Physics R: Form TR8.17A TEST 8 REVIEW Name Date Period Test Review # 8 Light and Color. Color comes from light, an electromagnetic wave that travels in straight lines in all directions from a light source through both space and objects. Matter usually does not produce light, rather, it is reflected, absorbed, or passes through matter. What happens to light when it strikes matter depends on the material of which the matter is composed. All matter reflects and absorbs some light. Materials through which no light passes, such as wood or brick, are opaque. Materials that only let some light pass, such as waxed paper or frosted glass are translucent. Materials through which most light passes, such as glass, are transparent. Light often seems colorless, but sunlight and electric lights contain a mixture of frequencies. The color of an object depends on which frequencies are reflected and which are absorbed. Green leaves reflect green light and absorb all the other frequencies. We interpret light based on the way we see it. The three primary colors of light are red, blue, and green. Light that appears to be any color can be made with different mixtures of the three primary colors of light. White light is a mixture of all three. Mixing pigments is different than mixing light because the appearance of pigments is based on the colors of light they absorb and reflect. Primary pigment colors result from absorption of a primary color of light. The three primary colors of pigments are cyan, magenta, and yellow. Cyan absorbs red, and reflects blue and green. Magenta absorbs green, and reflects red and blue. Yellow absorbs blue, and reflects red and green. A pigment of any color can be made with different mixtures of the three primary colors of pigments. But a mixture of all three pigments is not white, because it is absorbing all the primary colors. Instead, it is grey or black. Plane Mirrors. When you stand in front of a mirror, you see your reflection. The reflected light is organized into a clear image because it follows the law of reflection. The law of reflection says the angle of incidence is equal to the angle of reflection. A line perpendicular to a surface is called a normal. An incoming ray of light forms an angle with the normal called the angle of incidence. A reflected ray of light forms an angle with the normal called the angle of reflection. According to the law of reflection, the two angles are equal. Not all surfaces are flat and smooth like the surface of a plane mirror. The light that bounces off these surfaces still follows the law of reflection. Rough surfaces scatter the light so parallel rays are no longer parallel. A clear image does not form. This is called diffuse reflection. Smooth surfaces reflect light in such a way that parallel rays are still parallel. A clear image does form. This is called regular or specular reflection. Plane mirrors have a smooth, flat surface. As a result, specular reflection occurs. A clear image forms from the organized reflections that enter your eyes. Your brain assumes that light travels in a straight line without changing direction. This makes the image look as far behind the mirror as the object is in front of the mirror, but left and right switch places. Concave Mirrors. A mirror that is curved inward is a concave mirror. It causes light rays to come together or converge. Concave mirrors can be used as magnifying mirrors, but they also form other types of images depending on the location of the object relative to the mirror. A straight line drawn perpendicular to the center of the mirror is called the optical axis or principal axis. Light rays that travel parallel to the principal axis and strike the mirror converge on a single point on the principal axis called the focal point. The distance from the center of the mirror to the focal point is the focal length. The image that forms from a concave mirror depends on the position of the object relative to the focal point. If an object is further from the mirror than the focal length, the image that forms is upside down, in front of the mirror, real, and the size of the image decreases as the object moves further away. If an object is closer to the mirror than the focal length, the image that forms is upright, behind the mirror, virtual, and the size of the Primary colors of light [A] Angle of reflection; [B] Normal; [C] Angle of incidence
TEST 8 REVIEW Page 2 image decreases as the object moves closer. If a source of light is placed at the focal point, a focused beam of light is formed as in a flashlight or headlights. Convex Mirror. A convex mirror is mirror with a surface that curves outward. Normals drawn perpendicular to the curved surface of a convex mirror diverge or spread out. The law of reflection tells us then, that the reflected rays of a convex mirror coming from parallel incident rays will also diverge. A convex mirror causes light rays to spread out or diverge. Since the reflected rays of a convex mirror diverge, the mirror does not focus them. The rays of a convex mirror appear to come from a focal point behind the mirror. As a result, a virtual image forms. The image of a convex mirror is virtual, always upright, and appears behind the mirror. It is always smaller than the object. Convex mirrors extend the field of view. They can be used as blind spot car mirrors, at traffic intersections, and for surveillance.. Refraction and Lenses. When light travels from one medium to another at an angle to the boundary