Part 62 (2/2)

(D) Computation of location of images.

Exercises

1. Why is an image of a candle formed by an aperture, not sharply defined?

2. When a photographer takes your picture and moves the camera nearer you, must he move the ground gla.s.s screen toward the lens or away from it? Explain.

3. How can you find the princ.i.p.al focal length of a lens.

4. How can you test a spectacle lens to see whether it is convex concave?

5. When will a convex lens produce a virtual image? Have you ever seen one? Where?

6. When a photographer wishes to obtain a full length view of a person, where does he place the camera?

7. The focal length of the lens is 24 cm. How far from the lens must an object be placed in order that a real image may be three times as long as the object?

8. There is a perfect image of an object on the ground gla.s.s of a camera. The center of the lens is 20 cm. in front of the image and the object 75 cm. from the lens. What is the focal length of the lens?

9. An object is 60 cm. from the lens, the image 120 cm. from it. Find the focal length.

10. How can you find experimentally the princ.i.p.al focal length of a lens?

11. A lens is used to project an enlarged image of a candle upon a screen. Which is farther from the lens, the candle or the image?

Explain.

(6) OPTICAL INSTRUMENTS

=389. The Eye.=--The most common optical instrument is the _eye_. While the structure of the eye is complicated, the principle of it is simple, involving the formation of an image by a double convex lens. (See Fig.

386, in which is shown a front to back, vertical cross-section of the eye.) The eye appears to be made of portions of two spheres, one of which, smaller than the other, is placed in front. This projecting part is transparent, but refracts the light which strikes it obliquely, so as to turn it into the eye. This enables us to see objects at the side when looking straight ahead. Test this by looking directly in front of you and see how far back on each side of the head you can notice a movement of the forefinger of each hand.

[Ill.u.s.tration: FIG. 386.--Cross-section of the eye.]

=390. Action of the Eye in Vision.=--When we look at an object, a small, real, inverted image is formed upon the _retina_ at the back of the interior of the eye. The retina is an expansion of the optic nerve and covers the inner surface at the back of the eyeball. Seeing is due to the action of light in forming images upon the retina. Our eyes are so constructed that when they are relaxed the lens is adjusted to form clear images of _distant_ objects upon the retina. If we look from distant to near objects without changing the shape of the eye lens, a sharp image of the latter cannot be formed and we get a blurred impression. It is difficult, however, to look at objects without automatically adjusting the eye lens so that it will make a sharp image.

Test this by looking out of a window at a distant object, then without moving the head or eyes look at the gla.s.s of the window; you will notice a slight change of some sort _in_ the eye itself as the vision is adjusted. This adjustment is made by muscles that pull or compress the eye lens so as to make it thicker for near objects and thinner for distant ones. The eye ordinarily does not see objects nearer than 10 in.

clearly. This means that the greatest possible thickening of lens will not form clear images upon the retina if the object is nearer than 10 in. (25 cm.).

[Ill.u.s.tration: FIG. 387.--The visual angle, _AOB_ is greater at _AB_ than at _A'B'_.]

=391. The Visual Angle.=--To examine objects carefully we usually bring them as close to the eye as possible, for the nearer to the eye the object is brought, the larger is the visual angle formed by it (see Fig.

387), and the larger is its image upon the retina. _The visual angle of an object is the angle at the eye lens between the rays that have come from the ends of the object._ Consequently the more distant the object, the smaller is its visual angle. Now if we wish to examine small objects with great care, we frequently find that it is necessary to bring them close to the eye so that they have a visual angle of adequate size. If they must be brought closer than 10 in. a double convex lens is placed in front of the eye. This a.s.sists the eye lens in converging the light so that a clear image may be formed when the object is close, say an inch or so from the eye. This is the principle of the magnifying gla.s.s used by watch-makers and of the _simple microscope_. The action of the latter is ill.u.s.trated by Fig. 388. The convex lens forms a virtual, enlarged image ”_A'-B'_” of the object ”_A-B_” which it observed instead of the object itself.

[Ill.u.s.tration: FIG. 388.--Action of the simple microscope.]

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