G. P. (George Payn) Quackenbos.

A natural philosphy: embracing the most recent discoveries in the various branches of physics .. online

. (page 24 of 42)
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direction, while others are brought to rest within the body,
and others again are transmitted through it with certain
modifications.

We have treated of that portion of the light which is
reflected ; we must now look at that which enters the trans-
parent body.

635. "When a boy rowing a boat brings his oar into the water, it no longer
looks straight, but broken at the point where it enters. The same appear-
ance is presented when he plunges a spoon or cane obliquely in a pail of wa-
ter. On taking out the oar, the spoon, and the cane, they look perfectly
straight again. It is evident, therefore, that the rays coming from the parts

rays? "Where does the virtual focus lie? Illustrate the effect of convex mirrors on
parallel rays, with Fig. 240. 633. What is the effect of convex mirrors on diverging
rays? On converging rays? 634. "When light strikes a transparent body, what be-
comes of it ? Express this according to the Undulatory Theory. 635. Give some fa-
miliar examples which prove that rays are bent on passing from one medium to an*



246 OPTICS.

immersed are turned from their course on entering the air, so that the points
from which they come appear to lie where they do not really lie. Rays thus
turned from their course are said to be refracted.

636. Refraction is that change of direction which a ray
of light experiences on passing obliquely from one medium
to another.

For an example, see the ray A in Fig. 241. If there were no water in the
vessel, it. would go on in a straight line to B ; when the vessel is filled, it is
refracted to C.

637. That branch of Optics which treats of the laws and
principles of refracted ligjit, is called Dioptrics.

638. REFRACTIVE POWER OP DIFFERENT MEDIA. All
media do not have the same refractive power. Rays of
light falling from the air on water, alcohol, glass, and ice,
are turned from their course in different degrees by each.

A medium that has great refractive power is said to be
dense; one that has but little, is called rare. The terms
dense and rare, therefore, applied to media in Optics, have
a different meaning from that which they convey in other
departments of Natural Philosophy.

As a general rule, those media are the densest that have the greatest spe-
cific gravity ; and, of media having about the same specific gravity, the most
inflammable is the densest. The following substances are arranged accord-
ing to their refractive power, chromate of lead, a transparent solid, being the
densest: Chromate of lead, diamond, phosphorus, sulphur, mother-of-pearl,
quartz, amber, plate-glass, olive oil, alcohol, water, ice, air, oxygen, hy-
drogen.

639. LAWS OF REFRACTED LIGHT. 1. In a uniform
medium, there is no refraction. It is only on passing from
one medium (or stratum of a medium) to another, that a
ray is turned from its course.

2. Only such rays as enter a medium obliquely are re-
fracted, not such as enter at right angles.

3. "WJien a ray passes obliquely from a rarer to a denser

other. What term is applied to such rays? 636. What is Refraction ? Illustrate this
definition with Fig. 241. 63T. What is Dioptrics? 638. What is said of the refractive
power of different media ? What is a Dense Medium ? What is a Eare Medium?
What is said of the meaning of the terms dense and rare in Optics? As a general
rule, what media are the densest ? Mention some substances in the order of their
refractive power ? 639. What is the first law of refracted light? The second ? The





REFRACTION. 247

medium, it zs refracted towards a line perpendicular to
the surface. In Fig. 241, let the ray A pass from air, a
rarer medium, into water, a denser medium, and instead of
going on in a straight line to B, it will be
refracted to C, nearer the perpendicular.

4. When a ray passes from a denser me-
dium into a rarer, it is refracted from the
perpendicular. In Fig. 241, let the ray B
pass obliquely from water into air, and in-
stead of going on in a straight line to A, it
will be refracted to D, farther from the perpendicular.

640. An interesting experiment which every pupil may perform for him-
self, admirably illustrates refraction, and proves the last law to be true.
Place a coin on the bottom of an empty vessel (see Fig. 242.

Fig. 242), and fix the eye in such a position that
it just misses seeing it on account of the vessel's
side coming between. Keep the eye there, and
let water be poured in ; the coin will then become
visible, the rays from its surface being refracted
so as to meet the eye. The coin will appear to lie
at N, some distance above the bottom of the ves-
sel ; because the rays from it that last meet the eye, if continued in straight
lines, would go on to that point.

