[Ill.u.s.tration: FIG. 402.--Violet light comes to a focus sooner than red.]

=401. The Achromatic Lens.=--The existence of these different kinds of gla.s.s makes possible a combination of lenses in which dispersion is entirely overcome with the loss of only about one-half of the refraction. Such a combination is shown in Fig. 403. It is called an _achromatic lens_, since images formed by it are not colored but white (_a_ = without, _chroma_ = color). _The achromatic lens consists of a double convex lens of crown gla.s.s combined with a plano-concave lens of flint gla.s.s._ Achromatic lenses are used in all high-grade optical instruments such as telescopes and microscopes. The colored images that are sometimes seen in cheap opera gla.s.ses show the result of not using achromatic lenses.

[Ill.u.s.tration: FIG. 403.--An achromatic lens. _C_ is of crown gla.s.s; _F_, of flint gla.s.s.]

=402. The Color of Bodies.=--Project the spectrum of sunlight upon a white surface in a darkened room.

Now place in different parts of the spectrum objects of various colors. Red objects will show brilliant red when at the red end of the spectrum but look black at the blue end, while blue objects appear blue only at the blue end.

These facts indicate that the color of an object depends upon two things: (a) _the light that falls upon it and_ (b) _the light which it sends to the eye_. A _black_ surface absorbs all color while a _white_ one reflects all wave lengths to the eye in the same proportion that they come to it. A white object will appear red in red light, and blue in blue light since it reflects both of these. A _colored_ object reflects light of its own color but absorbs all others. The color then of a body is due to the light which it does not absorb, but which comes from it to the eye.

_403. The color of transparent bodies_, such as colored gla.s.s, is due to the presence of a _dye_ or _pigment_ contained in the body. This pigment absorbs a part of the light, the part transmitted giving the color. This may be shown by holding a sheet of colored gla.s.s in a beam of light either before or after it has pa.s.sed through a prism. Some colors, as red, may be found to be nearly _pure_, only the red pa.s.sing through, while green gla.s.s often transmits in addition to the green some yellow and some red light.

=404. Complementary Colors.=--If two prisms are placed in reversed position near each other (see Fig. 401), a beam of light dispersed by one is recombined into white light by the other. If now a card is held between the two prisms so as to cut off some of the colored light, say the red, the remaining light will be found to form a _greenish blue_. If the card is removed, the light becomes _white_ again. That is, red and _peac.o.c.k blue_ light together form white. Any two colors that together form white light are called _complementary_. Other complementary colors are light yellow and blue, green and crimson, orange and greenish blue, violet and greenish yellow. We must not confuse the combining of colors (light) and the combining of _pigments_, the latter consisting of bodies that absorb light. Yellow pigment absorbs all but yellow and some green, while blue pigment absorbs all but blue and some green. Mixing these two pigments causes the absorption of all colors but _green_. Blue and yellow _paint_ mixed produce _green_, while blue and yellow _light_ give white.

=405. The solar spectrum=, as the spectrum of sunlight is called, may be observed in the _rainbow_. The latter is produced through the dispersion of light by spherical raindrops. Its formation may be imitated by sending a small circular beam of light through a screen against a round gla.s.s flask filled with water. (See Fig. 404.) The light pa.s.ses through the water and is dispersed when it enters and when it leaves, producing a color upon the screen at _R_-_V_. The course of the light within the drop is indicated in Fig. 405. The violet ray comes to the eye more nearly horizontal and is therefore below red, as we look at the rainbow.

=406. Fraunhofer Lines.=--Some of the most important features of the solar spectrum are not seen in the rainbow or in the band of light usually observed upon a screen. By the use of a narrow slit and a convex lens to carefully focus the slit upon a white screen it is seen that the solar spectrum is crossed by many _dark_ lines. These are called Fraunhofer lines, to honor the German scientist who in 1814 first accurately determined _their_ position. Two experiments _with a spectroscope_ will help to make clear the meaning of the Fraunhofer lines.

[Ill.u.s.tration: FIG. 404.--A rainbow formed by a beam of light striking a flask of water.]

[Ill.u.s.tration: FIG. 405.--The course of a beam of light within a drop of water.]

=407. The Spectroscope and Its Uses.=--The spectroscope (Fig. 406) is an instrument for observing spectra. It consists of a prism, a slit, and a convex lens _T_ for focusing an image of the slit accurately upon a screen (Fig. 407) where the spectrum is observed through the eyepiece _E_.

[Ill.u.s.tration: FIG. 406.--The spectroscope.]

(A) A Bunsen flame is placed in front of the slit and a heated platinum wire which has been dipped in common salt or some sodium compound placed in the Bunsen flame; the latter becomes yellow and a vivid yellow line is observed on the screen in the spectroscope. Other substances, as barium and strontium salts, when heated to incandescence in the Bunsen flame, give characteristic bright lines. In fact each _element_ has been found to have its own characteristic set of colored lines. This fact is made use of in _spectrum a.n.a.lysis_, by which the presence of certain elements in a substance can be definitely proved upon the appearance of its particular lines in the spectrum.

