1© Manhattan Press (H.K.) Ltd. Final image at infinity Eye-ring Eye-ring 12.6 Refracting telescope

2 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 233) Refracting telescope Refracting telescope - consists of two converging lenses (objective & eyepiece)

3 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 233) Final image at infinity Go to More to Know 14 More to Know 14

4 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 234) Final image at infinity Increase M by: 1. use objective of longer focal length 2. use eyepiece of shorter focal length Go to More to Know 15 More to Know 15 Go to Example 13 Example 13

5 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 234) Eye-ring In general, the eye-ring defines the smallest region which all refracted lights by both lenses have passed through.

6 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 235) Eye-ring Observer views through eye-ring - receives maximum amount of light Go to More to Know 16 More to Know 16 d – distance of eye- ring from eyepiece

7 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 236) 12.1 Reflection and refraction 1. The Laws of Reflection: (a) The incident ray, the normal to the surface and the reflected ray are all lie in one plane. (b) The angle of reflection is equal to the angle of incidence.

8 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 236) 12.1 Reflection and refraction 2. The properties of the image formed by a plane mirror: (a) virtual (an image cannot be formed on a screen) (b) erect (c) same size as the object (d) laterally inverted (e) the image distance is equal to the object distance

9 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 236) 12.1 Reflection and refraction 3. When light travels across the interface of two media, refraction occurs because of the change in light speed. 4. The Laws of Refraction: (a) The incident ray, the normal and the refracted ray at the point of incidence are all lie in one plane.

10 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 236) 12.1 Reflection and refraction 4. (b) At the interface between any two different media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for any particular wavelength of the ray. This is named as Snell’s Law. n 1 sinθ 1 = n 2 sinθ 2

11 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 236) 12.1 Reflection and refraction 5. Refractive index of a medium = where real depth is the separation between the interface and the object and apparent depth is the separation between the interface and the image. 6. Refractive index of a prism = where A is the refracting angle of the prism and D min is the minimum deviation.

12 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 237) 12.1 Reflection and refraction 7. Refraction by rectangular glass block Lateral displacement (d) = where θ 1 is the angle of incidence, θ 2 is the angle of refraction and a is the width of the rectangular glass block.

13 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 237) 12.1 Reflection and refraction 8. When the angle of incidence > c (critical angle of a medium), total internal reflection occurs. The light rays must travel from a medium with high refractive index to one with low refractive index.

14 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 237) 12.2 Converging and diverging lenses 9. A converging (convex) lens causes light rays to converge while a diverging (concave) lens causes light rays to diverge. 10. The parts of lenses: (a) optical centre (O) (b) principal axis (c) principal focus (F) (d) focal length (f) (e) focal plane

15 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 237) 12.2 Converging and diverging lenses 11. There are three special incident light rays for lenses: (a) a ray passes through the optical centre O (b) a ray is parallel to the principal axis (c) a ray passes through F (for converging lens) or is directed towards F (for diverging lens)

16 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 238) 12.2 Converging and diverging lenses 12. The nature of the images formed by a converging lens: where u is the object distance and v is the image distance. Object position Image position Nature of image at infinityat Freal, inverted, diminished u > 2ff < v < 2freal, inverted, diminished u = 2fv = u = 2freal, inverted, same size as the object

17 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 238) 12.2 Converging and diverging lenses 12. The nature of the images formed by a converging lens: Object position Image position Nature of image f < u < 2fv > ureal, inverted, magnified u = fat infinitycannot be determined u < fbehind objectvirtual, erect, magnified

18 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 238) 12.2 Converging and diverging lenses 13. The nature of the images formed by a diverging lens: (a) virtual (b) erect (c) diminished (d) v < f

19 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 238) 12.2 Converging and diverging lenses 14. The lens formula relates the object distance (u), the image distance (v) and the focal length (f ). (a) If the object and the image are real, then the values of u and v are positive. (b) If the object and the image are virtual, then the values of u and v are negative. (c) The value of f of a converging lens is positive while the value of f of a diverging lens is negative.

20 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 238) 12.2 Converging and diverging lenses 15. The linear magnification (m) is: 16. When two thin lenses of focal lengths f 1 and f 2 are in contact, where f is the combined focal length.

21 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 238) 12.3 Properties of vision 17. The far point is the point furthest from the eye where an object can be seen clearly by the eye without straining it. 18. The near point is the point of the least distance from the eye such that an object can be seen clearly by the eye without straining it. 19. The least distance of distinct vision is the distance of the near point from a normal eye.

22 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 239) 12.3 Properties of vision 20. A short-sighted person cannot see distant objects clearly and it can be corrected by a diverging lens. 21. A long-sighted person can only see distant objects clearly and it can be corrected by a converging lens. 22. Visual angle is the angle subtended at the eye by the object.

23 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 239) 12.4 Magnifying glass 23. A magnifying glass is a converging lens. 24. The angular magnification (M) is the power of the magnification of the magnified image by a magnifying glass. where D is the distance between the near point and the eye (least distance of distinct vision) and f is the focal length of the magnifying glass.

24 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 239) 12.5 Microscope 25. Two converging lenses (objective and eyepiece) are used in a microscope. 26. An intermediate image is formed by the objective within the focus of the eyepiece. Therefore, the eyepiece acts as a magnifying glass to form a magnified image at the least distance of distinct vision (near point).

25 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 239) 12.5 Microscope 27. The angular magnification (M) of a compound microscope is: M = Linear magnification of eyepiece (m e ) x Linear magnification of objective (m o )

26 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 239) 12.6 Refracting telescope 28. A refracting telescope consists of two converging lenses (objective and eyepiece) as the microscope. 29. An intermediate image is formed by the objective on the focal plane of the eyepiece. Therefore, the eyepiece acts as a magnifying glass to form a final image at infinity.

27 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 239) 12.6 Refracting telescope 30. The angular magnification (M) of a telescope is: where f o is the focal length of the objective and f e is the focal length of the eyepiece. 31. The eye-ring of a telescope or a microscope defines the smallest region which all refracted lights by the objective and eyepiece have passed through.

28 © Manhattan Press (H.K.) Ltd Refracting telescope (SB p. 240)

29 © Manhattan Press (H.K.) Ltd. End

30 © Manhattan Press (H.K.) Ltd. Normal adjustment of telescope A telescope in normal adjustment forms the final image at the far point of the user (at infinity) because it is always used to view distant objects. Return to Text 12.6 Refracting telescope (SB p. 233)

31 © Manhattan Press (H.K.) Ltd. Length of telescope tube The length of the telescope tube must be greater than the sum of f o and f e. Return to Text 12.6 Refracting telescope (SB p. 234)

32 © Manhattan Press (H.K.) Ltd. Large aperture of telescope Telescopes always have a large aperture because they can minimize the effect of diffraction and collect more light from the distant object. Return to Text 12.6 Refracting telescope (SB p. 235)

33 © Manhattan Press (H.K.) Ltd. Q: Q: A telescope whose objective has a focal length of 60 cm and its eyepiece has a focal length of 1.5 cm. Calculate (a) the separation between the lenses forming an image at infinity, and (b) the angular magnification of the telescope. Solution 12.6 Refracting telescope (SB p. 234)

34 © Manhattan Press (H.K.) Ltd. Solution: Solution: (a) The lens separation is estimated as: = f o + f e = = 61.5 cm Return to Text 12.6 Refracting telescope (SB p. 234)