Your camera’s zoom lens has an adjustable focal length

Problem 64PE Consider a 250-W heat lamp fixed to the ceiling in a bathroom. If the filament in one light burns out then the remaining three still work. Construct a problem in which you determine the resistance of each filament in order to obtain a certain intensity projected on the bathroom floor. The ceiling s 3.0 m high. The problem will need to involve concave mirrors behind the filaments. Your instructor may wish to guide you on the level of complexity to consider in the electrical components.

Problem 1CQ Using the law of reflection, explain how powder takes the shine off of a person’s nose. What is the name of the optical effect?

Suppose a man stands in front of a mirror as shown in Figure 25.50. His eyes are \(1.65 \ m\) above the floor, and the top of his head is \(0.13 \ m\) higher. Find the height above the floor of the top and bottom of the smallest mirror in which he can see both the top of his head and his feet. How is this distance related to the man’s height? Figure 25.50 A full-length mirror is one in which you can see all of yourself. It need not be as big as you, and its size is independent of your distance from it. Equation Transcription: Text Transcription: 1.65 m 0.13 m

Problem 2CQ Diffusion by reflection from a rough surface is described in this chapter. Light can also be diffused by refraction. Describe how this occurs in a specific situation, such as light interacting with crushed ice.

Show that when light reflects from two mirrors that meet each other at a right angle, the outgoing ray is parallel to the incoming ray, as illustrated in the following figure. Figure 25.51 A corner reflector sends the reflected ray back in a direction parallel to the incident ray, independent of incoming direction.

Problem 3CQ Why is the index of refraction always greater than or equal to 1?

Problem 3PE Light shows staged with lasers use moving mirrors to swing beams and create colorful effects. Show that a light ray reflected from a mirror changes direction by 2? when the mirror is rotated by an angle ? .

Problem 5CQ Will light change direction toward or away from the perpendicular when it goes from air to water? Water to glass? Glass to air?

A flat mirror is neither converging nor diverging. To prove this, consider two rays originating from the same point and diverging at an angle \(\theta\). Show that after striking a plane mirror, the angle between their directions remains \(\theta\). Figure 25.52 A flat mirror neither converges nor diverges light rays. Two rays continue to diverge at the same angle after reflection. Equation Transcription: Text Transcription: theta theta

Problem 5PE What is the speed of light in water? In glycerine?

Problem 6PE What is the speed of light in air? In crown glass?

Problem 6CQ Explain why an object in water always appears to be at a depth shallower than it actually is? Why do people sometimes sustain neck and spinal injuries when diving into unfamiliar ponds or waters?

Problem 7CQ Explain why a person’s legs appear very short when wading in a pool. Justify your explanation with a ray diagram showing the path of rays from the feet to the eye of an observer who is out of the water.

Calculate the index of refraction for a medium in which the speed of light is \(2.012\times10^8\mathrm{\ m}/\mathrm{s}\), and identify the most likely substance based on Table 25.1. Equation Transcription: Text Transcription: 2.012 x 10^8 m/s

Why is the front surface of a thermometer curved as shown? Figure 25.47 The curved surface of the thermometer serves a purpose.

In what substance in Table 25.1 is the speed of light \(2.290\times10^8\mathrm{\ m}/\mathrm{s}\)? Equation Transcription: Text Transcription: 2.290 x 10^8 m/s

Problem 9PE There was a major collision of an asteroid with the Moon in medieval times. It was described by monks at Canterbury Cathedral in England as a red glow on and around the Moon. How long after the asteroid hit the Moon, which is 3.84×105 km away, would the light first arrive on Earth?

Problem 9CQ Suppose light were incident from air onto a material that had a negative index of refraction, say –1.3; where does the refracted light ray go?

Problem 10CQ A ring with a colorless gemstone is dropped into water. The gemstone becomes invisible when submerged. Can it be a diamond? Explain.

A scuba diver training in a pool looks at his instructor as shown in Figure 25.53. What angle does the ray from the instructor’s face make with the perpendicular to the water at the point where the ray enters? The angle between the ray in the water and the perpendicular to the water is \(25.0^{\circ}\). Figure 25.53 A scuba diver in a pool and his trainer look at each other. Equation Transcription: Text Transcription: 25.0 degree

Problem 11CQ A high-quality diamond may be quite clear and colorless, transmitting all visible wavelengths with little absorption. Explain how it can sparkle with flashes of brilliant color when illuminated by white light.

