(h) Place two pieces of heavy plate glass, one flat on top of the other, and arrange a sodium flame so that its image can be seen by reflection from the plate glass surfaces. Note the appearance of the image. What is the effect of changing the pressure between the two glass surfaces?

In order to understand these experiments the references must be studied carefully. Explain what diffraction is and why the phenomena occur. What is interference and why is it caused by the thin air film between the two glass plates?

EXPERIMENT NO. 511

THE DIFFRACTION GRATING.

References: Stewart, Physics, Sect. 656-658; Kimball, College Physics, Sect. 933, 938-939; Duff, College Physics, Sect. 465; Spinney, Text-Book of Physics, Sect. 519, 521.

This experiment is designed to illustrate how the wavelength of light can be measured by the use of a diffraction grating. The method described is a rough one, but it is the same in principle as the accurate method involving the use of a spectrometer.

The apparatus consists of a small, plane, replica grating with a suitable support; a board with a vertical slit at its middle point, with a meter stick fastened longitudinally on the board with the 50-centimeter mark at the middle of the slit; a fish-tail gas burner; a Bunsen burner with an asbestos tube extension; solutions of sodium chloride and lithium chloride; wires with tufts of asbestos paper to feed solutions into the flame. This experiment is best worked in a room at least partially darkened.

The board is held on stands with its plane vertical and the attached meter rod horizontal. About 80 cm. away the grating is held in a clampstand. It must be placed so that its center lies on a line through the slit perpendicular to the board, and its plane is perpendicular to the line (i. e., the grating and board are parallel). This is accomplished by first placing it so that its center is equally distant from points on the meter rod at equal distances from the slit. The grating is then turned till it is perpendicular to the line joining its middle point to the slit.

(a) Set the fish-tail burner back through the grating toward the board.

of the slit and look

On each side will be

seen a continuous spectrum, farther out a second-order spectrum, longer and fainter. If the light is bright enough, a third-order spectrum will be seen still farther out.

The theory of the grating shows that, if a represents the wave-length of the light, the angle at which we must look to see that light, n the order of the spectrum, and d the distance between successive rulings on the grating, then

nλ = d sin Ə.

The number of lines of the grating per inch is marked on the grating, hence we can calculate d. Moreover, n has the value 1, 2, etc., according as we are using the first-order spectrum, the second-order, etc. Consequently, by measuring the angle → we can calculate λ. However, a continuous spectrum offers no very definite wave-length to measure. Consequently, we replace the fish-tail burner by a sodium burner, which gives out only one certain definite wave-length. We will then see, on looking through the grating, instead of two continuous spectra on each side of the slit, two bright yellow lines, one being the first-order spectrum of sodium, the other the second-order spectrum. We can now determine the wave-length of sodium light if we can determine the angle for the first-order spectrum and for the second-order. To do this, note at what point on the meter rod the spectral image seems to lie and measure the distance of this point from the slit, itself. This distance, divided by the perpendicular distance from the grating to the meter rod gives tan, from which → and sin → can be obtained by the use of tables. Determine → for the first spectrum on each side, take the average, and calculate λ. Do the same for the second spectrum on each side.

After this, determine the wave-length of the red light from lithium in the same way, except that probably only the first-order spectrum can be used. The lithium chloride solution must be fed into the sodium flame on the end of an iron or platinum wire. The red lithium line will then appear near the yellow sodium line.

(b) An electric arc formed between corners of two rather thick copper plates gives at least seven colored lines distinct enough to be measured by this method. The arc should be run with as little current as possible, so as to have a very small source of light. This tends to prevent eye injury by strong ultraviolet light and also facilitates exact readings of the positions of the various spectra. Several students may carry on measurements on the same arc at the same time. No slit is needed. At any position from which the arc can be seen, set up the grating with a meter stick 60 to 80 centimeters nearer the arc and parallel to the grating. To gauge the correctness of the wavelengths determined, consult tables of wave lengths found in Kaye and Laby, Tables of Physical and Chemical Constants, The Handbook of Chemistry and Physics, and other similar works.

