The place you get your hair cut has two nearly parallel mirrors 5.0 m apart. As you sit in the chair, your head is 2.0 m from the nearer mirror. Looking toward this mirror, you first see your face and then, farther away, the back of your head. (The mirrors need to be slightly nonparallel for you to be able to see the back of your head, but you can treat them as parallel in this problem.) How far away does the back of your head appear to be? Neglect the thickness of your head.
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33. Geometric Optics
Mirror Equation
Problem 74
Textbook Question
Suppose the mirrors in a Michelson interferometer are perfectly aligned and the path lengths to mirrors M₁ and M₂ are identical. With these initial conditions, an observer sees a bright maximum at the center of the viewing area. Now one of the mirrors is moved a distance x. Determine a formula for the intensity at the center of the viewing area as a function of x, the distance the movable mirror is moved from the initial position.

1
Understand the basic operation of a Michelson interferometer: It splits a beam of light into two paths, reflects them back with mirrors, and then recombines them. The interference pattern observed depends on the path difference between the two beams.
Recognize that the initial condition of a bright maximum at the center indicates constructive interference, meaning the path difference between the two beams is an integer multiple of the wavelength, starting with zero (i.e., the path lengths are initially equal).
Identify that moving one mirror a distance x changes the path length for that beam by 2x (since the light travels to the mirror and back). This changes the path difference between the two beams.
Recall the condition for constructive interference (bright fringes) is when the path difference is an integer multiple of the wavelength, \( n\lambda \), where \( n \) is an integer. The new path difference is \( 2x \).
Use the formula for intensity in terms of path difference: \( I(x) = I_0 \cos^2(\frac{\pi \cdot 2x}{\lambda}) \), where \( I_0 \) is the maximum intensity, and \( \lambda \) is the wavelength of the light used. This formula accounts for the variation in intensity due to the change in path difference caused by moving the mirror.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Interference of Light
Interference occurs when two or more light waves overlap, resulting in a new wave pattern. In the context of the Michelson interferometer, constructive interference leads to bright fringes when the path difference between the two beams is an integer multiple of the wavelength, while destructive interference results in dark fringes. Understanding this principle is crucial for analyzing how moving one mirror affects the observed intensity.
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Path Length Difference
The path length difference is the difference in distance traveled by two light beams before they recombine. In a Michelson interferometer, if one mirror is moved, the path length for that beam changes, affecting the interference pattern. The intensity at the center of the viewing area can be expressed as a function of this path length difference, which is directly related to the distance x that the mirror is moved.
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Intensity Formula
The intensity of light resulting from interference can be described by the formula I = I₀(1 + cos(Δφ)), where I₀ is the maximum intensity and Δφ is the phase difference between the two beams. The phase difference is related to the path length difference and the wavelength of the light used. By determining how the phase changes as the mirror is moved, one can derive the intensity at the center of the viewing area as a function of the distance x.
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