Problem 11PE Components of some computers communicate with each other through optical fibers having an index of refraction n = 1.55 . What time in nanoseconds is required for a signal to travel 0.200 m through such a fiber?

Problem 13CQ The most common type of mirage is an illusion that light from faraway objects is reflected by a pool of water that is not really there. Mirages are generally observed in deserts, when there is a hot layer of air near the ground. Given that the refractive index of air is lower for air at higher temperatures, explain how mirages can be formed.

(a) Using information in Figure 25.53, find the height of the instructor’s head above the water, noting that you will first have to calculate the angle of incidence. (b) Find the apparent depth of the diver’s head below water as seen by the instructor.

Is it possible that total internal reflection plays a role in rainbows? Explain in terms of indices of refraction and angles, perhaps referring to Figure 25.48. Some of us have seen the formation of a double rainbow. Is it physically possible to observe a triple rainbow? Figure 25.48 Double rainbows are not a very common observance. (credit: InvictusOU812, Flickr)

Problem 13PE Suppose you have an unknown clear substance immersed in water, and you wish to identify it by finding its index of refraction. You arrange to have a beam of light enter it at an angle of 45.0º , and you observe the angle of refraction to be 40.3º . What is the index of refraction of the substance and its likely identity?

Problem 14CQ It can be argued that a flat piece of glass, such as in a window, is like a lens with an infinite focal length. If so, where does it form an image? That is, how are di and do related?

Problem 14PE On the Moon’s surface, lunar astronauts placed a corner reflector, off which a laser beam is periodically reflected. The distance to the Moon is calculated from the round-trip time. What percent correction is needed to account for the delay in time due to the slowing of light in Earth’s atmosphere? Assume the distance to the Moon is precisely 3.84×108 m , and Earth’s atmosphere (which varies in density with altitude) is equivalent to a layer 30.0 km thick with a constant index of refraction n = 1.000293 .

Problem 15CQ You can often see a reflection when looking at a sheet of glass, particularly if it is darker on the other side. Explain why you can often see a double image in such circumstances.

Problem 16CQ When you focus a camera, you adjust the distance of the lens from the film. If the camera lens acts like a thin lens, why can it not be a fixed distance from the film for both near and distant objects?

Problem 17CQ A thin lens has two focal points, one on either side, at equal distances from its center, and should behave the same for light entering from either side. Look through your eyeglasses (or those of a friend) backward and forward and comment on whether they are thin lenses.

Problem 17PE Unreasonable Results Suppose light travels from water to another substance, with an angle of incidence of 10.0º and an angle of refraction of 14.9º . (a) What is the index of refraction of the other substance? (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?

Suppose Figure 25.54 represents a ray of light going from air through crown glass into water, such as going into a fish tank. Calculate the amount the ray is displaced by the glass \((\Delta x)\), given that the incident angle is \(40.0^{\circ}\) and the glass is \(1.00 \ cm\) thick. Equation Transcription: Text Transcription: (Delta x) 40.0 1.00 cm

Figure 25.54 shows a ray of light passing from one medium into a second and then a third. Show that \(\theta_{3}\) is the same as it would be if the second medium were not present (provided total internal reflection does not occur). Figure 25.54 A ray of light passes from one medium to a third by traveling through a second. The final direction is the same as if the second medium were not present, but the ray is displaced by \(\Delta x\) (shown exaggerated). Equation Transcription: Text Transcription: theta_3 Delta x

Problem 18CQ Will the focal length of a lens change when it is submerged in water? Explain.

Problem 18PE Construct Your Own Problem Consider sunlight entering the Earth’s atmosphere at sunrise and sunset—that is, at a 90º incident angle. Taking the boundary between nearly empty space and the atmosphere to be sudden, calculate the angle of refraction for sunlight. This lengthens the time the Sun appears to be above the horizon, both at sunrise and sunset. Now construct a problem in which you determine the angle of refraction for different models of the atmosphere, such as various layers of varying density. Your instructor may wish to guide you on the level of complexity to consider and on how the index of refraction varies with air density.

