single light ray is focused by a convex lens to limit its width. The ray enters a plastic disc, which models the water drop, is reflected internally and returns at an angle of about 40 degrees. The dispersion is apparent at a distance from the drop, so the light can be cast on a wall or a distant screen to see the colors.

FS0
N1-45: RAINBOW - WATER FLASK

N1-45

Demonstrate a large angle rainbow.

Light from a bright point source falls onto a spherical water flask. The flask acts as a raindrop, producing a nearly 360 degree rainbow. The rainbow is bright enough that it can be seen throughout the entire lecture hall.
Both the primary rainbow and the secondary rainbow, with colors reversed, can be seen. Slight adjustment of the flask on the sliding carriage may be necessary to maximize the brightness.

Please handle with care, the flask is fragile.

N1, OM1, LS1
N1-61: REFRACTION OF LIGHT IN DIAMOND

N1-61

Show how a diamond produces colors from white light.

A bright point light source with condenser lens and baffle passes through a small hole in the screen and strikes a small mounted artificial diamond. Because of its multiple prismatic facets, the diamond separates the light into its spectral colors, directing much of the light backwards onto the screen.
N3-21: ADDITIVE FILTERS AND THEIR SPECTRA

N3-21

Show colors of additive filters along with their spectra.

Two sets of red, green and blue additive color filters are used to simultaneously observe the color of the filter and its spectrum. A baffle placed on an overhead projector has a cutout into which the the filter is placed to show the color of the filter. The prism spectrum setup of demonstration N1-01: PRISMATIC SPECTRUM OF WHITE LIGHT - POINT SOURCE is used with the filter placed in a holder just before the slit. The color of the light passed by the filter on the overhead projector is located on the screen adjacent to the spectrum for easy comparison.
Positive color filters are seen to pass a band of wavelengths around their apparent color, as seen in the photographs above.
N3-22: SUBTRACTIVE FILTERS AND THEIR SPECTRA

N3-22

Compare colors and spectra of negative filters.

Using the spectrum setup of demonstration N1-01: PRISMATIC SPECTRUM OF WHITE LIGHT - POINT SOURCE, negative color filters are positioned in the spectrum setup just before the slit to see the spectrum of light passing through the negative filters. Placing an identical filter on an overhead projector baffle allows us to view the color of the filter and its spectrum simultaneously.
Negative filters attain their color by removing the complementary color from white light.
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.
N3-24: FILTERGRAPH

N3-24

Show colors with their spectra.

A number of transparencies are available to show on the spectrograph screen. Each transparency has a large color filter along with its spectrum. Overlay two filters to see their sum and the sum spectrum (wavelengths where both are present). A large number of filters are available showing positive and negative colors as well as various mixtures.

OS special
N3-25: SERIES RED FILTERS

N3-25

Show saturation using series filters.

A series of red filters is arranged so that light can be seen as it emerges from each successive filter. (Each square filter in the series occupies less of the area of the first filter.) Pass light through the filters or look at a white light through the filters to see how saturation changes the color.