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Ch. 32 - Light: Reflection and Refraction
Giancoli Douglas - Physics for Scientists and Engineers 5th edition
Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Not the one you use?Change textbook
Chapter 31, Problem 72

A 1.80-m-tall person stands 4.20 m from a convex mirror and notices that he looks precisely half as tall as he does in a plane mirror placed at the same distance. What is the radius of curvature of the convex mirror? (Assume that θ ≈ θ .) [Hint: The viewing angle is half.]

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Identify the given values: the height of the person (h = 1.80 m), the distance from the mirror to the person (d_o = 4.20 m), and the fact that the image height in the convex mirror is half the actual height (h_i = 0.90 m).
Use the mirror equation to relate the object distance (d_o), the image distance (d_i), and the focal length (f) of the mirror: \(\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}\).
Apply the magnification equation which relates the height of the image (h_i), the height of the object (h), and the distances (d_i and d_o): magnification (m) = \(\frac{h_i}{h} = -\frac{d_i}{d_o}\). Since the image height is half the object height, m = -0.5.
Solve the magnification equation for the image distance (d_i) using the magnification value and the object distance.
Substitute the value of d_i back into the mirror equation to solve for the focal length (f), and then use the relationship between the focal length and the radius of curvature (R) of the mirror, which is R = 2f, to find the radius of curvature of the convex mirror.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Convex Mirror Properties

A convex mirror is a curved mirror that bulges outward, causing light rays to diverge. This results in a virtual image that is smaller than the object and appears behind the mirror. The image formed by a convex mirror is always upright and diminished, which is crucial for understanding how the height of the image relates to the actual height of the person.
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Magnification

Magnification is the ratio of the height of the image to the height of the object. In this scenario, the person appears half as tall in the convex mirror compared to a plane mirror, indicating a magnification factor of 0.5. Understanding magnification helps in determining how the image size changes with respect to the object size, which is essential for solving the problem.
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Mirror Equation

Radius of Curvature

The radius of curvature is the distance from the mirror's surface to its center of curvature, which is the point where the mirror's surface would meet if extended. For mirrors, the relationship between the radius of curvature (R) and the focal length (f) is given by the equation f = R/2. This concept is vital for calculating the radius of curvature of the convex mirror based on the image characteristics provided in the question.
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Related Practice
Textbook Question

Two plane mirrors are facing each other 2.2 m apart as in Fig. 32–60. You stand 1.5 m away from one of these mirrors and look into it. You will see multiple images of yourself. (a) How far away from you are the first three images of yourself in the mirror in front of you? (b) Are these first three images facing toward you or away from you?


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Textbook Question

A triangular prism made of crown glass (n = 1.52) with base angles of 26.0° is surrounded by air. If parallel rays are incident normally on its base as shown in Fig. 32–66, what is the angle Φ between the two emerging rays?

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Textbook Question

We wish to determine the depth of a swimming pool filled with water by measuring the width (x = 5.20m) and then noting that the bottom edge of the pool is just visible at an angle of 13.0° above the horizontal as shown in Fig. 32–61. Calculate the depth of the pool.


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Textbook Question

The critical angle of a certain piece of plastic in air is θC = 35.8°. What is the critical angle of the same plastic if it is immersed in water?

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Textbook Question

The label on a laser says it produces light of wavelength 670 nm. The laser beam passes through a block of plastic for which n = 1.57. What is the wavelength of the light inside the plastic?

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Textbook Question

When light passes through a prism, the angle that the refracted ray makes relative to the incident ray is called the deviation angle δ, Fig. 32–64. Show that this angle is a minimum when the ray passes through the prism symmetrically, perpendicular to the bisector of the apex angle Φ, and show that the minimum deviation angle, δm, is related to the prism’s index of refraction n by


n=sin12(ϕ+δm)sinϕ/2.n = \(\frac{\sin \frac{1}{2}\)(\(\phi\) + \(\delta\)_m)}{\(\sin\) \(\phi\)/2}.


[Hint: For θ in radians, (d/dθ)(sin1θ)=1/1θ2(d/d\(\theta\)) (\(\sin\)^{-1}\(\theta\)) = 1/\(\sqrt{1 - \theta^2}\).]

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