Physics Part 58

1. Tone quality. Fundamental and overtones. Chladni's plate.

2. Manometric flame apparatus.

3. Phonograph recorder and reproducer.

4. Air columns and wind instruments.

5. How we hear.

Exercises

1. What determines the pitch of the note of a toy whistle?

2. The lowest note of the organ has a wave length of about 64 ft. What is the length of a closed pipe giving this note? Of an open pipe?

3. What is the first overtone of C? What are the second and third overtones? Give vibration numbers and pitch names or letters.

4. Why is the music of a band just as harmonious at a distance of 400 ft. as at 100 ft.?

5. A resonant air column 60 cm. long closed at one end will respond to what rate of vibration at 10C.?

6. Can you find out how the valves on a cornet operate to change the pitch of the tone?

7. How is the trombone operated to produce tones of different pitch?

8. The lowest note on an organ has a wave length of about 64 ft. What must be the length of a closed pipe giving this note?

9. What is the approximate length of an open organ pipe which sends out waves 4 ft. long?

Review Outline: Sound

Sound--definition, source, medium, speed, nature.

Waves--longitudinal, transverse, ill.u.s.trations.

Characteristics of { intensity--area, amplitude, density, distance.

Musical Sounds: { pitch--scales; major, tempered, triads, _N_ = { _V_/_L_ quality--fundamental and overtones.

Sympathetic Vibrations--resonance, interference, beats, discord.

Musical Instruments--string, air column, membrane or plate.

Laws of; (a) vibrating strings (3), (b) vibrating air columns (2).

CHAPTER XVI

LIGHT

(1) LIGHT, ITS RECTILINEAR PROPAGATION, SHADOWS

=352. A Comparison of Sound and Light.=--Light from the standpoint of physics is considered much as is _sound_, as a _mode_ of _motion_; one affecting the ear, the other producing the result called _vision_. There are other differences also worth considering. (a) While sound travels as vibrations of some _material_ medium, light travels only as vibrations of the _ether_; solids, liquids, and gases act so as to hinder rather than to a.s.sist in its movement. That is, light travels best in a vacuum or in a s.p.a.ce devoid of ordinary matter. (b) The _speed_ of light is so great that at ordinary distances on the earth its motion is practically instantaneous. Experiments have shown that its speed is about 186,000 miles to 300,000 kilometers a second.

=353. Luminous and Illuminated Bodies.=--If we consider the objects within a room, some of them, as books and furniture, would be invisible if all light from external sources were excluded. On the other hand, some other objects, such as a lighted lamp, a burning coal, or a red hot iron, would be seen if no outside light were present. Such bodies are said to be luminous. Most luminous bodies are hot and become non-luminous on cooling. There are, however, some bodies that are luminous at ordinary room temperatures, as the firefly and some phosph.o.r.escent paints. When light emitted by a luminous body strikes an object, a portion of it is always _reflected_. It is this reflected light that makes the illuminated object _visible_. If the object is a sheet of gla.s.s, some of the light is _transmitted_. If a substance is so clear that objects can be seen through it, the substance is _transparent_, but if objects cannot be seen through it, the substance is said to be _translucent_. Objects transmitting no light are _opaque_.

Some of the light falling upon a body is neither reflected nor transmitted, but is _absorbed_ and tends to warm the body. The light falling upon a body is therefore either _reflected_, _transmitted_, or _absorbed_. Thus Fig. 345 represents light coming from _S_ to a piece of gla.s.s _GL_. A portion of the light represented by _R_ is reflected.

Another part _A_ is absorbed and disappears, while still another part _T_ is transmitted and pa.s.ses on.

[Ill.u.s.tration: FIG. 345.--The light is transmitted (_T_), reflected (_R_), or absorbed (_A_).]

There is no sharply drawn line between transparent and opaque bodies.

Very thin sheets of gold transmit a greenish light, and experiments have shown that substances as transparent as clear water absorb enough light so that at considerable depths in an ocean or lake little or no light is ever found. All light whether from luminous bodies or reflected from non-luminous objects shows certain properties which will now be considered.

