Engagement Suggestion

Background

Three sources are permanently mounted on a roll-around cart, from top to bottom: (1)a clear glass long-filament incandescent light bulb which produces a continuous white light spectrum, (2) a mercury lamp which produces a line spectrum, and (3) a cadmium lamp which produces a line spectrum

These spectra are seen using 1"x2" sections of a large roll of replica diffraction grating material with 13,200 lines per inch. The pieces of grating material are relatively cheap, and may be given to the students. Tell your students to go away and look at the spectra of various lights.

The three lamps are mounted in a vertical line so the colors of the lines are the same as those in the adjacent white light spectrum. Point out that the spectra of mercury and cadmium are very different, and generalize that observation to suggest uniqueness of the spectra for each material.

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.

This is a very bright spectral source which shows bright lines as well as a continuous spectrum. Light from the superpressure mercury source passes through a condenser lens and iris and is focused onto a slit by a 10 cm focal length cylindrical convex lens. A 20 cm focal length spherical convex lens focuses the slit onto a distant screen, with the flint glass prism placed in the light just after the lens. Note that the two violet rays at the left of the spectrum are actually ultraviolet, but can be seen because of fluorescent materials in the white paper used as the photography backdrop.

DO NOT REMOVE the condenser lens, because of possible UV radiation hazard.

A bright point source with condenser lens and iris is focused by a 20 cm focal length cylindrical convex lens onto a slit, which is in turn focused onto a distant screen by a 20 cm focal length spherical convex lens. A white light spectrum is obtained by inserting an equilateral prism just after the spherical lens. A chlorophyll solution, prepared by smushing up grass in a container of methanol, is placed in the beam just before the slit. The system now shows the absorption spectrum of chlorophyll.

The color of chlorophyll is not a narrow band of green, but rather is achieved by removing the complementary color, magenta, through absorbing some of the red and much of the blue components. The absorption spectrum of chlorophyll is compared with the full spectrum of white light above.

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.

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

This process is quite slow due to the low intensity light source; demonstration P2-02 is recommended over this one in most circumstances.

NOTE: This arrangement does not work to show that a positive zinc plate will NOT be discharged by the UV source. If you wish to show the effect with both positive and negative charge on the plate you must use the carbon arc lamp, Demonstration P2-02.

Place the clean projection tray on the overhead projector with the ruler underneath. Cover the tray with a layer of water (over 1/8") and allow the water to settle. Lightly dust the surface with lycopodium powder, and adjust the projector so the powder and ruler are both in focus (center photograph). Hold the dropper just above the center of the tray and carefully release one drop of oleic acid solution onto the water surface. A circular "hole" quickly appears in the powder film, reaching its fixed maximum diameter in a few seconds (photograph at right). This is the monomolecular layer of oleic acid molecules held together by surface tension. Measure the diameter of the film so that the approximate thickness can be determined.

The following comments on the nature of oleic acid that makes this experiment possible were taken from the Science Teachers' Resource Center, Chemistry section, laboratory #31.

Molecules that are repelled by water are called hydrophobic. Molecules that are attracted to water are called hydrophilic. Cooking oil is hydrophobic; it won't mix with water. Some molecules have one end that is hydrophobic and one end that is hydrophilic. There are such molecules in the cells in your body. They are used to take hydrophobic nutrients into the cell that is mostly water. Soaps are this way also so that they can dissolve both hydrophobic and hydrophilic substances and be washed away by water.

Oleic acid is a substance with one hydrophobic and one hydrophilic end. When a small amount of oleic acid is placed on the surface of water, it stands on end with the hydrophilic end towards the water and the hydrophobic end away. If you could see them, they would look like fans at a crowded concert.

In this lab, we will find the length of one oleic acid molecule by spreading a small amount over the surface of water and measuring the diameter of the circle. The oleic acid spreads itself into a one-molecule thick layer in the shape of a VERY flat cylinder.

Read more: Determination of the Size of a Fatty Acid Molecule, by David A. Katz is a very nice article on the web describing this experiment. (pdf)