The arc may be replaced by a condensed spark between copper terminals with equally satisfactory results. Other metals may also be used if desired. A mercury arc gives a number of good lines suitable for such measurement.

(c) The fish-tail burner may be set behind the slit and the wave lengths of the extreme red and violet measured for the continuous spectrum. This gives an idea of the range of wavelengths covered by the visible spectrum.

(d) A variation of this experiment is to measure the grating-space, assuming the wave-length of the light as known.

The student should ask the instructor what parts of this experiment are to be performed.

EXPERIMENT NO. 512

SPECTRA.

References: Stewart, Physics, Sect. 629-633, 635; Kimball, College Physics, Sect. 896-900, 902-906; Duff, College Physics, Sect. 470-473; Spinney, Text-Book of Physics, Sect. 508, 509, 511-515.

This experiment is designed to show the student the three most common types of spectra and to show that different substances are characterized by different bright-line spectra.

Small direct-vision spectroscopes are very satisfactory for use in this experiment and are relatively inexpensive. Obviously the ordinary spectroscope or spectrometer can be used, and, in the absence of either type of spectroscope, a common prism can be used for certain parts of the experiment. The room should be darkened.

(a) A continuous spectrum is given out by an incandescent solid or liquid. Look through the prism or spectroscope at an incandescent lamp, at a fish-tail gas burner or a lamp flame. Compare the spectra and tell exactly what they look like. If a prism alone is used, a metal screen containing a slit should be put in front of the light and observed through the prism from a distance of fifteen to twenty feet. Note that the smoky flames are clouds of solid carbon particles.

(b) Bright-line spectra are given out by incandescent gases or vapors under relatively low pressures.

Observe through the spectroscope the Bunsen burner with the air adjusted so as to give a noisy blue cone in the flame. How does this spectrum differ from those seen in (a)?

Place a non-luminous Bunsen flame in front of the spectroscope. Hold in the flame a narrow strip of asbestos paper which has been dipped in sodium chloride solution. Repeat with a fresh strip dipped in lithium chloride solution. Describe what is seen in each case. The tests may be repeated with solutions of thallium sulphate and of salts of barium, calcium, strontium and as many other metals as are provided. Throw away each strip of asbestos after using it once with any of these last named materials as it is very easy to contaminate and spoil the solutions and these materials are expensive. Each partner should observe all these spectra and record what he sees. Try to identify one of the solutions by observing its spectrum.

Other interesting light sources to examine are metallic arcs, a mercury vapor lamp, vacuum tubes containing different gases. Note that each of these substances produces its own characteristic group of lines.

(c) Absorption spectra are formed when white light passes through a gas which absorbs certain wave-lengths characteristic of its own spectrum, or through a solid or liquid which absorbs certain wide bands of wave-lengths.

Examine the spectrum of sunlight. Make the slit of the spectroscope very narrow. If a direct-vision spectroscope is used, look at a white cloud or merely at the window through which sunlight is shining. If the ordinary type of spectroscope is used, a replica grating is preferable to a prism on account of its greater resolving power. Reflect a beam of sunlight into the slit parallel to the axis of the collimator, or better, put a converging lens in this beam of light so as to cast an image of the sun on the slit of the collimator. Dark lines will be seen extending across the continuous spectrum in a direction parallel to the length of the slit of the spectroscope. These are Fraunhofer lines. Caution! If there are dust particles in the narrow slit, dark lines due to them may be seen extending longitudinally across the spectrum. Do not confuse these with the Fraunhofer lines. How are the Fraunhofer lines caused?

Place an incandescent lamp, a fish-tail burner or a lamp flame in front of the spectroscope slit and observe the effects of interposing a piece of red glass. Try a piece of cobalt blue glass. Record in each case what part of the spectrum is cut off. Note that in place of narrow lines of darkness, there now are wide bands. If time and materials permit, use also solutions of copper sulphate, potassium permanganate, potassium dichromate.

In the report tell exactly what was observed in each case, making diagrams to illustrate the locations of lines and bands. See the pictures of spectra in your text-book for suggestions.