simple science

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Simple Science

111. Chandelier Crystals and Prisms
When a ray of light passes through plane glass, like a window pane, it is shifted somewhat, but its direction does not change; that is, the emergent ray is parallel to the incident ray. But when a beam of light passes through a triangular glass prism, such as a chandelier crystal, its direction is greatly changed, and an object viewed through a prism is seen quite out of its true position.

Whenever light passes through a prism, it is bent toward the base of the prism, or toward the thick portion of the prism, and emerges from the prism in quite a different direction from that in which it entered. Hence, when an object is looked at through a prism, it is seen quite out of place. In Figure, the candle seems to be at S, while in reality it is at A.

FIG. - When looked at through the prism, A seems to be at S.
112. Lenses
If two prisms are arranged as in Figure 69, and two parallel rays of light fall upon the prisms, the beam A will be bent downward toward the thickened portion of the prism, and the beam B will be bent upward toward the thick portion of the prism, and after passing through the prism the two rays will intersect at some point F, called a focus.

If two prisms are arranged as in Figure 70, the ray A will be refracted upward toward the thick end, and the ray B will be refracted downward toward the thick end; the two rays, on emerging, will therefore be widely separated and will not intersect.

Lenses are very similar to prisms; indeed, two prisms placed as in Figure, and rounded off, would make a very good convex lens. A lens is any transparent material, but usually glass, with one or both sides curved. The various types of lenses are shown in Figure

The first three types focus parallel rays at some common point F, as in Figure 69. Such lenses are called convex or converging lenses. The last three types, called concave lenses, scatter parallel rays so that they do not come to a focus, but diverge widely after passage through the lens.

FIG. - Rays of light are converged and focused at F.

FIG. - Rays of light are diverged and do not come to any real focus.
113. The Shape and Material of a Lens
The main or principal focus of a lens, that is, the point at which rays parallel to the base line AB meet, depends upon the shape of the lens. For example, a thick lens, such as A, focuses the rays very near to the lens; B, which is not so thick, focuses the rays at a greater distance from the lens; and C, which is a very thin lens, focuses the rays at a considerable distance from the lens. The distance of the principal focus from the lens is called the focal length of the lens, and from the diagrams we see that the more convex the lens, the shorter the focal length.

The position of the principal focus depends not only on the shape of the lens, but also on the refractive power of the material composing the lens. A lens made of ice would not deviate the rays of light so much as a lens of similar shape composed of glass. The greater the refractive power of the lens, the greater the bending, and the nearer the principal focus to the lens.

There are many different kinds of glass, and each kind of glass refracts the light differently. Flint glass contains lead; the lead makes the glass dense, and gives it great refractive power, enabling it to bend and separate light in all directions. Cut glass and toilet articles are made of flint glass because of the brilliant effects caused by its great refractive power, and imitation gems are commonly nothing more than polished flint glass.

FIG. - The different types of lenses.

FIG. - The more curved the lens, the shorter the focal length, and the nearer the focus is to the lens.
114. How Lenses Form Images
Suppose we place an arrow, A, in front of a convex lens. The ray AC, parallel to the principal axis, will pass through the lens and emerge as DE. The ray is always bent toward the thick portion of the lens, both at its entrance into the lens and its emergence from the lens.

We know that two rays determine the position of any point of our image; hence in order to locate the image of the top of the arrow, we need to consider but one more ray from the top of the object. The most convenient ray to choose would be one passing through O, the optical center of the lens, because such a ray passes through the lens unchanged in direction, as is clear from Figure. The point where AC and AO meet after refraction will be the position of the top of the arrow. Similarly it can be shown that the center of the arrow will be at the point T, and we see that the image is larger than the object. This can be easily proved experimentally. Let a convex lens be placed near a candle; move a paper screen back and forth behind the lens; for some position of the screen a clear, enlarged image of the candle will be made.

If the candle or arrow is placed in a new position, say at MA, the image formed is smaller than the object, and is nearer to the lens than it was before. Move the lens so that its distance from the candle is increased, and then find the image on a piece of paper. The size and position of the image depend upon the distance of the object from the lens. By means of a lens one can easily get on a visiting card a picture of a distant church steeple.