[Ill.u.s.tration: FIG. 407.--Diagram of a spectroscope.]

[Ill.u.s.tration: FIG. 408.--The bright line spectrum of iron and its coincidences with some of the dark lines of the solar spectrum.]

(B) If light from, for example, an arc light is sent over a gas flame containing _sodium_ vapor, a _dark line_ appears in the spectrum--in the exact position in which the yellow sodium line appeared. It seems that the sodium vapor removes from white light the same wave lengths that it itself produces. This absorption is supposed to be due to sympathetic vibration; just as a tuning fork is set in vibration by the waves of another fork in unison with it, at the same time absorbing the wave energy, so in the gas flame the sodium particles absorb the wave motion of the same vibration rate as that emitted by them. The fact that the spectrum of sunlight contains a great many dark lines is believed to indicate that the sun is surrounded by clouds formed by the vaporization of the various substances in the sun itself. By comparing the dark lines of the solar spectrum with the _bright-line spectra_ of various substances found in the earth, such an exact correspondence of the lines is found that the presence of the vapor of these substances about the sun is considered proved. (See Fig. 408 which shows the exact correspondence between the bright-line spectrum of iron vapor and the dark lines appearing in a portion of the sun's spectrum.) The spectra of the stars also contain certain dark lines. Thus the presence of the corresponding substances in distant stars is considered as determined.

=408. Theory of Color Vision.=--By combining light of the _three colors_ _red_, _green_ and _blue-violet_ in proper proportions, it has been found possible to produce any color effect, even white. This leads to the conclusion that in the retina of the eye are three different kinds or sets of sensitive nerve endings, sensitive respectively to red, to green, and to blue light. This idea is given corroboration by some facts of color blindness. Thus some persons have no sensation of _red_, this color not being distinguished from green. Others are color blind to green or blue. It is supposed that in color blind persons one of the sets of nerve endings sensitive to one of these three colors is lacking.

=409. Three-color Printing.=--Since all colors may be produced by mixing the three colors, light red, green, and blue-violet, these are called _the three primary colors_. The so-called primary pigments or paints are simply the complements of the three primary colors. They are, in order, peac.o.c.k blue, crimson, and light yellow. The three pigments when mixed yield black, since combined they absorb all kinds of visible light. The process of three-color printing, now so generally employed in printing colored pictures for books, calendars, etc., consists in combining upon white paper three colored impressions, using successively the three primary pigments (yellow, crimson and blue) from plates prepared as follows:

Three photographs of a given colored object are taken, each through a different sheet of gelatine called a filter, stained the color of one of the primary colors. From these photographs half-tone blocks are made in the usual way. The colored picture is made by carefully superposing impressions from these blocks, using in each case an ink whose color is the complement of the "filter" through which the original picture was taken. An ill.u.s.tration of the process is given upon the plate in the frontispiece of this book.

Important Topics

1. Color, due to wave length; dispersion by prism, sphere in rainbow, complementary colors, color of opaque and transparent bodies.

2. Spectra, solar; formation of rainbow; bright-line spectra, how formed, how used; dark-line, how formed, used.

3. Theory of color vision. Three color printing.

Exercises

1. How does a white flower look when viewed through a blue gla.s.s?

Through a red gla.s.s? Through a red and blue gla.s.s at the same time?

2. Why does a red ribbon appear black when seen by blue light and red when seen by red light?

3. In what part of the sky must you look to see a rainbow in the morning? In the afternoon? Explain.

4. How would you arrange two similar prisms so as to produce double the deviation produced by one?

5. The color of an object depends upon what two things?

6. What kind of a spectrum should moonlight give? Why?

7. A mixture of green and red lights gives a sensation of yellow. Can you suggest why a mixture of blue and yellow lights gives the sensation of white?

(8) NATURE OF LIGHT, INTERFERENCE, POLARIZATION

=410. The Corpuscular Theory.=--The theory of the nature of light that was most generally accepted until about the year 1800, held that light consists of streams of minute particles, called corpuscles, moving at enormous velocities. This _corpuscular theory_ was in accord with the facts of reflection and the _rectilinear_ motion of light, but was abandoned after the discovery of the _interference of light_, as it could not account for the latter phenomenon.

=411. The Wave Theory of Light.=--The theory that _light is_ a _form of wave motion_ was first advanced by Huygens, a Dutch physicist, in the seventeenth century. This theory was opposed at the start since (A) _no medium_ was known to exist which would convey wave motion through s.p.a.ce, as from the sun to the earth, and (B) the _rectilinear motion_ of light was _unlike_ that of any _other_ form of known wave motions, such as that of water or of sound waves which are able to bend around corners.

