Unit 6. Lenses and Their Application.

Active vocabulary

1. Axial symmetry аксиальна (осьова) симетрія

2. Biconvex [baɪ'kɔnveks]/ двоопуклий

double convex ['dʌblˌkɔn'veks]

3. Biconcave [baɪ'kɔnkeɪv] двовгнутий

4. Collimated beam [kɔlɪ'meɪtɪd'biːm] коллімірованний пучок

5. Compound lens [kəm'paund'lenz] складна лінза

6. Concave [kɔŋ'keɪv] вгнутий

7. (To) concentrate ['kɔnsəntreɪt] зосереджувати(ся) (на - on, upon)

8. Converge [kən'vɜːʤ] сходитись

9. Converging lens [kən'vɜːʤɪŋ'lenz] збиральна лінза

10. Convex [ˌkɔn'veks] опуклий, випуклий

11. Convex-concave [ˌkɔn'veks kɔŋ'keɪv] випукло-вгнутий

12. Corrective lens [kə'rektɪv'lenz] коригувальна лінза

13. Curvature ['kɜːvəʧə] кривизна, вигин, викривлення

14. (To) diverge [daɪ'vɜːʤ] розходитись, відхиляти

15. Diverging lens [daɪ'vɜːʤɪŋ'lenz] розсіювальна лінза

16. Focal length ['fəukl'leŋθ] фокусна відстань

17. Focal plane ['fəukl'pleɪn] фокальна площина

18. Focal point ['fəukl'pɔɪnt] фокальна (фокусна) точка

19. Lens axis ['lenz 'æksɪs] оптична вісь лінзи

20. Magnifying glass ['mægnɪfaɪɪŋglɑːs] збільшувальне скло, лупа

21. Meniscus (lens) [mə'nɪskəs] меніск, меніскова лінза

22. Optical aberration ['ɔptɪkəlˌæbə'reɪʃn] оптична аберація

23. Optical power ['ɔptɪkəl'pauə] оптична сила

24. Plano-concave [ˌpleɪnəu'kɔnkeɪv] плоско-вгнутий

25. Plano-convex [ˌpleɪnəu'kɔnveks] плоско-опуклий

26. Principal axis ['prɪnsəp(ə)l 'æksɪs] головна вісь

27. Simple lens ['sɪmpl'lenz] проста лінза

28. (To) transmit (light) [træns'mɪt] пропускати (світло)

29. Virtual image ['vɜːʧuəl'ɪmɪʤ] віртуальне (уявне) зображення

Pre-reading task

Exercise 63. Work in small groups. List as many applications of lenses in different areas as you can. Share your ideas with other groups.

Reading

Exercise 64. Read, translate the text below.

LENSES

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, concentrating or diverging the beam. A simple lens is a lens consisting of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Manufactured lenses are typically made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses: for instance, a microwave lens can be made from paraffin wax.

Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex (or double convex, or just convex) if both surfaces are convex. A lens with two concave surfaces is biconcave (or just concave). If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is convex-concave or meniscus.

Fig. 6.13. Types of lenses

If the lens is biconvex or plano-convex, a collimated or parallel beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens (known as the focal length). In this case, the lens is called a positive or converging lens.

If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is called a diverging lens or negative lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens.

The principal axis of a lens is a line joining the centres of curvature of its surface. For a converging lens, the focal point at which a beam of light parallel to the principal axis converges. The focal plane is a plane perpendicular to the principal axis that passes through either focal point of a lens. The focal length of a lens, whether converging or diverging, is the distance between the center of the lens and its focal point.

If the lens is convex-concave (a meniscus lens), whether it is converging or diverging depends on the relative curvatures of the two surfaces. It is this type of lens that is most commonly used in corrective lenses. It is commonly supposed that if the two surfaces have the same curvature the lens will have no optical power. This would be true for the case of an infinitely thin lens but is not quite true for lenses with nonzero thickness (http://en.wikipedia.org/wiki/Lens_(optics)).

Exercise 65. Match each term (1−9) to its definition (a−i).

1. Principal axis a) a lens causing rays of light that are initially parallel to meet at a single point

2. Focal point b) a piece of glass or plastic that refracts light

3. Focal plane c) a line joining the centres of curvature of a lens

4. Focal length d) plane perpendicular to the principal axis that passes through either focal point

5. Lens e) a lens causing rays of light to appear to originate from a single point

6. Aberration f) distance between the center of the lens and its focal point

7. Collimated beam g) location where a beam of light parallel to the principal axis converges

8. Converging lens h) distortion in an image

9. Diverging lens i) parallel ray(s) of light

Exercise 66. Circle the correct answer.

1. Converging lenses are _____ at the center and _____ at the edges.

a. thickest, thinnest b. thinnest, thickest

2. Diverging lenses are _____ at the center and _____ at the edges.

a. thickest, thinnest b. thinnest, thickest

3. Converging lenses will have ______ (positive, negative) focal lengths.

4. Diverging lenses will have ______ (positive, negative) focal lengths.

Exercise 67. Answer the following questions.

1. What is a lens?

2. What types of lenses do you know?

3. What is a convex lens and concave lens?

4. What is a plano-convex lens and a plano-concave lens?

5. What is a convex-concave lens?

6. What is a converging lens and a diverging lens?

Exercise 68. Read the information in the text, draw the table in your exercise books and complete it. Add your own ideas. You may use some applications from exercise 63.

APPLICATION OF LENS

A lens is a device which refracts and transmits light by converging or diverging the beam. They are made of glass or plastic depending on the applications. Certain lenses refract light outside of the visual spectrum, such as a paraffin wax lens refracts electromagnetic radiation in a microwave.

Magnifying Glass

A mounted, single convex lens with a handle is called as a magnifying glass and used to enlarge objects or images that may be too small for observation.

Security/ Surveillance

The fish-eye lens is designed for day/night operations in security/surveillance cameras. They may be hand held for bar code reading with auto-focus technology.

Camera

Normal cameras use different types of single or multiple lenses. They are also used in other imaging systems with strict axial symmetry and auto focus technologies. Some such applications are monocular used by jewelers, binoculars, telescopes, projectors. These lenses may produce a real image or a virtual image which may be captured on a film or projected on a screen. In such devices the lenses may be paired with concave mirrors to correct certain optical image aberrations.