=354. The Rectilinear Propagation of Light.=--If a beam of light pa.s.ses through a hole in a window shade into a darkened room, it is seen to follow a perfectly straight course. If a person while coughing holds a book before the face, the sound pa.s.ses around the book and is heard at any point in the room while the face is hidden by the book. In other words, light ordinarily does not pa.s.s around corners as sound does, but travels in _straight lines_. This fact is made use of when one aims a gun or merely looks at an object. So well established in our minds is the idea that an _object_ is in the direction from which we see the light coming to us from it, that we are sometimes deceived as to the real position of an object, when the course of the light from it has been changed by a mirror or some other reflecting surface. Many _illusions_ are produced in this way, of which the _mirage_ of the desert is one example. (See Art. 381.)

[Ill.u.s.tration: FIG. 346.--Shadow from a small source of light.]

[Ill.u.s.tration: FIG. 347.--Shadow when source of light is large.]

=355. Shadows.=--_A shadow is the s.p.a.ce from which light is cut off by an opaque body._ Thus if a book (see Fig. 346) is held between a screen, _N_, and a _small_ source of light, _L_, a shadow is produced which extends from the book to the screen. Notice that the shadow is a _s.p.a.ce_ and not an _area_. If a _large_ gas flame (see Fig. 347) is used as the source of light, the shadow of the book is no longer clear cut at the edges as before, but has a darker central part with a lighter fringe of partial shadow at the edges. The dark portion within the shadow has all the light excluded from it and is called the _umbra_. The lighter portion of the shadow at the edges has only a part of the light from the flame cut off. This portion is called the _penumbra_. when one stands in sunlight his shadow extends from his body to the ground or object on which the shadow falls. At night we are in the earth's shadow, which extends out into s.p.a.ce beyond the earth.

[Ill.u.s.tration: FIG. 348.--Character of the earth's shadow.]

=356. Eclipses.=--Since the sun is a very large object the shadow cast by the earth contains both umbra and penumbra. (See Fig. 348.) When the moon pa.s.ses into the shadow of the earth, there is said to be an eclipse of the moon, while if the moon's shadow falls upon the earth, the portion of the earth cut off from the sun's light has an eclipse of the sun.

=357. Images by Small Apertures.=--The straight line movement of light makes possible the _pin-hole_ camera, by which satisfactory photographs have been made. The action of this device may be ill.u.s.trated by placing a luminous body, a lighted candle, an incandescent lamp, or a gas flame, in front of a piece of cardboard, _S_, which has a small opening in it.

Light from the object (see Fig. 349) falls upon a screen, _S_{2}_, so as to produce an _inverted image_. Other applications of this principle will be given later.

In Fig. 349 let _PQ_ represent a gas flame, then light from point _P_ at the _top_ of the flame will pa.s.s in a straight line through the opening or aperture of the cardboard and strike at _P_{2}_ at the _bottom_ of the illuminated spot upon the screen. Light from _Q_ pa.s.sing in straight lines through the aperture will strike at _Q_{2}_ at the top of the lighted s.p.a.ce. This spot of light will have the same outlines as the luminous body _PQ_ and being formed as just described will be _inverted_.

[Ill.u.s.tration: FIG. 349.--Image formed by a small aperture is inverted.]

This spot of light, resembling in its outlines the flame, is called an _image_. _An image is defined as an optical counterpart of an object_.

Images are formed in a variety of devices, such as _apertures_, _mirrors_, and _lenses_. The _pin-hole camera_ is simply a light-tight box with a small aperture in one side. Light pa.s.sing through this aperture forms an image upon the opposite side of the interior of the box, of whatever object is in front of the camera. Light entering a room through a _large_ aperture such as a window produces a mult.i.tude of overlapping images which blend to form a somewhat evenly illuminated surface.

Important Topics

1. Light contrasted with sound (three differences).

2. Bodies: transparent, translucent, opaque.

3. Light: reflected, transmitted, absorbed.

4. Light travels in straight lines, evidence, shadows, umbra, penumbra.