Imagens das páginas
PDF
ePub

I now pass a beam of light (produced by throwing the oxyhydrogen flame on lime) through a rhombohedron of Iceland spar, and we obtain two images on the screen. By rotating the crystal on its axis, one of the images revolves around the other, but neither disappears during the revolution. Now this fact proves that the light which falls on the crystal is unpolarized; for if it had been polarized, one image would have disappeared in certain positions, as I shall hereafter prove.

10. Polarization.-When submitted to certain influences, the rays of common light acquire peculiar properties, designated by the term polarization. These peculiarities are not distinguishable by the unassisted eye.

A very common question, put by persons who have not studied the subject, is," What is polarized light?" and the philosopher feels very considerable difficulty in giving a concise and intelligible reply; so that the enquirer, perhaps after listening to a lengthened detail, frequently goes away, without obtaining as he says, a direct and short answer to his question.

There are two modes of reply: one is to describe, independent of all hypotheses, the properties by which polarized light is distinguished from common light; the other, is to adopt some hypothesis of the nature of light, and, therefore, to give an hypothetical explanation of the nature of polarized light. Whichever method we adopt and I shall give both-lengthened details are necessary to enable the uninitiated to comprehend the subject. There are four methods of polarizing light, viz.

a. Reflection.

b. Simple refraction.

c. Double refraction.

d. Transmission through a plate of tourmaline.

In the following table I have contrasted some of the distinguishing characteristics of common and polarized light:

A Ray of Common Light,

1. Is capable of reflection, at oblique angles of incidence, in every position of the reflector.

2. Penetrates a plate of tourmaline (cut parallel to the axis of the crystal) in every position of the plate.

3. Penetrates a bundle of parallel glass plates, in every position of the bundle.

4. Suffers double refraction by Iceland spar in every direction, except that of the axis of the crystal.

A Ray of Polarized Light,

1. Is capable of reflection, at oblique
angles of incidence, in certain posi-
tions only of the reflector.
2. Penetrates a plate of tourmaline
(cut parallel to the axis of the
crystal) in certain positions of the
plate, but in others is wholly in-
tercepted.

3. Penetrates a bundle of parallel
glass plates, in certain positions of
the bundle, but not in others.
4. Does not suffer double refraction
by Iceland spar in every direction,
except that of the axis of the
crystal. In certain positions, it
suffers single refraction only.

Thus, then, one mode of replying to the before-mentioned question would be, by recapitulating the facts stated in this second column. This reply would form what I may term a matterof-fact answer, being independent of all hypothesis.

The naked or unassisted eye cannot then distinguish common from polarized light. Every person must have repeatedly seen polarized light, but not knowing how to recognize it, has failed to distinguish it from common light. If you look at a polished mahogany table, placed between you and the window, part of the light reflected from the table is polarized. When you look obliquely at the goods in a linendraper's shop, through the plate-glass window, part of the light by which you see the articles is polarized. When you see two images by a crystal of Iceland spar the transmitted light is polarized. The atmospheric light is frequently polarized, especially in the earlier and later periods of the day when the solar rays fall very obliquely on the atmosphere. At the present season, the effect may be perceived at eight or nine o'clock in the morning and five or six o'clock in the afternoon, the observer standing with his back to the sun, or with his face north or south. I have found that the effect is best perceived when the sun is shining, and the atmosphere more or less misty.

It is obvious, therefore, that after we have polarized a ray of light, we must employ some agent to detect its peculiar properties. This agent is called the analyser. It would be better understood if it were termed the test. It may be a reflecting plate, a plate of tourmaline, a bundle of glass plates, a Nichol's prism, or a double refracting prism; in fact, the analyser or test must be a polarizer.

Thus, then, a polariscope consists of two parts: one for polarizing, the other for analysing or testing the light. There is no essential difference between the two parts, except what convenience or economy may lead us to adopt; and either part, therefore, may be used as polarizer or analyser; but whichever we use as the polarizer, the other then becomes the analyser.

a. Polarization by reflection.-This method of polarizing light was discovered by Malus, in 1808. He was viewing, through a double refracting prism, the light of the setting sun reflected from the glass windows of the Luxemburgh palace in Paris; and, on turning round the prism, he was surprised to observe a remarkable difference in the intensity of the two images: the most refracted alternately surpassing and falling short of the least refracted in brightness.

Polarizing reflectors are usually glass. This should be either ground or blackened at the back to prevent posterior reflection. Water is seldom made use of. Mica may be employed instead

FIG. 3.

b

Polarization by Reflection. a. Incident ray of common or unpo

larized light.

b. Plate of glass (polarizing plate). c. Reflected ray of polarized light.

of glass. A well-polished or var-
nished piece of wood (as a table,
top of a pianoforte, or a counter)
is very convenient. Marble also
answers tolerably well. The
shining back of a book is oft-
Metallic
entimes serviceable.
plates are objectionable; since
by one reflection only from them,
the light is found to be ellipti-

cally polarized; though by successive reflections it becomes plane polarized.

The polarizing angle varies for different substances, as the following table shows:

Angles of Polarization by reflection.

[merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small]

From a very extensive series of experiments, made to determine the maximum polarizing angles of various bodies, Dr. Brewster arrived at the following law: the index of refraction is the tangent of the angle of polarization. It follows, therefore, that the reflected polarized ray forms a right angle with the refracted ray.

