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Physics Chapter 13

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Tim DeKoninck

on 24 May 2010

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Transcript of Physics Chapter 13

Light and Reflection What to Expect In this chapter, you will learn about the characteristics of light and other forms of electromagnetic radiation. You will learn how flat and curved mirrors can be used to reflect light and create real and virtual images of objects. Characteristics of Light Flat Mirrors Electromagnetic Waves The spectrum includes more than visible light electromagnetic wave a wave that consists of oscillating electric and magnetic fields, which radiate outward from the source at the speed of light Physics Chapter 13 Curved Mirrors Color and Polarization Electromagnetic waves vary depending on frequency and wavelength The AM radio band extends from 5.4 × 10^5 Hz to 1.7 × 10^6 Hz.What are the longest and shortest wavelengths in this frequency range? Problems 1. Gamma-ray bursters are objects in the universe that emit pulses of gamma rays with high energies. The frequency of the most energetic bursts has been measured at around 3.0 × 10^21 Hz.What is the wavelength of these gamma rays? 2. What is the wavelength range for the FM radio band (88 MHz–108 MHz)? 3. Shortwave radio is broadcast between 3.50 and 29.7 MHz. To what range of wavelengths does this correspond? Why do you suppose this part of the spectrum is called shortwave radio? 4. What is the frequency of an electromagnetic wave if it has a wavelength of 1.0 km? 5. The portion of the visible spectrum that appears brightest to the human eye is around 560 nm in wavelength, which corresponds to yellow-green. What is the frequency of 560 nm light? 6. What is the frequency of highly energetic ultraviolet radiation that has a wavelength of 125 nm? Waves can be approximated as rays Illuminance decreases as the square of the distance from the source Light is... ...a particle ...a wave ...both? fields are perpendicular to the direction in which the wave moves electromagnetic wave Electromagnetic waves are distinguished by their different frequencies and wavelengths accounts for different colors distinguishes visible light from invisible electromagnetic radiation All electromagnetic waves move at the speed of light Speed of light traveling in a vacuum is 2.997 924 58 × 10^8 m/s. In this book, use 3.00 × 10^8 m/s. Reflection of Light the change in direction of an electromagnetic wave at a surface that causes it to move away from the surface The texture of a surface affects how it reflects light reflection Incoming and reflected angles are equal the angle between a ray that strikes a surface and the line perpendicular to that surface at the point of contact angle of incidence the angle formed by the line perpendicular to a surface and the direction in which a reflected ray moves angle of reflection Flat Mirror virtual image an image that forms at a point from which light rays appear to come but do not actually come Image location can be predicted with ray diagrams Ray diagrams are drawings that use simple geometry to locate an image formed by a mirror Concave Spherical Mirrors Concave mirrors can be used to form real images concave spherical mirror a mirror whose reflecting surface is a segment of the inside of a sphere real image an image formed when rays of light actually pass through a point on the image Image created by spherical mirrors suffer from spherical aberration spherical aberration occurs when certain rays do not exactly intersect at the image point. This phenomenon is particularly noticeable for rays that are far from the principal axis and for mirrors with a small radius of curvature occurs with real light rays and real spherical mirrors Image location can be predicted with the mirror equation 100 W bulb vs. a 25 W bulb Light bulbs are rated by their power input (measured in watts) and their light output rate at which light is emitted from a source is called the luminous flux measured in lumens (lm) luminous flux divided by the area of the surface, which is called the illuminance (measured in lm/m2, or lux) decreases as radius squared with distance from the source This line is called a ray, and this simplification is called the ray approximation. Huygens’ principle mirror equation focal point sign conventions positive sign = front side negative sign = back side Magnification relates image and object sizes Ray diagrams can be used for concave spherical mirrors The image distance in the diagram should agree with the value for q calculated from the mirror equation Concave mirrors can produce both real and virtual images 4. A pen placed 11.0 cm from a concave spherical mirror produces a real image 13.2 cm from the mirror.What is the focal length of the mirror? What is the magnification of the image? If the pen is placed 27.0 cm from the mirror, what is the new position of the image? What is the magnification of the new image? Is the new image real or virtual? Draw ray diagrams to confirm your results. 