Problem 19CQ What are the differences between real and virtual images? How can you tell (by looking) whether an image formed by a single lens or mirror is real or virtual?

Problem 19PE Unreasonable Results Light traveling from water to a gemstone strikes the surface at an angle of 80.0º and has an angle of refraction of 15.2º . (a) What is the speed of light in the gemstone? (b) What is unreasonable about this result? (c) Which assumptions are unreasonable or inconsistent?

Problem 20CQ Can you see a virtual image? Can you photograph one? Can one be projected onto a screen with additional lenses or mirrors? Explain your responses.

Problem 20PE Verify that the critical angle for light going from water to air is 48.6º , as discussed at the end of Example 25.4, regarding the critical angle for light traveling in a polystyrene (a type of plastic) pipe surrounded by air. Example 25.4 How Big is the Critical Angle Here? What is the critical angle for light traveling in a polystyrene (a type of plastic) pipe surrounded by air?

Problem 21CQ Is it necessary to project a real image onto a screen for it to exist?

Problem 22CQ At what distance is an image always located—at do , di , or f ?

Problem 21PE (a) At the end of Example 25.4, it was stated that the critical angle for light going from diamond to air is 24.4º . Verify this. (b) What is the critical angle for light going from zircon to air? Example 25.4 How Big is the Critical Angle Here? What is the critical angle for light traveling in a polystyrene (a type of plastic) pipe surrounded by air?

Problem 23CQ Under what circumstances will an image be located at the focal point of a lens or mirror?

Problem 23PE At what minimum angle will you get total internal reflection of light traveling in water and reflected from ice?

Problem 22PE An optical fiber uses flint glass clad with crown glass. What is the critical angle?

Problem 26CQ Figure 25.49 shows a light bulb between two mirrors. One mirror produces a beam of light with parallel rays; the other keeps light from escaping without being put into the beam. Where is the filament of the light in relation to the focal point or radius of curvature of each mirror?

Problem 24PE Suppose you are using total internal reflection to make an efficient corner reflector. If there is air outside and the incident angle is 45.0º , what must be the minimum index of refraction of the material from which the reflector is made?

You can determine the index of refraction of a substance by determining its critical angle. (a) What is the index of refraction of a substance that has a critical angle of \(68.4^{\circ}\) when submerged in water? What is the substance, based on Table 25.1? (b) What would the critical angle be for this substance in air? Equation Transcription: Text Transcription: 68.4 degree

Problem 25CQ Can a case 1 image be larger than the object even though its magnification is always negative? Explain.

A ray of light, emitted beneath the surface of an unknown liquid with air above it, undergoes total internal reflection as shown in Figure 25.55. What is the index of refraction for the liquid and its likely identification? Figure 25.55 A light ray inside a liquid strikes the surface at the critical angle and undergoes total internal reflection.

Problem 24CQ What is meant by a negative magnification? What is meant by a magnification that is less than 1 in magnitude?

Problem 27CQ Devise an arrangement of mirrors allowing you to see the back of your head. What is the minimum number of mirrors needed for this task?

A light ray entering an optical fiber surrounded by air is first refracted and then reflected as shown in Figure 25.56. Show that if the fiber is made from crown glass, any incident ray will be totally internally reflected. Figure 25.56 A light ray enters the end of a fiber, the surface of which is perpendicular to its sides. Examine the conditions under which it may be totally internally reflected.

(a) What is the ratio of the speed of red light to violet light in diamond, based on Table 25.2? (b) What is this ratio in polystyrene? (c) Which is more dispersive?

Problem 29CQ It can be argued that a flat mirror has an infinite focal length. If so, where does it form an image? That is, how are di and do related?

Problem 28CQ If you wish to see your entire body in a flat mirror (from head to toe), how tall should the mirror be? Does its size depend upon your distance away from the mirror? Provide a sketch.

Problem 29PE A beam of white light goes from air into water at an incident angle of 75.0º . At what angles are the red (660 nm) and violet (410 nm) parts of the light refracted?

Problem 30CQ Why are diverging mirrors often used for rear-view mirrors in vehicles? What is the main disadvantage of using such a mirror compared with a flat one?