The change caused by refraction in the apparent position of an object
often misleads persons standing on the bank of a sheet of water as to its
depth. Objects on the bottom seem to be several feet nearer the surface than
they are, and bathers, deceived by 4he appearance, venture beyond their
depth and are drowned.

04 1. ATMOSPHERIC REFRACTION. Rays from the heav-
enly bodies, on entering our atmosphere obliquely from a
rarer medium, are refracted towards the perpendicular.
Hence we never see these bodies in their real position, ex-
cept when they are directly over head.

The sun is visible to us some time before he really rises above the horizon,
and remains visible at night after he has sunk below it. We owe our twi-
light to successive reflections and refractions of his rays by atmospheric
strata of dilfereut densities, after he has disappeared.

third ? The fourth ? Illustrate the third and the fourth law with Fig. 241. 640. What
interesting experiment illustrates refraction ? How are persons standing on the bank
of a sheet of water often deceived? 641. When do wo see the heavenly bodies in
their real position ? Why, at other times, do we not see them in their real position ?



248 OPTICS.

642. Mirage. Different strata of the atmosphere differ
in their refractive power. Accordingly, rays from an olx
ject below the horizon (that is, concealed from us by the
roundness of the earth) may, under peculiar circumstances,
by successive refractions through different strata, be made
to describe a curve to our eyes, and will in that case ap-
pear to come from a distant point in the air lying in the
direction of the line described by the ray as it entered the
eye. Such is the origin of the phenomenon called Mirage



Mirage is the appearance in the air of an erect or in-
verted image of some distant object which is itself in visible.
It is most frequently seen on the water, but has also ap-
peared to persons travelling through deserts, with such viv-
idness as to make them believe that they saw trees and
springs before them in the distance.

Mirage is sometimes remarkably distinct at sea. Captain Scoresby, on
one occasion, in a whaling-ship, recognized his father's vessel, when distant
from him more than 30 miles (and consequently below the horizon), by its
inverted image in the air, though he did not previously know that it was
cruising in that part of the ocean. Another notable case occurred on the
coast of Sussex, England. Cliffs were distinctly seen in the air ; and the
sailors, crowding to the beach, recognized different parts of the French shore,
distant from 40 to 50 miles. These phenomena are comparatively frequent
in the Strait of Messina, and as there exhibited have been called Fata Mor-
gana [fah'-tah mor-gah'-naJi].

643. REFRACTION BY PKISMS AND LENSES. Prisms and
lenses are much used in experimenting on light and in the
construction of optical instruments.

Fig 243 644. Prisms. A Prism (see Fig. 243)

is a solid piece of glass, having for its sides



XI \| three plane surfaces and for its ends two

A PRISM. equal and parallel triangles.

645. A ray of light falling on a prism must pass through
two of its surfaces. If it strike both of them obliquely, it

To what do we owe our twilight ? 6-12. Explain how an object below the horizon is
rendered visible. What phenomenon is thus produced ? What is Mirage ? Where
is it seen ? What case of mirage is recorded by Captain Scoresby ? What other nota-
ble case is mentioned ? Where are these phenomena frequent ? 643. What are much
used in experimenting on light ? 644 What is a Prism ? 645. What is the effect of



REFRACTION BY P.RISMS AND LENSES.



249




will be twice refracted ; if it strike one surface perpendic-
ularly and the other obliquely, it will be refracted but once.
In either case, the object from which it comes will appear
to lie in a position more or less removed from its real one.

Fig. 244 shows the refractive effect of a prism. Fi 244.

A ray from E, entering the prism ABC, from

air, a rarer medium, is refracted to D, and on . - o

passing back into the rarer medium, at that point
is refracted to the eye. The object from which it
comes appears to lie at F, in the direction from
which the ray entered the eye. Had there been

but one refraction, it would still have appeared elevated above its real posi-
tion, but not so much.

646. Lenses. A lens is a transparent body which has
two polished surfaces, either both curved or one curved and
the other plane. The general effect of lenses is to refract
rays of light, and magnify or diminish objects seen through
them. They are generally made of glass ; but in specta-
cles rock crystal is sometimes used instead of glass, because
it is harder and less easily scratched.

647. Classes of Lenses. Lenses are divided into six
classes according to their shape. Fig. 245 shows these six
classes. The name of each is given on one side, and a de-
scription of it on the other.

Fig. 245.



