QuestionsCh33

QuestionsCh33 - CHAPTER 33: Lenses and Optical Instruments...

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CHAPTER 33: Lenses and Optical Instruments Answers to Questions 1. The film must be placed behind the lens at the focal length of the lens. 2. The lens moves farther away from the film. When the photographer moves closer to his subject, the object distance decreases. The focal length of the lens does not change, so the image distance must increase: i o d d f 1 1 1 + = 3. Yes. Diverging lenses, by definition, cause light rays to diverge and will not bring rays from a real object to a focal point as required to form a real image. However, if another optical element (for example, a converging lens) forms a virtual object for the diverging lens, it is possible for the diverging lens to form a real image. real image 4. A real image formed by a thin lens is on the opposite side of the lens as the object, and will always be inverted as shown in the top diagram. A virtual image is formed on the same side of the lens as the real object, and will be upright, as shown in the bottom diagram. virtual image 5. Yes. In the thin-lens equation, the variables for object distance and image distance can be interchanged and the formula remains the same. 6. Yes, real images can be projected on a screen. No, virtual images cannot, because they are formed by diverging rays, which do not come to a focus on the screen. Both kinds of images can be photographed. The lenses in a camera are designed to focus either converging or diverging light rays down onto the film. 7. ( a ) Yes. The image moves farther from the lens. ( b ) Yes. The image also gets larger. 8. The mirror equation and the lens equation are identical. According to the sign conventions, d > 0 indicates a real object or image and d < 0 indicates a virtual object or image, for both mirrors and lenses. But the positions of the objects and images are different for a mirror and a lens. For a mirror, a real object or image will be in front of the mirror and a virtual object or image will be behind the mirror. For a lens, a real image will be on the opposite side of the lens from a real object, and a virtual image will be on the same side of the lens as the real object.
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9. No. The lens will be a diverging lens when placed in water because the index of refraction of the lens is less than the index of refraction of the medium surrounding it. Rays going from water to lens material will bend away from the normal instead of toward the normal, and rays going from the
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This note was uploaded on 03/22/2009 for the course PHY 54L taught by Professor Thomas during the Spring '09 term at Duke.

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QuestionsCh33 - CHAPTER 33: Lenses and Optical Instruments...

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