Материал: Андрианова English Thesaurus of Student-Physicist 2013

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

3. Fill in the gaps with the missing words from the list:

reflection, dual nature, refraction, index of refraction, angle of incidence

1.Light must be regarded as having a _____: in some experiments light acts as a wave and in others it acts as a particle.

2.In _____, part of the light encountering the second medium bounces off that medium.

3.In _____, the light passing into the second medium bends through an angle with respect to the normal to the boundary.

4.Experiments show that the angle of reflection equals the

_____.

5.The _____ is inversely proportional to the wave speed.

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UNIT 23

MIRRORS AND LENSES

Read the following texts. Study the thesaurus on fig. 23, answer the questions after the texts

The development of the technology of mirrors and lenses led to a revolution in the progress of science. These devices, relatively simple to construct from cheap materials, led to microscopes and telescopes, extending human sight and opening up new pathways to knowledge, from microbes to distant planets.

This Unit covers the formation of images when plane and spherical light waves fall on plane and spherical surfaces. Images can be formed by reflection from mirrors or by refraction through lenses. In our study of mirrors and lenses, we continue to assume that light travels in straight lines (the ray approximation), ignoring diffraction.

Flat mirrors

Consider the flat mirror. Light rays leave the source and are reflected from the mirror. Regardless of the system under study, images are formed at the point where rays of light actually intersect or where they appear to originate. Images are classified as real or virtual. In the formation of a real image, light actually passes through the image point. For a virtual image, the light doesn't pass through the image point, but appears to come (diverge) from there. In fact, the images seen in flat mirrors are always virtual (for real objects). Real images can be displayed on a screen (as at a movie), but virtual images cannot.

Finally, note that a flat mirror produces an image having an apparent left-right reversal. You can see this reversal standing in front of a mirror and raising your right hand. Your image in the mirror raises his left hand. Likewise, your hair appears to be parted on the opposite side, and a mole on your right cheek appears to be on your image's left cheek..

Images formed by spherical mirrors

A spherical mirror, as its name implies, has the shape of a segment of a sphere. A spherical mirror with a silvered inner, concave surface, is called a concave mirror. After reflecting from the mirror, rays converge to meet at point, called the image point. The rays then continue and diverge from this point as if there were an object there. As a result, a real image is formed. Whenever reflected light actually passes through a point, the image formed there is real.

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We often assume that all rays that diverge from the object make small angles with the principal axis. Rays that make a large angle with the principal axis converge to other points on the principal axis, producing a blurred image. This effect, called spherical aberration, is present to some extent with any spherical mirror.

A convex mirror is silvered so that light is reflected from the outer, convex surface. This is sometimes called a diverging mirror because the rays from any point on the object diverge after reflection, as though they were coming from some point behind the mirror.

We won't derive any equations for convex spherical mirrors. If we did, we would find that the equations developed for concave mirrors can be used with convex mirrors if particular sign conventions are used. We call the region in which light rays move the front side of the mirror, and the other side, where virtual images are formed, the back side.

We can conveniently determine the positions and sizes of images formed by mirrors by constructing ray diagrams. This kind of graphical construction tells us the overall nature of the image and can be used to check parameters calculated from the mirror and magnification equations. Making a ray diagram requires knowing the position of the object and the location of the center of curvature.

Thin lens

A typical thin lens consists of a piece of glass or plastic, ground so that each of its two refracting surfaces is a segment of either a sphere or a plane. Lenses are commonly used to form images by refraction in optical instruments, such as cameras, telescopes, and microscopes. The equation that relates object and image distances for a lens is virtually identical to the mirror equation, and the method used to derive it is also similar.

The distance from the focal point to the lens is called the focal length. The focal length is the image distance that corresponds to an infinite object distance. Recall that we are considering the lens to be very thin. As a result, it makes no difference whether we take the focal length to be the distance from the focal point to the surface of the lens or the distance from the focal point to the center of the lens, because the difference between these two lengths is negligible. A thin lens has two focal points, one on each side of the lens. One focal point corre-

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sponds to parallel rays traveling from the left and the other corresponds to parallel rays traveling from the right.

Note that a converging lens has a positive focal length under this convention and a diverging lens has a negative focal length. Hence the names positive and negative are often given to these lenses.

Lens and mirror aberrations

One of the basic problems of systems containing mirrors and lenses is the imperfect quality of the images, which is largely the result of defects in shape and form. The simple theory of mirrors and lenses assumes that rays make small angles with the principal axis and that all rays reaching the lens or mirror from a point source are focused at a single point, producing a sharp image. This is not always true in the real world. Where the approximations used in this theory do not hold, imperfect images are formed.

The departures of real (imperfect) images from the ideal predicted by the simple theory are called aberrations. Two common types of aberrations are spherical aberration and chromatic aberration. Spherical aberration results from the fact that the focal points of light rays passing far from the principal axis of a spherical lens (or mirror) are different from the focal points of rays with the same wavelength passing near the axis. Chromatic aberration for a diverging lens is opposite that for a converging lens. Chromatic aberration can be greatly reduced by a combination of converging and diverging lenses.

1.What devices do mirrors use?

2.Explain how does a flat mirror produce an image?

3.What kinds of spherical mirrors do you know?

4.What equations developed for concave mirrors can be used for convex ones?

5.In what devices thin lenses are used?

6.Why one of the basic problems of systems containing mirrors and lenses is the imperfect quality of the images?

7.What are the two common types of aberrations?

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