DOUBLE CONVEX LENS.
PLANO-CONVEX LENS.
MENISCUS.

DOUBLE CONCAVE LENS.
PLANO-CONCAVE LENS.
CONCAVO-CONVEX LENS.





Both sides convex.

One side convex, the other plane.

( One side convex, the other concave.
\ Thickest in the middle.

Both sides concave.

One side concave, the other plane.

j One side concave, the other convex.
( Of uniform thickness, or thickest at the
ends.



a prism on a ray of light? Show this effect with Fig. 244. 646. What is a lens?
What is the general effect of lenses ? Of what are they made ? 647. Into how many
classes are lenses divided ? Name them. Describe the Double Convex Lens. The
Plano-convex. The Meniscus. The Double Concave Lens. The Plano-concave.



250 OPTICS.

The first three of the above lenses, which are thickest in the middle, are
called Convex Lenses, and their effect is to make rays passing through them
incline more towards each other. The next two (the double concave and
plano-concave) which are thinnest in the middle, are called Concave Lenses,
and their effect is to make rays passing through them incline further from
each other.

The concavo-convex lens, when its two surfaces are parallel (as in the
above Figure) does not change the direction of rays passing through it, for
the convergent effect of the convex surface is nullified by the divergent effect
of the concave surface. When the convex surface has a greater curvature
than the concave, this lens becomes a meniscus. When the concave surface
has the greater curvature, it becomes a concave lens, and participates in the
properties of that class.

648. Refraction by Convex Lenses. The general effect
of convex lenses is threefold: 1. They make rays passing
through them incline more towards each other than before.
2. They enable us to see objects which are invisible to the
naked eye on account of their distance. 3. They magnify
objects seen through them.

649. A double convex lens of glass, with sides equally
convex, brings parallel rays passing through it to a focus at
the centre of the sphere, of which the surface of the lens
first struck by the rays forms a part. This is shown in Fig.
246. Converging rays would be brought to a focus be-
tween the centre and the lens ; diverging rays, on the other
side of the centre.

Fig. 246. Tig. 247.





The Concavo-convex. What are the first three of these lenses called ? What is their
effect ? What are the double concave and the plano-concave lens called ? What is
their effect ? What is the effect of the concavo-convex lens, when its two surfaces are
parallel? When the convex surface has a greater curvature than the concave?
When the concave surface has a greater curvature than the convex? 648. What is
the general effect of convex lenses ? 649. What is the effect of a double convex glass
lens on parallel rays passing through it ? On converging rays ? On diverging rays ?



REFRACTION BY LENSES. 251

A plano-convex lens brings parallel rays to a focus at a
distance from the lens about equal to the diameter of the
sphere of which the convex surface of the lens forms a part.
This is shown in Fig. 247.

650. Convex lenses collect heat as well as light at their focus. Hence
they are sometimes called Burning Glasses. Hold an old person's eye-glass
in the sun-shine a short distance from your hand. A bright spot of light
marks the focus, and the heat at that point soon becomes too great to be
borne. All the rays that fall on the surface of the lens being concentrated
in this one point, the heat at the focus is as many times greater than the heat
of ordinary sun-light as the area of the lens is greater than the area of the fo-
cus. If the area of the lens be 100 square inches, and that of the focus J / 4 of
an inch, the ordinary heat of the sun will be increased 400 times.

651. The second effect of convex lenses follows from the
first. Light, it will be remembered, diminishes in intensity
according to the square of the distance from the luminous
body ; hence rays from exceedingly remote stars become
so faint by the time they reach the eye as not to produce
the sensation of vision. A convex glass concentrates a great
number of these faint rays, and thus renders the distant
object visible to an eye placed at its focus.

652. The third effect of convex lenses is to magnify ob-
jects seen through them. Hence they are sometimes called
Magnifying Glasses. The glasses used by old persons, as
well as by engravers and others who have to deal with mi-
nute objects, are convex lenses.

653. Refraction by Concave Lenses. The effects of
concave lenses are opposite to those of convex. 1. They
make rays passing through them incline farther from each
other. 2. They diminish objects seen through them.

654. All the above laws relating to prisms and lenses apply to rays pass-
ing into them from a rarer medium, such as air. If they come from a denser
medium, the results will be reversed, convex lenses will have a diverging
and diminishing effect, while concave lenses will have a converging and
magnifying effect.