between the media, it bends. The bending of light waves as they pass through different media is called refraction. A lens is a transparent object with at least one curved side that both transmits and refracts light. There are two basic types of simple lenses concave and convex. A concave lens is depressed or caved in. A planoconcave lens is concave on one side, while a biconcave lens is concave on both sides. A convex lens bulges out. A planoconvex lens is convex on one side, while a biconvex lens is convex on both sides. Fiber Optics. As light passes from one medium to another, some is refracted, but some is also reflected. The amount refracted and the amount reflected depends upon the angle of incidence. As light passes from a medium through which it travels slower to a medium through which it travels faster (from water to air, for example), the refracted beam is bent away from the normal. As the angle of incidence increases, the refracted beam bends further from the normal, and closer to the boundary between the media, while the amount refracted decreases and the amount reflected increases. If the light beam strikes the boundary at an angle to the normal greater than the critical angle, it is all reflected as if the boundary were a mirror. This is called total internal reflection. The size of the critical angle depends on the materials involved. Optical fibers are thin transparent fibers of glass or plastic covered with another material called cladding. Cladding is a material through which light can travel faster than through the fiber. When light strikes the boundary between the fiber and the cladding, total internal reflection can occur. As a result, light travels through the fiber. Because of total internal reflection, light can't leak from one fiber to another. Optical fibers can carry information coded into light signals. That is the science of fiber optics Lenses. A lens is a transparent object with at least one curved side that causes light to bend. The more curved the sides of the lens are, the more light bends. In this way, lenses can be used to control the bending of light. A convex lens is a lens that is thicker at the center than at the edge. Parallel light rays traveling through a convex lens converge at the focal point. The more curved the lens is, the closer the focal point is to the lens, and the shorter the focal distance is. Image formation is similar to that of a concave mirror. If the object is further than two focal lengths from the lens, the image is real, inverted, and smaller than the object, but the image gets larger as the object is moved towards the lens. If the object is closer to the lens than one focal length, the image is virtual, right side up, and larger than the object, as in a magnifying glass.
TEST 8 REVIEW Page 3 A concave lens is a lens that is thicker at the edge than at the center. A concave lens causes light rays to diverge, so they are not brought into focus. The image formed is virtual, upright, and smaller than the object. Drawing Ray Diagrams. FOR LENSES: Step 1: Draw a ray from the top of the object parallel to the central or principal axis to the center of the lens. Then, bend the ray toward the normal and continue towards the far focus for a converging lens, and toward the near focus for a diverging lens. Step 2: Draw a second ray from the top of the object through the center of the lens on the principal axis. Step 3: Draw the image of the object from where the rays cross to the principal axis. FOR MIRRORS: Step 1: Draw a ray from the top of the object parallel to the central or principal axis to the surface of the mirror. Then, draw the reflected ray through the point where the incident ray hits the mirror as if it comes from the far focus for a diverging mirror, and through the near focus for a converging mirror. Step 2: Draw a second ray from the top of the object through the far focus for a diverging mirror and through the near focus for a converging mirror. Draw the reflected ray parallel to the principal axis from the point where the second ray intersects the mirror s surface. Step 3: Draw the image of the object from where the reflected rays appear to cross to the principal axis.
TEST 8 REVIEW Page 4 Optical Devices. Some common optical devices, devices that use mirrors and/or lenses, include microscopes, telescopes, cameras, projectors, and lasers. Microscopes are used to magnify small objects. A compound microscope has two sets of convex lenses, an eyepiece and an objective. The objective produces an enlarged, inverted, real image in the microscope tube. The eyepiece magnifies the enlarged image further enlarging it. Telescopes are used to examine objects that are far away. Telescopes need to have a large objective lens in order to collect as much light as possible so far away objects are bright enough to be seen. There are two types of telescopes a refracting telescope and a reflecting telescope. A refracting telescope works the same way that a microscope works. A reflecting telescope has a concave mirror instead of a convex objective lens to gather light. The concave mirror reflects light onto a secondary plane mirror. The secondary mirror reflects light through the eyepiece which magnifies the image. Cameras focus an image on a light sensitive layer to take a photograph. A camera uses a convex lens with a short focal length to focus an inverted, real image that is smaller than the object on the light sensitive layer. For many cameras, diaphragm with a variable f-stop is used to control the exposure. A projector is used to shine an image on a surface. A concave mirror reflects light from an intense source. A condenser consisting of two planoconvex lenses directs light through a slide or movie frame to a projection lens. The projection lens focuses an image on a screen. Lasers provide a high energy narrow beam of light. Most light beams spread out because they are composed of different frequency waves that are out of phase with each other. The more a light beam spreads, the less energy is focused on a given point. Lasers are monochromatic, so the waves stay in phase and they don t spread. This makes the laser beam narrower and higher energy. Answer the questions below by circling the number of the correct response. 