Chapter XVI: Light (2)

=392. Defects of Vision.=--There are several defects of vision that may be corrected by spectacles or eye-glasses. One of these is "near-sightedness." It is due either to an eyeball that is elongated, or to an eye lens that is too convex, or to both conditions. This condition brings light from distant objects to a focus too soon (as shown in Fig. 389). Only light from near objects will focus upon the retina in such cases. With _normal_ vision light from _distant_ or _near_ objects may be focused without unusual effort upon the retina, see Fig. 390. The remedy for near-sightedness is to use concave lenses which will assist in properly refracting the light so the focus will be formed on the retina (Fig. 391). "Far-sightedness" is the reverse of near-sightedness; the eyeball is either too short, or the lens too flat, or both conditions obtain, so that the light entering the eye is brought to a focus behind the eyeball (Fig. 392). The remedy is convex lenses which will assist in properly converging the light, see Fig. 393. A third defect is called _astigmatism_. This is caused by some irregularity or lack of symmetry in the eye. It is corrected by a _cylindrical_ lens that compensates for this defect of the eye. A diagram similar to Fig. 394 is used as a test for astigmatism. If the lines appear with unequal distinctness, some irregularity of refraction (astigmatism) is indicated.

=393. The Photographic Camera.=--This is a light-tight box, provided with a convex lens in front, covering an aperture and a ground glass screen at the back. The distance between the lens and the screen is adjusted until a sharp image is obtained upon the latter, which is then replaced by a sensitive plate or film. The sensitized surface of the plate or film contains a salt of silver which is changed by the action of light. After the plate has been "exposed" to the action of light, it is "developed" by the use of chemicals producing a _negative_ image. From "negative," by the use of sensitized paper, "positive" prints may be secured which resemble the object photographed.

=394. The projecting lantern= (see Fig. 395) employs a strong source of light, as an electric arc lamp _L_, to strongly illuminate a transparent picture, or _lantern slide_, _S_, a real image (_I_) of which is formed upon a large screen. Two large plano-convex lenses (_C_), called condensing lenses, are placed near the lamp to concentrate the light upon the "slide" _S_. The convex lens forming the image is called the "objective" (_O_).

=395. The compound microscope= consists of two lenses. One called the _objective_ is placed near the object to be viewed. This lens has a short focal length usually less than a centimeter. It forms a _real image_ of the object. _A´_-_B´_. The other lens, the _eyepiece_ forms a virtual image of this real image. _A´´_-_B´´_. (See Fig. 396.)

=396. The telescope= consists of two lenses, the eyepiece and the objective. As in the compound microscope, the objective of the telescope forms a real image of the distant object, the eyepiece forming an enlarged virtual image of the real image. It is the virtual image that is viewed by the observer. (See Fig. 397.) In order to collect sufficient light from distant stars the objective is made large, sometimes 50 in. in diameter.

The length of the telescope tube depends upon the focal length of the objective, since the distance between the two lenses must equal the _sum_ of their focal lengths.

=397. The opera glass= consists of a convex lens as objective and a _concave_ lens as an eyepiece. The former tends to form a real image but the latter diverges the rays before a real image can be formed, the action of the two lenses producing an enlarged virtual image (as in Fig. 398) which is viewed by the one using the glass. The compact size of the opera glass is due to the fact that the distance between the two lenses is the _difference_ of the focal lengths.

=398. The Prism Field Glass or Binocular.=--This instrument. has come into use in recent years. It possesses the wide field of view of the spy glass but is as compact as the opera glass. This compact form is secured by causing the light to pass back and forth between two right-angle prisms (as shown in Fig. 399). This device permits the use of an objective lens with a focal length three times that of the tube, securing much greater magnifying power than the short instrument would otherwise possess. A further advantage is secured by the total reflection from the two prisms, one of which is placed so as to reverse the image right for left and the other inverts it, so that when viewed in the eyepiece it is in its proper position.

Important Topics

1. The eye: parts, formation of image, kind, how, where.

2. Eye defects, how remedied. Visual angle.

3. Simple microscope, camera; images, kind, how formed.

4. Compound microscope, telescope and opera glass; images, action of each lens.

Exercises

1. Name three instruments in which lenses form virtual images and three in which _real_ images are formed.

2. In what direction is an oar in water apparently bent? Explain by a diagram.

3. What optical instruments have you used? Is the _visible_ image formed by each of these _real_ or _virtual_?

4. The focal length of a copying camera lens is 14 in. Where must a drawing be placed so that an image of the same size may be formed upon the ground glass screen? What must be the distance of the screen from the lens?

5. What are two methods by which you can determine the focal lengths of the lens of a photographic camera?

6. The critical angle for water is 48-1/2 degrees. Show by a diagram how much of the sky can be seen by a diver who looks upward through the water.