Problem 30PE By how much do the critical angles for red (660 nm) and violet (410 nm) light differ in a diamond surrounded by air?

Problem 31PE (a) A narrow beam of light containing yellow (580 nm) and green (550 nm) wavelengths goes from polystyrene to air, striking the surface at a 30.0º incident angle. What is the angle between the colors when they emerge? (b) How far would they have to travel to be separated by 1.00 mm?

Problem 33PE A ray of 610 nm light goes from air into fused quartz at an incident angle of 55.0º . At what incident angle must 470 nm light enter flint glass to have the same angle of refraction?

Problem 32PE A parallel beam of light containing orange (610 nm) and violet (410 nm) wavelengths goes from fused quartz to water, striking the surface between them at a 60.0º incident angle. What is the angle between the two colors in water?

Problem 34PE A narrow beam of light containing red (660 nm) and blue (470 nm) wavelengths travels from air through a 1.00 cm thick flat piece of crown glass and back to air again. The beam strikes at a 30.0º incident angle. (a) At what angles do the two colors emerge? (b) By what distance are the red and blue separated when they emerge?

Problem 36PE What is the power in diopters of a camera lens that has a 50.0 mm focal length?

A narrow beam of white light enters a prism made of crown glass at a \(45.0^{\circ}\) incident angle, as shown in Figure 25.57. At what angles, \(\theta_{\mathrm{R}}\) and \(\theta_{\mathrm{V}}\) , do the red \((660 \ nm)\) and violet \((410 \ nm)\) components of the light emerge from the prism? Figure 25.57 This prism will disperse the white light into a rainbow of colors. The incident angle is \(45.0^{\circ}\) , and the angles at which the red and violet light emerge are \(\theta_{\mathrm{R}}\) and \(\theta_{\mathrm{V}}\) . Equation Transcription: Text Transcription: 45.0 degree theta_R theta_V (660 nm) (410 nm) 45.0 degree theta_R theta_V

Problem 39PE You note that your prescription for new eyeglasses is –4.50 D. What will their focal length be?

Problem 37PE Your camera’s zoom lens has an adjustable focal length ranging from 80.0 to 200 mm. What is its range of powers?

Problem 38PE What is the focal length of 1.75 D reading glasses found on the rack in a pharmacy?

Problem 40PE How far from the lens must the film in a camera be, if the lens has a 35.0 mm focal length and is being used to photograph a flower 75.0 cm away? Explicitly show how you follow the steps in the Problem-Solving Strategy for lenses.

Problem 41PE A certain slide projector has a 100 mm focal length lens. (a) How far away is the screen, if a slide is placed 103 mm from the lens and produces a sharp image? (b) If the slide is 24.0 by 36.0 mm, what are the dimensions of the image? Explicitly show how you follow the steps in the ProblemSolving Strategy for lenses.

Problem 42PE A doctor examines a mole with a 15.0 cm focal length magnifying glass held 13.5 cm from the mole (a) Where is the image? (b) What is its magnification? (c) How big is the image of a 5.00 mm diameter mole?

Problem 44PE A camera with a 50.0 mm focal length lens is being used to photograph a person standing 3.00 m away. (a) How far from the lens must the film be? (b) If the film is 36.0 mm high, what fraction of a 1.75 m tall person will fit on it? (c) Discuss how reasonable this seems, based on your experience in taking or posing for photographs.

Problem 43PE How far from a piece of paper must you hold your father’s 2.25 D reading glasses to try to burn a hole in the paper with sunlight?

Problem 45PE A camera lens used for taking close-up photographs has a focal length of 22.0 mm. The farthest it can be placed from the film is 33.0 mm. (a) What is the closest object that can be photographed? (b) What is the magnification of this closest object?

Problem 46PE Suppose your 50.0 mm focal length camera lens is 51.0 mm away from the film in the camera. (a) How far away is an object that is in focus? (b) What is the height of the object if its image is 2.00 cm high?

Problem 47PE (a) What is the focal length of a magnifying glass that produces a magnification of 3.00 when held 5.00 cm from an object, such as a rare coin? (b) Calculate the power of the magnifier in diopters. (c) Discuss how this power compares to those for store-bought reading glasses (typically 1.0 to 4.0 D). Is the magnifier’s power greater, and should it be?