1. Light waves travel the fastest through which of the following? (1) air (2) water (3) diamond (4) a vacuum 2. Which of the following determines the color of light? (1) a prism (2) its wavelength (3) its refraction (4) its incidence 3. If an object reflects red and green light, what color does the object appear to be? (1) yellow (2) green (3) red (4) purple 11. Which statement is always true about the image formed by a concave lens? (1) It is upside down and larger than the object. (2) It is upside down and smaller than the object. (3) It is upright and larger than the object. (4) It is upright and smaller than the object. Use the figure below to answer questions 12 and 13. 4. If an object absorbs all the light that hits it, what color is it? (1) white (2) black (3) blue (4) green 5. What type of image is formed by a plane mirror? (1) upright (2) magnified (3) inverted (4) all of these 6. How is the angle of incidence related to the angle of reflection? (1) It's greater. (2) It's the same. (3) It's smaller. (4) It's not focused. 7. Which of the following can be used to magnify objects? (1) a concave lens (2) a convex mirror (3) a convex lens (4) all of these 8. What is an object that reflects some light and transmits some light called? (1) colored (2) opaque (3) diffuse (4) translucent 9. Which of the following types of mirror can form an image larger than the object? (1) convex (2) plane (3) concave (4) all of these 10. Which of the following describes an object that allows no light to pass through it? (1) transparent (2) opaque (3) translucent (4) diffuse 12. Which of the following describes the process occurring in the upper panel of the figure? (1) refraction (2) diffuse reflection (3) specular reflection (4) total internal reflection 13. The surface in the lower panel of the figure would be like which of the following? (1) a mirror (2) a sheet of paper (3) waxed paper (4) a painted wall 14. Why does a leaf look green? (1) It reflects green light. (2) It absorbs green light. (3) It reflects all colors of light. (4) It reflects all colors except green. 15. Through which of the following does light travel the slowest? (1) air (2) water (3) diamond (4) vacuum 16. What is the bending of a light wave due to a change in speed? (1) reflection (2) refraction (3) diffraction (4) transmission
TEST 8 REVIEW Page 5 Use the figure below to answer questions 17 and 18. 26. A laser has a narrow high energy beam compared to other light sources because it is (1) white light, (2) red light, (3) green light, (4) monochromatic light. 27. Telescopes need a large objective lens in order to (1) have high magnification, (2) gather a lot of light, (3) form a real image, (4) form a virtual image. 28. A camera needs a convex lens with a short focal length in order to produce a (1) tiny virtual image, (2) magnified virtual image, (3) tiny real image, (4) magnified real image. 17. If the girl is standing 1 m from the mirror, where will her image seem to be located? (1) 2 m behind the mirror (2) 1 m behind the mirror (3) 2 m in front of the mirror (4) 1 m in front of the mirror 18. Which of the following describes the image of the girl formed by the plane mirror? (1) It will be upside down. (2) It will be in front of the mirror. (3) It will be larger than the girl. (4) It will be reversed left to right. 22. 4 23. 2 24. 4 25. 2 26. 4 27. 2 28. 3 15. 3 16. 2 17. 2 18. 4 19. 1 20. 3 21. 3 Answers 8. 4 9. 3 10. 2 11. 4 12. 2 13. 1 14. 1 1. 4 2. 2 3. 1 4. 2 5. 1 6. 2 7. 3 19. A lens is an optical device that is used to: (1) refract light in a controlled manner. (2) reflect light in a controlled manner. (3) separate light into its different colors. (4) scatter light in a diffuse manner. 20. An image that is constructed in your brain from diverging rays reflected off an object is called a: (1) real image. (2) diffuse image. (3) virtual image. (4) refracted image. 21. What color pigments must be mixed into a can of white paint to make the paint appear red? (1) Blue and green (2) Yellow and cyan (3) Magenta and yellow (4) Pink and purple 22. If a cloth that appears blue in white light is viewed in a room filled with only red light, it will appear: (1) blue, (2) white. (3) red (4) black 23. The eyepiece of a compound microscope acts like a magnifying glass for the image produced by the objective lens. The final image you see by looking in the microscope must be: (1) real. (2) virtual. (3) aberrated. (4) projected. 24. Which of the following does NOT use lenses? (1) microscope (2) camera (3) projector (4) laser 25. A refracting telescope is different from a reflecting telescope in that the objective of a reflecting telescope is a (1) lens, (2) concave mirror, (3) convex mirror, (4) plane mirror.