7. How is near-sightedness caused? How is it corrected? Illustrate by a diagram.

8. How is the eye accommodated (focused) as an object gradually approaches it?

9. Explain why a simple microscope assists in looking at the parts of a flower or insect.

10. Why do people who have good eyesight when young require glasses as they grow old?

(7) COLOR AND SPECTRA

"Copyright by Underwood & Underwood, N. Y."

Guglielmo Marconi (Italy). Inventor of wireless telegraphy.]

"Copyright by Underwood & Underwood, N. Y."

Alexander Graham Bell, Washington, D. C. Inventor of the telephone.]

=399. Color.=--Much of the pleasure experienced in gazing at beautiful objects is due to the _color_ shown by them. The blue sky, the green grass, and the varied tints of flowers, and of the rainbow all excite our admiration The study of color begins naturally with the production of the _spectrum_, the many-colored image upon a screen produced by passing a beam of light through a prism. The spectrum is best shown when the light enters by a narrow slit (Fig. 400). The spectrum was first produced by Sir Isaac Newton in 1675 by the means just described. The names usually given to the more prominent colors of the spectrum are violet, indigo, blue, green, yellow, orange, and red. The initials of these names, combined, spell _vibgyor_, a word without meaning except to assist in remembering the order of the colors in a spectrum. If the light that has passed through a prism is sent through a second prism placed in reverse position (see Fig. 401), the light passing through both prisms is found to be white. This experiment _indicates that white light is composed of light of all colors_.

=400. Dispersion.=--The separation of the colors by a prism is called dispersion. In experimenting to find a reason for dispersion, it has been learned that lights of different colors are of different wave lengths. Color in light is therefore analogous to pitch in sound. We hear through many octaves, but we see through about one octave. That is, the shortest visible waves of violet light are about 0.000038 cm. in length while the longest visible red rays are 0.000076 cm., or the longest visible light waves are about twice the length of the shortest visible ones. It appears from the evidence of experiments upon dispersion that _light waves of different lengths are refracted differently_. This causes the images formed by refraction through simple glass lenses to be fringed with color and to lose some of their sharpness and definiteness of outline, since the violet light is brought to a focus sooner than the red. (See Fig. 402.) This seriously affects the value of such lenses for optical purposes. Fortunately it is found that _different kinds of glass have a different rate of dispersion for the same amount of refraction_.

=401. The Achromatic Lens.=--The existence of these different kinds of glass 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 glass combined with a plano-concave lens of flint glass._ 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 glasses show the result of not using achromatic lenses.

=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 glass, 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 glass in a beam of light either before or after it has passed through a prism. Some colors, as red, may be found to be nearly _pure_, only the red passing through, while green glass 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 _peacock 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 glass flask filled with water. (See Fig. 404.) The light passes 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.

=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_.

(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 analysis_, by which the presence of certain elements in a substance can be definitely proved upon the appearance of its particular lines in the 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, peacock 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 illustration 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 glass? Through a red glass? Through a red and blue glass 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 space, 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 assumed 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.

=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 glass 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 glass. If light of one color, such as that transmitted by red glass, 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 glass, 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 glass. Now the wave moving from _R_ to the plates has some of its light reflected from each glass 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 illustrate the reasoning that has led to this conclusion, suppose a rope to be passed through two vertical gratings. (See Fig. 411, 1.) If the rope be set in _transverse_ vibration by a hand, the waves produced will readily pass 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.

=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 passes the second crystal. This experiment shows that the light which has passed through one tourmaline crystal will pass 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 illustrates what is called _polarization of light_. The beam that after passing through _a_ (Fig. 412) is unable to pass 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

1. Interference of light: evidence, reasoning involved, illustration.

2. Polarization of light: evidence, reasoning involved.

3. Nature of light, differences between sound and light.

Exercises

1. Make a list of the differences between sound and light and state briefly the evidence upon which the knowledge of these differences is based.

2. Why will a thickness of film that will produce interference of red light be different from that producing interference for green or blue?

3. Using the formula _n_ = _v_/_l_ compute the vibration rate for violet light if its wave length is considered as 0.00004 cm.

4. Explain how the fact of polarization affects the wave theory of light.

5. Show how it is possible by comparing the spectrum of the sun with that of a star to tell whether the star is approaching or receding from the earth.

Review Outline: Light

Light; speed, source, medium.

Straight Line Motion; shadow, umbra, penumbra, eclipse, image.

Photometry; Law of intensity, candle power, foot-candle.

Mirrors; Law of reflection; image--real, virtual; plane, curved, parabolic, mirrors.

Refraction; cause and effects; plate, prism, lens; total reflection.

Lenses; six forms, principal focus, center, lens equation, 1/_F_ = 1/_D_{o}_ + 1/_D_{i}_.

Optical instruments; eye, defects and correction, camera, microscope, etc.

Spectra; 3 kinds, dispersion, production of color effects, spectroscope, uses.

Nature of Light; wave theory, interference, polarization, significance.