OPTICAL INSTRUMENTS VERY SHORT ANSWER QUESTIONS Q-1. The difference in the focal lengths of the two lenses is larger in which case microscope or telescope? Q-2. What is the formula for angular magnification for a telescope in normal adjustment? Q-3. Who designed the compound microscope? Q-4. Name an optical instrument in which we make use of the phenomenon of total internal reflection. Q-5. What type of lens is used as eye-lens in Galileo telescope? Q-6. Give two uses of optical instruments? Q-7. What is the size and nature of the image of an object placed in contact with a convex lens? Q-8. What is the ideal position of the eye for observing the image in a microscope or telescope? Q-9. What happens when the focal length of the objective of a telescope is increased? Q-10. Who designed the first telescope? Q-11. What do you understand by the term eye-ring? Q-12. What is the formula for the magnifying power of a compound microscope? Q-13. Why no cross-wires can be used in Galileo telescope? Q-14. In a simple microscope, why the focal length of the lens should be small? Q-15. Name one essential difference between a terrestrial telescope and an astronomical telescope. Q-16. How can you increase the magnifying power of a telescope? Q-17. Why do we prefer a magnifying glass of smaller focal length? Q-18. Express the angular magnification of an astronomical telescope. Q-19. What is the magnification produced by a single convex lens used as a simple microscope in normal use? Q-20. What happens to the magnifying power of a compound microscope when length of the tube is increased? Q-21. What is the main limitation of Galileo s telescope? Q-22. A short-sighted person may read a book without spectacles. Comment. 1
Q-23. If a telescope is inverted, will it be able to work as a microscope? Q-24. Two lenses of focal lengths 6 cm and 50 cm are to be used for making a telescope. Which will you use for the objective? Q-25. On what factors does the magnifying power of a refracting type astronomical telescope depend? Q-26. On what factors does the magnifying power of a compound microscope depend? Q-27. How does the magnifying power of a telescope change on decreasing the aperture of its objective lens? Q-28. Is angular magnification of a telescope equal to ratio of diameters of objective and eye lens? Q-29. What is the length of a telescope in normal adjustment? Q-30. What is meant by normal adjustment in case of an astronomical telescope? Q-31. Q-32. Write two advantages of reflecting type telescope over astronomical telescope. You are given three lenses of power 0.5 D, 4 D,. State, with reason, which two lenses will you select for construction a good astronomical telescope. SHORT ANSWER QUESTIONS Q-33. Why do we prefer an objective of large aperture in a telescope? Q-34. Why God has gifted us two eyes instead of one? Q-35. What is the difference between the angular magnification and magnification produced by a lens? Q-36. Why do we prefer reflecting type telescope for astronomical purposes? Q-37. Why the Sun appears to be a small disc to an observer on the surface of Earth? Q-38. Show with the help of a diagram the inversion of rays from an object AB by a prism. Q-39. What is the function of the camera shutter? Q-40. What is the adjustment needed in a camera to take pictures of objects at different distances? Q-41. The objective of telescope A has a diameter 3 times that of the objective of telescope B. Show that the range of A is three times the range of B. (Range tells you how far away a star of some standard absolute brightness can be spotted by the telescope). Q-42. A hypermetropic person has problem in driving without spectacles. Comment on this statement. 2
Q-43. Can we increase the range of a telescope by increasing the diameter of the objective lens? Q-44. Q-45. Q-46. Q-47. Using the data given below, state which two of the given lenses will you prefer to construct a best possible (i) telescope (ii) microscope Also, indicate which of the selected lenses is to be used as an objective and as an eyepiece in each case? Lenses Power Aperture L 1 6D 1 cm L 2 3D 8 cm L 3 10D 1 cm Draw a labelled diagram showing course of rays for a simple microscope. Draw a ray diagram to illustrate image formation by a Newtonian type reflecting telescope. Four double convex lenses with the following specifications are available. Which two of the given four lenses should be selected as the objective and eyepiece to construct an astronomical telescope and why? What will be the magnifying power and normal length of the telescope tube so constructed? Lens Focal length Aperture A B C D 100 cm 100 cm 10 cm 5 cm 10 cm 5 cm 2 cm 2 cm Q-48. In the previous question, which two of the four lenses should be selected as objective and eyepiece of a compound microscope and why? How can the magnifying power of such a microscope be increased? Q-49. Draw a ray diagram of an astronomical telescope in the near point adjustment. Write down the expression for its magnifying power. Q-50. Using the data given below, state as to which of the given lenses will you prefer to use as (i) an eyepiece and (ii) an objective, to design a compound microscope. Give reason for your answer. Lens Power Aperture A B C D 20 D 1.0 D 0.05 m 0.1 m Q-51. Using the data given below, state as to which of the given lenses will you prefer to use as (i) an eyepiece and (ii) an objective, to construct an astronomical telescope. Give reason for your answer. 3