1. Find the image distance and magnification of the mirror in the sample problem when the object distances are 10.0 cm and 5.00 cm. Are the images real or virtual? Are the images inverted or upright? Draw a ray diagram for each case to confirm your results. A concave spherical mirror has a focal length of 10.0 cm. Locate the image of a pencil that is placed upright 30.0 cm from the mirror. Find the magnification of the image.Draw a ray diagram to confirm your answer. 3. A concave makeup mirror is designed so that a person 25.0 cm in front of it sees an upright image at a distance of 50.0 cm behind the mirror.What is the radius of curvature of the mirror? What is the magnification of the image? Is the image real or virtual? Problems 2. A concave shaving mirror has a focal length of 33 cm. Calculate the image position of a cologne bottle placed in front of the mirror at a distance of 93 cm. Calculate the magnification of the image. Is the image real or virtual? Is the image inverted or upright? Draw a ray diagram to show where the image forms and how large it is with respect to the object. Convex Spherical Mirrors convex spherical mirror a mirror whose reflecting surface is an outward-curved segment of a sphere 2. A convex mirror with a focal length of 0.25 m forms a 0.080 m tall image of an automobile at a distance of 0.24 m behind the mirror.What is the magnification of the image? Where is the car located, and what is its height? Is the image real or virtual? Is the image upright or inverted? 3. A convex mirror of focal length 33 cm forms an image of a soda bottle at a distance of 19 cm behind the mirror. If the height of the image is 7.0 cm, where is the object located, and how tall is it? What is the magnification of the image? Is the image virtual or real? Is the image inverted or upright? Draw a ray diagram to confirm your results. 1. The image of a crayon appears to be 23.0 cm behind the surface of a convex mirror and is 1.70 cm tall. If the mirror’s focal length is 46.0 cm, how far in front of the mirror is the crayon positioned? What is the magnification of the image? Is the image virtual or real? Is the image inverted or upright? How tall is the actual crayon? Problems An upright pencil is placed in front of a convex spherical mirror with a focal length of 8.00 cm. An erect image 2.50 cm tall is formed 4.44 cm behind the mirror. Find the position of the object, the magnification of the image, and the height of the pencil. 4. A convex mirror with a radius of curvature of 0.550 m is placed above the aisles in a store. Determine the image distance and magnification of a customer lying on the floor 3.1 m below the mirror. Is the image virtual or real? Is the image inverted or upright? 5. A spherical glass ornament is 6.00 cm in diameter. If an object is placed 10.5 cm away from the ornament, where will its image form? What is the magnification? Is the image virtual or real? Is the image inverted or upright? 6. A candle is 49 cm in front of a convex spherical mirror that has a focal length of 35 cm.What are the image distance and magnification? Is the image virtual or real? Is the image inverted or upright? Draw a ray diagram to confirm your results. diffuse reflection specular reflection Parabolic Mirrors occurs especially with rays that reflect at the mirror’s surface far from the principal axis. certain rays in ray diagrams do not intersect exactly at the image point also occurs with real light rays and real spherical mirrors. Color Additive primary colors produce white light when combined Subtractive primary colors filter out all light when combined Polarization of Light Waves Light can be linearly polarized through transmission the alignment of electromagnetic waves in such a way that the vibrations of the electric fields in each of the waves are parallel to each other linear polarization Light can be polarized by reflection and scattering additive primary colors: red green blue complementary color pixels primary pigments
AKA: primary subtractive colors cyan magenta yellow Combining yellow pigment and its complementary color, blue, should produce a black pigment Yet earlier, blue and yellow were combined to produce green?!? 4. Critical Thinking The light reflected from the surface of a pool of water is observed through a polarizer. How can you tell if the reflected light is polarized? 1. A lens for a spotlight is coated so that it does not transmit yellow light. If the light source is white, what color is the spotlight? 2. A house is painted with pigments that reflect red and blue light but absorb all other colors.What color does the house appear to be when it is illuminated by white light? What color does it appear to be under red light? Highlights 3. What primary pigments would an artist need to mix to obtain a pale yellow green color? What primary additive colors would a theater-lighting designer need to mix in order to produce the same color with light?
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