What is the effect of a plano-convex lens on parallel rays ? 650. What are convex
Senses sometimes called, and why ? How may their concentration of heat be shown f
How does the heat at the focus compare with that of ordinary sun-light ? 651. Show
how i convex lens enables us to see distant heavenly bodies that would otherwise be
Invisible. 652. What is the third effect of convex lenses ? What are they sometimes



252



OPTICS.



655. Glasses with Parallel Surfaces. When rays pass through a refracting
medium having parallel surfaces, they leave it, not exactly in the same line,
but in a direction parallel to that in which they entered it. The last refrac-
tion nullifies the change of direction produced by the first. Hence we see
objects through a pane of window-glass very nearly in their real position. Ir-
regularities in the glass cause objects seen through it to look distorted.

656. The Multiplying Glass. If a plano-convex lens
have its convex surface ground into several flat surfaces, an
object seen through it will be 'multiplied as many times as
there are flat surfaces.



Fig. 243.




T1IE MULTIPLYING



In Fig. 248, A B represents a multiplying glass, and
D an object viewed through it. The ray D C, striking
both surfaces perpendicularly, reaches the eye without
refraction ; but D I and D F, falling obliquely, suffer
two refractions, which bring them also to the eye at
the focus. As objects are always seen in the direction
in which their rays enter the eye, three objects like D
will be visible : one at D, in its real position ; the
others, in the direction of the dotted lines, at G and II.

657. DOUBLE REFRACTION. Certain
substances (chiefly minerals) have the prop-
erty of causing rays which pass through them to take two
distinct paths, and thus produce two images. This phe-
nomenon is called Double Refraction.

Fig. 249. A crystal of carbonate of lime,

commonly called Iceland Spar, is
one of the best substances for ex-
hibiting double refraction. Let it be
placed over a piece of paper con-
taining lines, and each line will be
seen double, as shown in Fig. 249.

Keeping the same side on the
paper, and turning the crystal round
on its axis, we find that the double
lines continue parallel, but that the
distance between them varies, diminishing till tkey coincide, then increas-
ing; then diminishing till they coincide again, and then once more increas-

called in consequence ? 653. What are the general effects of concave lenses ? 654. In
what case do the above laws relating to prisms and lenses apply ? Suppose the rays
pass into them from a denser medium, what will be the result ? 655. What effect has
a refracting medium with parallel surfaces on incident rays ? How do we see objects
through a pane of window-glass ? 656. How is the multiplying glass formed ? How
many times is an object seen through it multiplied? Show this with Fig. 248.
657. What is Double Kefraction ? How is it exhibited with Iceland spar ? What phe-




POLAEIZATION OF LIGHT. 253

ing. During each revolution of the crystal, the lines will coincide twice. A
single pencil of rays is thus refracted into two distinct pencils, one of which,
following the usual law of refraction, is called the Ordinary Pencil, while the
other, deviating from that law, is called the Extraordinary Pencil.

Polarization of ILigSit.

658. Light is said to be polarized, when, on being re-
flected or refracted by a surface which it strikes at a cer-
tain angle, it is absorbed by a similar surface perpendicular
to the former one, though it is reflected or transmitted by
one forming any other angle with it.

Let A and B (Fig. 250) be two tubes open at both j,. ^

ends, and so adjusted to each other that B turns stiff-
ly within A. Jn each tube fix a piece of polished A B ^
glass, M, N, roughened and blackened on the back,
so as to form an angle of 33 degrees with the axis of /
the tubes. Bring the instrument into such a position

that the light from a luminous body, falling on M, may be reflected along the
axis and strike N. Now, keeping the tube A stationary, turn within it the
tube B, carrying the reflector N. The reflection from N, if observed, will be
seen to keep varying in intensity. In the two positions in which N is paral-
lel to M, the reflection will be brightest ; at the points midway between these,
that is, when N is perpendicular to M, there is no reflection at all. We
express this by saying that the light reflected from M is polarized.

659. The polarizing angle, that is, the angle which the
incident ray must make with a perpendicular to the first
reflecting surface, in order to be polarized, is different in
the case of different substances. For glass, it is about 57
degrees.