Lens Power Aperture L 1 L 2 L 3 L 4 1 D 20 D 0.1 m 0.05 m Q-52. Draw a labelled ray diagram of a reflecting type telescope. Write its any one advantage over refracting type telescope. Q-53. Draw a labelled ray diagram to show the image formation in a refracting type astronomical telescope for the near point adjustment. Why should the diameter of the objective of a telescope be large? Q-54. Draw a labelled ray diagram of a compound microscope, showing the formation of image at the near point of the eye. Q-55. Draw a labelled ray diagram of an astronomical telescope, forming the image at infinity. Also define the magnifying power of the astronomical telescope in this position. SHORT ANSWER QUESTIONS Q-56. A person looks at different trees in an open space with the following details. Arrange the trees in decreasing order of their apparent sizes. Tree Height(m) Distance from the eye(m) A B C D 2.0 2.5 1.8 2.8 50 80 70 100 Q-57. A telescope increases the brightness of a point source (such as a distant star) but not the brightness of the background compared to what is seen by the unaided eye. Thus it facilitates observation by improving the contrast between a star and its background. Explain the statement carefully. Q-58. Why are apertures of camera lenses so small while the apertures of telescopes are as large as feasible? Q-59. Explain the term depth of field. Why does the depth of field increase if aperture is reduced? Which shot in your view will require a greater depth of view-photograph of a scenic spot, or your identity photograph? 4
Q-60. (a) The far point of a myopic person is 80 cm in front of the eye. What is the power of the lens required to enable him to see very distant objects clearly? (b) In what way does the corrective lens help the person above? Does the lens magnify very distant objects? Explain carefully. (c) The person above prefers to remove his spectacles while reading a book. Explain why? Q-61. (a) The near point of a hypermetropic person is 75 cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25 cm from the eye? (b) In what way does the corrective lens help the person above? Does the lens magnify objects held near the eye? (c) The person above prefers to remove his spectacles while looking at the sky. Explain why? Q-62. (a) Suppose the person in Question 60 uses spectacles of power 0.80 dioptre, how far can he see clearly? (b) If the person in Question 61 uses spectacles of power +1.0 dioptre, what is the nearest distance of distinct vision for him? Q-63. (a) List some advantages of a reflecting telescope, especially for high resolution astronomy. (b) A reflecting telescope has a large mirror for its objective with radius of curvature equal to 80 cm. What is the magnifying power of the telescope it the eyepiece used has a focal length of 1.6 cm? Q-64. Which two main considerations are kept in mind while designing the objective of an astronomical telescope? Obtain an expression for the angular magnifying power and the length of the tube of an astronomical telescope in its normal adjustment position. CONCEPTUAL PROBLEMS Q-65. Can we increase the range of a telescope by increasing the diameter of the objective? Q-66. What do you understand by the term focal plane of a lens? Q-67. Why the objective and eyepiece of a compound microscope have short focal lengths? Q-68. What is least distance of distinct vision? Q-69. What would be the effect on the length of telescope if the focal length of the objective is increased? 5
Q-70. Q-71. Prism Binocular is nothing but a pair of astronomical telescopes. Comment on this statement. The objective of telescope A has a diameter 3 times that of the objective of telescope B. How much greater amount of light is gathered by A compared to B? Q-72. What is a telephoto lens? How does it differ from an ordinary camera lens? How does it increase the field of view? Q-73. A camera is set at the aperture size f/8 and the exposure time of (1/60)s. How much exposure time is required for receiving the same amount of light if the aperture size is set at f/5.6? How is the depth of field affected by this change? Q-74. The field of view of a camera is increased by using so called wide-angle lens. In what way does this lens differ from an ordinary camera lens? How does it increase the field of view? Q-75. Show that for a given brightness of the image on a camera film, the exposure time t is inversely proportional to the square of the aperture size a and directly proportional to the square of the focal length f of the camera lens. Q-76. The values of refractive indices for crown glass and dense flint glass for red and violet light are : μ red 1.516 (crown glass), 1.694 (dense flint glass) : μ violet 1.533 (crown glass), 1.742 (dense flint glass). Which will be more suitable for spectacle lens? Give reasons. LONG ANSWER QUESTIONS Q-77. Draw a labelled ray diagram to show the formation of an image by a compound microscope. Find an expression for its magnifying power. Q-78. Describe the construction and working of a compound microscope. Derive an expression for its magnifying power. Q-79. Describe construction and working of an astronomical telescope. Calculate its magnifying power, when the image is formed at the least distance of distinct vision. Q-80. Draw a labelled diagram of a compound microscope when the image is formed at infinity. Deduce the expression for its magnifying power. How can the magnifying power be increased? 6
Q-81. Explain the working of a compound microscope. Find an expression for its magnifying power. Also define resolving power of compound microscope. Q-82. Explain the construction and working of an astronomical telescope. Derive an expression for its magnifying power when the final image is formed at infinity. Q-83. Draw a labelled ray diagram of an astronomical telescope. Write mathematical expression for its magnifying power. How does the magnifying power get affected on increasing the aperture of the objective lens and why? Q-84. Draw a labelled ray diagram of an astronomical telescope. Write mathematical expression for its magnifying power. How does the magnifying power get affected on increasing the aperture of the objective lens and why? Q-85. With the help of a labelled ray diagram, show the image formation by a compound microscope. Derive an expression for its magnifying power. 7