Problem 48PE What magnification will be produced by a lens of power –4.00 D (such as might be used to correct myopia) if an object is held 25.0 cm away?

Problem 50PE Suppose a 200 mm focal length telephoto lens is being used to photograph mountains 10.0 km away. (a) Where is the image? (b) What is the height of the image of a 1000 m high cliff on one of the mountains?

Problem 51PE A camera with a 100 mm focal length lens is used to photograph the sun and moon. What is the height of the image of the sun on the film, given the sun is 1.40×106 km in diameter and is 1.50×108 km away?

In Example 25.7, the magnification of a book held \(7.50 \ cm\) from a \(10.0 \ cm\) focal length lens was found to be \(3.00\). (a) Find the magnification for the book when it is held \(8.50 \ cm\) from the magnifier. (b) Do the same for when it is held \(9.50 \ cm\) from the magnifier. (c) Comment on the trend in \(m\) as the object distance increases as in these two calculations. Equation Transcription: Text Transcription: 7.50 cm 10.0 cm 3.00 8.50 cm 9.50 cm m

Problem 53PE What is the focal length of a makeup mirror that has a power of 1.50 D?

Problem 54PE Some telephoto cameras use a mirror rather than a lens. What radius of curvature mirror is needed to replace a 800 mm focal length telephoto lens?

Problem 52PE Combine thin lens equations to show that the magnification for a thin lens is determined by its focal length and the object distance and is given by m = f /( f – do) .

Problem 55PE (a) Calculate the focal length of the mirror formed by the shiny back of a spoon that has a 3.00 cm radius of curvature. (b) What is its power in diopters?

Problem 56PE Find the magnification of the heater element in Example 25.9. Note that its large magnitude helps spread out the reflected energy. Example 25.9: A Concave Reflector Electric room heaters use a concave mirror to reflect infrared (IR) radiation from hot coils. Note that IR follows the same law of reflection as visible light. Given that the mirror has a radius of curvature of 50.0 cm and produces an image of the coils 3.00 m away from the mirror, where are the coils?

Problem 59PE An object 1.50 cm high is held 3.00 cm from a person’s cornea, and its reflected image is measured to be 0.167 cm high. (a) What is the magnification? (b) Where is the image? (c) Find the radius of curvature of the convex mirror formed by the cornea. (Note that this technique is used by optometrists to measure the curvature of the cornea for contact lens fitting. The instrument used is called a keratometer, or curve measurer.)

Problem 57PE What is the focal length of a makeup mirror that produces a magnification of 1.50 when a person’s face is 12.0 cm away? Explicitly show how you follow the steps in the Problem-Solving Strategy for Mirrors.

Problem 58PE A shopper standing 3.00 m from a convex security mirror sees his image with a magnification of 0.250. (a) Where is his image? (b) What is the focal length of the mirror? (c) What is its radius of curvature? Explicitly show how you follow the steps in the Problem-Solving Strategy for Mirrors.

Problem 60PE Ray tracing for a flat mirror shows that the image is located a distance behind the mirror equal to the distance of the object from the mirror. This is stated di = –do , since this is a negative image distance (it is a virtual image). (a) What is the focal length of a flat mirror? (b) What is its power?

Problem 61PE Show that for a flat mirror hi = ho , knowing that the image is a distance behind the mirror equal in magnitude to the distance of the object from the mirror.

Problem 62PE Use the law of reflection to prove that the focal length of a mirror is half its radius of curvature. That is, prove that f = R / 2 . Note this is true for a spherical mirror only if its diameter is small compared with its radius of curvature.

Problem 63PE Referring to the electric room heater considered in the first example in this section, calculate the intensity of IR radiation in W/m2 projected by the concave mirror on a person 3.00 m away. Assume that the heating element radiates 1500 W and has an area of 100 cm2 , and that half of the radiated power is reflected and focused by the mirror.

Problem 4CQ Does the fact that the light flash from lightning reaches you before its sound prove that the speed of light is extremely large or simply that it is greater than the speed of sound? Discuss how you could use this effect to get an estimate of the speed of light.