660. If a polarized ray be received on a crystal of Ice-
land spar, there will be but a single refraction.

661. Light is polarized by reflection at a certain angle, as we have just
seen ; by transmission through substances that have the property of double
refraction, through some imperfectly crystallized substances, such as agate,
mother-of-pearl, &c., and also through a sufficient number of uncrystallized
plates. However produced, polarized light always has the same' properties
Its phenomena are striking, and seem to prove the truth of the undulatory

nomena are presented as the crystal is turned around ? "What are the two pencils
presented to the eye called? 658. "When is light said to be polarized ? Illustrate the
polarization of light with Fig. 250. 659. "What is meant by the polarizing angle?
What is this angle in the case of glass ? 660. If a polarized ray is received on a crys-
tal of Iceland spar, what follows ? 661. Mention the different ways in which light ia
polarized. "What is said of the properties and phenomena of polarized light, how-



254



OPTICS.



theory. It is thought that the undulations of ether ordinarily take place in
planes perpendicular to the direction in which they are propagated; but
that, when light is polarized, they take place in planes parallel to this direc-
tion. At certain angles, the undulations, thus changed from their usual di-
rection, are reproduced or transmitted by the second reflecting or refracting
surface, and reach the eye ; but, when the two surfaces form an angle of 90
degrees, they are stopped, and the sensation of vision is not produced.

662. The mineral called Tourmaline [toor '-ma-leen\ pos-
sesses the property of polarizing light in a high degree. It
is cut into plates one-twentieth of an inch thick, which are
fixed between plates of glass for convenience of use. If we
look at the sun through such a plate, we shall find that most
of the light is transmitted. Place a second plate behind
the first and parallel to it, and the light will still be trans-
mitted ; but turn the second plate so as to bring it at right
angles to the first, and no light will pass through.

663. Some crystals viewed by polarized light, exhibit systems of beautiful
rings, like those shown in Fig. 251. Plates of the mineral culled Selenite,

Fig. 251.





bearing different designs, placed so as to be seen by polarized light, display
the most gorgeous coloring, and may be made to undergo remarkable and
beautiful changes by causing one of the reflecting surfaces to revolve.

Chromatics.

664. Chromatics is that branch of Optics which treats
of colors.

ever it is produced? Explain the polarization of light according to the nndulatory
theory. C62. What mineral possesses the property of polarizing light in a high de-
gree ? How is tourmaline prepared ? What experiment may be performed with tour-
maline plates ? 663. What phenomena are seen when certain crystals are viewed



THE SOLAR SPECTRUM.



255



C65. THE SOLAR SPECTRUM. If a ray from the sun be
admitted into a dark room through a small aperture, it will
form a circular spot of white light on the surface receiving
it. But if, after entering the room, it be received on a
prism, as shown in Fig. 252, it will be decomposed into

Fig. 252.




THE SOLAR SPECTRUM.



seven different colors. When made to fall on a white sur-
face, these seven colors are distinctly seen, covering an
oblong space, which is called the Solar Spectrum (plural,
spectra}. They are known as the Primary Colors, and in
every spectrum they are arranged in the order shown in
the Figure. By combining the primary colors in different
proportions, other colors are produced.

The seven colors, it will be observed, do not occupy
equal spaces of the spectrum. Violet covers the greatest
part, more than one-fifth of the whole ; and orange the
least, less than one-thirteenth of the whole.

666. Ordinary sun-light (and all white light) is therefore composed of
seven colors combined in different proportions. In further proof of this, we
may re-unite the seven primary colors of the spectrum, and we shall have
simply a small circular spot of white light. To re-unite the colors, we may
receive the spectrum on a concave mirror or double convex lens, which brings
together at its focus the parts of the decomposed ray. Or, we may receive
the spectrum on another prism placed in contact with the first, as shown
in Fig. 252. In either case, we have the same circular spot of white light
that would have been formed if the ray had not been decomposed at all.

by polarized light? When plates of selenite are viewed by polarized light?
GG-k "What is Chromatics? CG5. Describe the solar spectrum, and the way in which
it is formed. Name the seven primary colors in order. How are the other col-
ors produced? Which color occupies most of the spectrum, and which the least?
6GG. Of what, then, is all white light composed? What further proof have wa



256 OPTICS.

We may produce white light by combining the seven primary colors in
another way. Divide the surface of a circular card into seven parts propor-
tioned to each other as the spaces which the different colors occupy in the



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 24 of 42)