Here, perhaps, is the most convenient place for referring to a suggested application of polarized light. I have stated that light is polarized by water, at an angle of 52° 45'. By the analyser (as a tourmaline, or Nichol's prism, or a bundle of glass plates) the whole of this reflected polarized light may be intercepted without offering any impediment to the unpolarized but refracted light which has traversed the water; so that objects may be more readily seen at the bottom of ponds, rivers, and the sea, by this expedient than otherwise, since the glare of the reflected light is prevented. Hence anglers, and those fond of fish-spearing, may employ this property of polarized light in the discovery of the objects of their sport; and commanders of vessels may avail themselves of it to detect rocks and shoals in the bottom of the ocean, which are not otherwise visible except by viewing them from the mast-head, by which the angle of reflexion is diminished, and consequently the quantity of light reflected is thereby lessened.

I proceed now to demonstrate the polarization of light by reflection, and the essential properties of the polarized ray. For this purpose, I obtain an intense light by throwing the flame of a jet of mixed oxygen and hydrogen gases on a cylinder of lime.

This light, which I shall, for brevity, call the lime-light, is condensed by two crossed lenses (called condensers), and thrown on a plate of glass, blackened at the back, and placed at an angle of 56° 45'. The light is polarized by this plate, and being then refracted by two plano-convex glasses (termed the power), is afterwards received on a semi-transparent calico screen, strained on a wooden frame, and moistened with water. A bundle of glass plates is sometimes used as a reflecting polarizer: it has the advantage of reflecting more light, but a portion of it is unpolarized.

The light thus polarized is not distinguishable by the eye from common light, but to prove its nature, I proceed to test it. For this purpose, I place a plate of tourmaline on the front of the polariscope, and on revolving it, you observe the light on the screen is alternately cut off and admitted. At 0 the tourmaline transmits the light, at 90° it absorbs it, at 180° it transmits it, at 270° it absorbs it. I remove the tourmaline, and substitute a bundle of glass plates, placed at an angle of 56° 45'. On revolving this, the light is observed to be alternately cut off and transmitted, as in the case of the tourmaline. I now substitute a double refracting prism for the bundle of glass, and on revolving this, it is seen that in two positions double refraction takes place, and two images are produced on the screen, while in two intermediate positions, one image is cut off. Thus, then, the light reflected from the blackened glass plate of the polariscope, is polarized, since it possesses the characters assigned to this kind of light.

FIG. 4.

d

Polarization by single Refraction. a. Ray of common or unpolarized light. b. Bundle of glass plates. c. Reflected polarized ray. d. Refracted polarized ray. This ray is oppositely polarized to c.

2. Polarization by single Refraction. If light be transmitted obliquely through a bundle of diaphanous laminæ, it suffers polarization. The very thin parallel glass plates used for microscopes, and sold by Bromley and Drake, at 315, Oxford Street, form the best medium. Sixteen or more of these are to be placed parallel, and the bundle then placed at an angle of 56° 45′ to the ray to be polarized.

Common crown or window glass serves well enough for ordinary purposes. The flattest, thinnest, most colourless, and perfect pieces are to be selected. A very convenient mode of using them is the following:-Take two one ounce paper pillboxes, remove the lids and the bottoms, and then paste together the two cylinders of the boxes, so as to form a tube. Into this, place obliquely sixteen pieces of window glass.

Having cut in each of the lids a circular hole, of about the size of a sixpence, place a lid at each end of the cylinder. The light by passing obliquely through the glass-plates in this cylinder, becomes polarized.

A bundle of mica plates may be used for polarizing, but it is inferior to the bundle of microscope glass above referred to.

3. Polarization by Double Refraction.-I have already demonstrated the double refractive power of Iceland spar; though I have not hitherto said anything of the nature of the light of which the two pencils are made up.

I now proceed to demonstrate that the two pencils or rays produced by this process are polarized; but the polarized state of the one ray is of an opposite kind to that of the other; so that the two rays are said to be oppositely polarized, That this is the case is proved by applying our tests to them. Thus, if I apply a plate of tourmaline, you observe that, as this is rotated, one of the luminous rays is alternately cut off, which would not be the case were the rays composed of common light; while, if both were similarly polarized they would be simultaneously and not alternately absorbed. If I substitute the bundle of glass plates for the tourmaline, one of the rays is reflected, and the other transmitted; and by revolving the bundle 90°, the ray which was reflected is now transmitted, and that which was transmitted is now reflected.

a

FIG. 5

Double Refraction by a rhombohedron

of

Iceland Spar. from a to b represents, the axis of the

a. b. The obtuse angles. A line drawn

crystal.

c. Incident ray of unpolarized or common light.

e. o. Oppositely polarized transmitted rays, e is called the extraordinary, o the ordinary ray.

f.g. A line, which when viewed through the rhombohedron, appears doubled,g

and hi.

Nichol's prism, or Nichol's eye-piece*, is a most valuable and convenient polarizer. It is an oblique rhombic prism, whose terminal planes form an angle of 68° with the adjoining obtuse lateral edges. It is formed by joining, by means of Canada balsam, two wedge-shaped pieces of Iceland spar. It is a double refractor, but the two rays are made to deviate so far, that only one image is seen in its usual position. The cause of this is the Canada balsam, whose index of refraction (1.549) is intermediate between that for the ordinary ray (1.654), and that for the extraordinary ray (1.483), so

that this substance changes the direction of the two rays in an

* See Jameson's Journal, vols. 6, 16, and 27.

C

« AnteriorContinuar »