FIG. - The image is larger than the object. By means of a lens, a watchmaker gets an enlarged image of the dust which clogs the wheels of his watch.

FIG. - Rays above O are bent downward, those below O are bent upward, and rays through O emerge from the lens unchanged in direction.

FIG. - The lens is held in such a position that the image of the candle is larger than the object.

FIG. - The image is smaller than the object.

FIG. - The lens is placed in such a position that the image is about the same size as the object.
115. The Value of Lenses
If it were not for the fact that a lens can be held at such a distance from an object as to make the image larger than the object, it would be impossible for the lens to assist the watchmaker in locating the small particles of dust which clog the wheels of the watch. If it were not for the opposite fact - that a lens can be held at such a distance from the object as to make an image smaller than the object, it would be impossible to have a photograph of a tall tree or building unless the photograph were as large as the tree itself. When a photographer takes a photograph of a person or a tree, he moves his camera until the image formed by the lens is of the desired size. By bringing the camera (really the lens of the camera) near, we obtain a large-sized photograph; by increasing the distance between the camera and the object, a smaller photograph is obtained. The mountain top may be so far distant that in the photograph it will not appear to be greater than a small stone.

Many familiar illustrations of lenses, or curved refracting surfaces, and their work, are known to all of us. Fish globes magnify the fish that swim within. Bottles can be so shaped that they make the olives, pickles, and peaches that they contain appear larger than they really are. The fruit in bottles frequently seems too large to have gone through the neck of the bottle. The deception is due to refraction, and the material and shape of the bottle furnish a sufficient explanation.

By using combinations of two or more lenses of various kinds, it is possible to have an image of almost any desired size, and in practically any desired position.
116. The Human Eye
In Section 114, we obtained on a movable screen, by means of a simple lens, an image of a candle. The human eye possesses a most wonderful lens and screen; the lens is called the crystalline lens, and the screen is called the retina. Rays of light pass from the object through the pupil P, go through the crystalline lens L, where they are refracted, and then pass onward to the retina R, where they form a distinct image of the object.

We learned in Section 114 that a change in the position of the object necessitated a change in the position of the screen, and that every time the object was moved the position of the screen had to be altered before a clear image of the object could be obtained. The retina of the eye cannot be moved backward and forward, as the screen was, and the crystalline lens is permanently located directly back of the iris. How, then, does it happen that we can see clearly both near and distant objects; that the printed page which is held in the hand is visible at one second, and that the church spire on the distant horizon is visible the instant the eyes are raised from the book? How is it possible to obtain on an immovable screen by means of a simple lens two distinct images of objects at widely varying distances?

The answer to these questions is that the crystalline lens changes shape according to need. The lens is attached to the eye by means of small muscles, m, and it is by the action of these muscles that the lens is able to become small and thick, or large and thin; that is, to become more or less curved. When we look at near objects, the muscles act in such a way that the lens bulges out, and becomes thick in the middle and of the right curvature to focus the near object upon the screen. When we look at an object several hundred feet away, the muscles change their pull on the lens and flatten it until it is of the proper curvature for the new distance. The adjustment of the muscles is so quick and unconscious that we normally do not experience any difficulty in changing our range of view. The ability of the eye to adjust itself to varying distances is called accommodation. The power of adjustment in general decreases with age.

FIG. - The eye.
117. Farsightedness and Nearsightedness
A farsighted person is one who cannot see near objects so distinctly as far objects, and who in many cases cannot see near objects at all. The eyeball of a farsighted person is very short, and the retina is too close to the crystalline lens. Near objects are brought to a focus behind the retina instead of on it, and hence are not visible. Even though the muscles of accommodation do their best to bulge and thicken the lens, the rays of light are not bent sufficiently to focus sharply on the retina. In consequence objects look blurred. Farsightedness can be remedied by convex glasses, since they bend the light and bring it to a closer focus. Convex glasses, by bending the rays and bringing them to a nearer focus, overbalance a short eyeball with its tendency to focus objects behind the retina.

