The Telescope

A simple telescope uses a lens to form a real image of a distant object. A second lens called the eyepiece is positioned so that the image produced by the first lens is within one focal length of the eyepiece. The eyepiece forms an enlarged visual image of the real image.

A third lens or a pair of reflecting prisms is used in the terrestrial telescope, which produces an image that is right-side up. A pair of terrestrial telescopes, side by side, makes up a pair of binoculars.

There are two different types of telescopes; both are designed to aid in viewing distant objects, such as the planets in our Solar System. The refracting telescope uses a combination of lenses to form an image, and the reflecting telescope uses a curved mirror and a lens.

A refracting telescope has an objective and an eyepiece. The two lenses are arranged so that the objective forms a real, inverted image of a distant object very near the focal point of the eyepiece. In order to provide the largest possible magnification, the image distance for the eyepiece is infinite. When we look through a telescope at such relatively nearby objects as the Moon and the planets, magnification is important. However, individual stars in our galaxy are so far away that they always appear as small points of light. A large research telescope that is used to study very distant objects must have a great diameter to gather as much light as possible. This problem can be overcome by replacing the objective with a concave mirror, which results in a reflecting telescope. Because light is reflected from the mirror and does not pass through a lens, the mirror can have rigid supports on the back side.

The Compound Microscope

A

Fig. 6.14. A compound microscope

compound microscope (Fig. 6.14) uses two converging lenses of short focal length. The first lens, the objective, has a very short focal length and a second lens, the eyepiece, has a focal length of a few centimeters. The two lenses are separated by a distance that is much greater than either these focal lengths. The objective lens produces a real image of a closer object. The eyepiece, which serves as a simple magnifier, produces a virtual, enlarged image. Combinations of eyepieces with different focal lengths and different objectives can produce a wide range of magnifications (from ‘Physics for scientists and Engineers’).

Exercise 78. Match each term (1−9) to its definition (a−i).

1. Microscope a) the lens or combination of lenses in an optical instrument nearest the eye of the observer

2. Shutter b) a mirror with a curved reflective surface, which may be either convex or concave

3. Refracting c) a device for recording visual images in the telescope form of photographs, movie film, or video signals

4. Reflecting d) an optical instrument used for viewing very telescope small objects

5. Eyepiece e) the lens or mirror in a microscope or other optical instrument that first receives light rays from the object and forms the image

6. Curved mirror f) a mechanical device on a camera that opens and closes to control the time of a photographic exposure

7. Aperture g) a telescope in which a mirror is used to collect and focus light

8. Objective h) a telescope that uses a converging lens to collect light

9. Camera i) an adjustable opening in an optical instrument (a camera or telescope), that limits the amount of light passing through a lens or onto a mirror

Exercise 79. Look for the words and word combinations in tehe text which have the opposite meanings to the words below.

blurred image ______________________

close object, nearby object ______________________

simple ______________________

to increase, enhance ______________________

virtuall image ______________________

diverging lens ______________________

decrease, narrow, shorten ______________________

convex ______________________

Exercise 80. Answer the questions to text text.

1. What optical instruments do you know?

2. What are the main components of a photographic camera?

3. What image does the lens in a camera form?

4. What is the shutter?

5. What is exposure time?

6. What is an aperture used for?

7. What types of telescopes do you know?

8. What is the difference between a refracting telescope and a reflecting telescope? Which one is the more effective? Why?

9. What types of lenses does a compound microscope use?

10. What are the main components of a microscope?