In answer to the first objection, Huygens a.s.sumed the presence of a medium which he named _ether_, while the second objection has been completely overcome during the past century by the discovery that _light may deviate from a straight line_. It is now known that the _excessive shortness_ of light waves is the reason for its straight-line motion.

Further, long ether waves, as those of wireless telegraphy, are found to bend around obstacles in a manner similar to those of water or sound.

[Ill.u.s.tration: FIG. 409.--Two plates pressed together by a screw clamp.]

[Ill.u.s.tration: FIG. 410.--Ill.u.s.trating the interference of light by a thin film of air.]

=412. The interference of light= is one of the phenomena for which the wave theory offers the only satisfactory explanation. Interference of light may be shown by taking two pieces of plate gla.s.s and forcibly pressing them together by a screw clamp, as shown in Fig. 409. After a certain pressure has been reached, colored rings will appear about the compressed spot when viewed by light _reflected_ from the upper surface of the gla.s.s. If light of one color, such as that transmitted by red gla.s.s, falls upon the apparatus, the rings are seen to be alternately red and dark bands. The explanation of this phenomenon according to the wave theory is as follows: The two sheets of gla.s.s, although tightly pressed together, are separated in most places by a thin wedge of air (see Fig. 410), which represents in an exaggerated form the bending of the plates when pressed by the clamp. Several waves are represented as coming from the right and entering the gla.s.s. Now the wave moving from _R_ to the plates has some of its light reflected from each gla.s.s surface. Consider the two portions of the wave reflected at each of the surfaces between the plates, _i.e._, from the two surfaces of the wedge of air. If the portion of the wave reflected from the second surface of the air wedge combines with that reflected from the first surface, in the _same phase_ as at _C_, the two reflected waves strengthen each other. While if the two reflected portions of the wave meet in opposite phases as at _A_ and _B_, a decrease or a complete extinction of the light results. This is called _interference_. If light of one wave length is used, as red light, the regions of reinforcement and interference are shown by red and dark rings, while if white light is used, the ring where red light interferes, yields its complementary color, greenish blue. Where interference of greenish blue occurs, red is found, etc. Many phenomena are due to interference, such as (A) the color of thin films of oil on water, where the portions of light reflected from the two surfaces of the oil film interfere resulting in the production of color; (B) the color of soap bubbles. When first formed, soap-bubble films are not thin enough to show interference well, but as the bubbles increase in size or become thinner on standing, the conditions for interference are reached and, as the film becomes thinner, a regular succession of colors is noticed.

=413. Differences Between Light and Sound.=--Among the important differences between light and sound that have been considered are the following: the former are (a) _waves_ in the ether, (b) _of very short wave length_, and (c) their _motion is in straight lines_. Another difference (d) is in _the mode of vibration_.

Sound waves are _longitudinal, while light waves are transverse_. Light waves consist of vibrations of the ether at right angles to the line of motion. To ill.u.s.trate the reasoning that has led to this conclusion, suppose a rope to be pa.s.sed through two vertical gratings. (See Fig.

411, 1.) If the rope be set in _transverse_ vibration by a hand, the waves produced will readily pa.s.s through to the gratings _P_ and _Q_ and continue in the part extending beyond _Q_. If, however, _Q_ is at right angles to _P_, no motion will be found beyond _Q_. Now if a stretched coiled spring with longitudinal vibrations should take the place of the rope, it is evident that the crossed position of the two gratings would offer no obstacles to the movement of the vibration. In other words, crossed gratings offer no obstruction to longitudinal vibrations, while they may completely stop transverse vibrations.

[Ill.u.s.tration: FIG. 411.--Transverse waves will pa.s.s through both gratings in (1) where the openings in the two gratings are at right angles. The waves pa.s.sing _P_ are stopped by _Q_ (2).]

[Ill.u.s.tration: FIG. 412.--Effect of tourmaline crystals on light.]

=414. Polarization of Light.=--It is found that two crystals of tourmaline behave toward light just as the two gratings behave with respect to the transverse waves of the rope. Thus, if a small opening in a screen is covered with a _tourmaline_ crystal, light comes through but slightly diminished in intensity. If a second crystal is placed over the first one so that the two axes are in the same direction as in Fig.

412_P_, light is as freely transmitted through the second crystal as through the first, but if the crystals are crossed (Fig. 412_S_) no light pa.s.ses the second crystal. This experiment shows that the light which has pa.s.sed through one tourmaline crystal will pa.s.s through another only when the latter is held in a certain position, hence it is believed that a tourmaline crystal is capable of transmitting light that is vibrating in one particular plane. The direct conclusion from this is that _light waves_ are _transverse rather than longitudinal_.

The experiment just described ill.u.s.trates what is called _polarization of light_. The beam that after pa.s.sing through _a_ (Fig. 412) is unable to pa.s.s through _b_, if the two axes are crossed, is called a _polarized beam_. The conclusion that light waves are transverse is therefore based upon the phenomenon of the polarization of light. This was first discovered by Huygens in 1690.

Important Topics