A nearsighted person is one who cannot see objects unless they are close to the eye. The eyeball of a nearsighted person is very wide, and the retina is too far away from the crystalline lens. Far objects are brought to a focus in front of the retina instead of on it, and hence are not visible. Even though the muscles of accommodation do their best to pull out and flatten the lens, the rays are not separated sufficiently to focus as far back as the retina. In consequence objects look blurred. Nearsightedness can be remedied by wearing concave glasses, since they separate the light and move the focus farther away. Concave glasses, by separating the rays and making the focus more distant, overbalance a wide eyeball with its tendency to focus objects in front of the retina.

FIG. - The farsighted eye.

FIG. - The defect is remedied by convex glasses.

FIG. - The nearsighted eye. The defect is remedied by concave glasses.
118. Headache and Eyes
Ordinarily the muscles of accommodation adjust themselves easily and quickly; if, however, they do not, frequent and severe headaches occur as a result of too great muscular effort toward accommodation. Among young people headaches are frequently caused by over-exertion of the crystalline muscles. Glasses relieve the muscles of the extra adjustment, and hence are effective in eliminating this cause of headache.

An exact balance is required between glasses, crystalline lens, and muscular activity, and only those who have studied the subject carefully are competent to treat so sensitive and necessary a part of the body as the eye. The least mistake in the curvature of the glasses, the least flaw in the type of glass (for example, the kind of glass used), means an improper focus, increased duty for the muscles, and gradual weakening of the entire eye, followed by headache and general physical discomfort.
119. Eye Strain
The extra work which is thrown upon the nervous system through seeing, reading, writing, and sewing with defective eyes is recognized by all physicians as an important cause of disease. The tax made upon the nervous system by the defective eye lessens the supply of energy available for other bodily use, and the general health suffers. The health is improved when proper glasses are prescribed.

Possibly the greatest danger of eye strain is among school children, who are not experienced enough to recognize defects in sight. For this reason, many schools employ a physician who examines the pupils' eyes at regular intervals.

The following general precautions are worth observing: -

1. Rest the eyes when they hurt, and as far as possible do close work, such as writing, reading, sewing, wood carving, etc., by daylight.

2. Never read in a very bright or a very dim light.

3. If the light is near, have it shaded.

4. Do not rub the eyes with the fingers.

5. If eyes are weak, bathe them in lukewarm water in which a pinch of borax has been dissolved.
120. The Magic of the Sun
Ribbons and dresses washed and hung in the sun fade; when washed and hung in the shade, they are not so apt to lose their color. Clothes are laid away in drawers and hung in closets not only for protection against dust, but also against the well-known power of light to weaken color.

Many housewives lower the window shades that the wall paper may not lose its brilliancy, that the beautiful hues of velvet, satin, and plush tapestry may not be marred by loss in brilliancy and sheen. Bright carpets and rugs are sometimes bought in preference to more delicately tinted ones, because the purchaser knows that the latter will fade quickly if used in a sunny room, and will soon acquire a dull mellow tone. The bright and gay colors and the dull and somber colors are all affected by the sun, but why one should be affected more than another we do not know. Thousands of brilliant and dainty hues catch our eye in the shop and on the street, but not one of them is absolutely permanent; some may last for years, but there is always more or less fading in time.

Sunlight causes many strange, unexplained effects. If the two substances, chlorine and hydrogen, are mixed in a dark room, nothing remarkable occurs any more than though water and milk were mixed, but if a mixture of these substances is exposed to sunlight, a violent explosion occurs and an entirely new substance is formed, a compound entirely different in character from either of its components.

By some power not understood by man, the sun is able to form new substances. In the dark, chlorine and hydrogen are simply chlorine and hydrogen; in the sunlight they combine as if by magic into a totally different substance. By the same unexplained power, the sun frequently does just the opposite work; instead of combining two substances to make one new product, the sun may separate or break down some particular substance into its various elements. For example, if the sun's rays fall upon silver chloride, a chemical action immediately begins, and as a result we have two separate substances, chlorine and silver. The sunlight separates silver chloride into its constituents, silver and chlorine.

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