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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 55

The critical angle for a certain liquid–air surface is 52.6°. What is the index of refraction of the liquid?

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1
Identify the relationship between the critical angle and the index of refraction. The critical angle \( \theta_c \) is related to the indices of refraction of the two media by the equation: \( \sin(\theta_c) = \frac{n_2}{n_1} \), where \( n_1 \) is the index of refraction of the liquid and \( n_2 \) is the index of refraction of air (approximately 1.00).
Rearrange the equation to solve for \( n_1 \), the index of refraction of the liquid: \( n_1 = \frac{n_2}{\sin(\theta_c)} \).
Substitute the known values into the equation. Use \( n_2 = 1.00 \) (index of refraction of air) and \( \theta_c = 52.6° \).
Convert the critical angle from degrees to radians if necessary, depending on the calculator or software being used. Ensure that the sine function is applied correctly.
Calculate \( \sin(52.6°) \) and divide \( n_2 \) by this value to find \( n_1 \), the index of refraction of the liquid.

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Key Concepts

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

Critical Angle

The critical angle is the angle of incidence above which total internal reflection occurs when light travels from a denser medium to a less dense medium. It is defined as the angle at which the refracted light would travel along the boundary between the two media. If the angle of incidence exceeds this critical angle, no light is refracted, and all of it is reflected back into the denser medium.
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Snell's Law

Snell's Law describes the relationship between the angles of incidence and refraction when light passes between two different media. It is mathematically expressed as n1 * sin(θ1) = n2 * sin(θ2), where n1 and n2 are the indices of refraction of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively. This law is essential for calculating the index of refraction when given the critical angle.
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Index of Refraction

The index of refraction (n) is a dimensionless number that describes how much light slows down when it enters a medium compared to its speed in a vacuum. It is defined as n = c/v, where c is the speed of light in a vacuum and v is the speed of light in the medium. The index of refraction is crucial for understanding how light behaves at the interface of different materials, particularly in determining the critical angle.
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Related Practice
Textbook Question

(III) A light ray is incident on a flat piece of glass with index of refraction n as in Fig. 32–24. Show that if the incident angle θ is small, the emerging ray is displaced a distance d = tθ(n - 1)/n , where t is the thickness of the glass, θ is in radians, and d is the perpendicular distance between the incident ray and the (dashed) line of the emerging ray (Fig. 32–24).

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Textbook Question

A parallel beam of light containing two wavelengths, λ₁ = 461 nm and λ₂ = 656 nm, enters the silicate flint glass of an equilateral prism as shown in Fig. 32–56. At what angle does each beam leave the prism (give angle with normal to the face)? See Fig. 32–28.

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Textbook Question

(II) A ray of light, after entering a light fiber, reflects at an angle of 14.5° with the long axis of the fiber, as in Fig. 32–57. Calculate the distance along the axis of the fiber that the light ray travels between successive reflections off the sides of the fiber. Assume that the fiber has an index of refraction of 1.55 and is 1.60 x 10-4 m in diameter.

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Textbook Question

(III) A beam of light enters the end of an optic fiber as shown in Fig. 32–59. (a) Show that we can guarantee total internal reflection at the side surface of the material (at point A), if the index of refraction is greater than about 1.42. In other words, regardless of the angle α , the light beam reflects back into the material at point A, assuming air outside. (b) What if the fiber is immersed in water?

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Textbook Question

A light beam strikes a piece of glass at a 55.00° incident angle. The beam contains two wavelengths, 450.0 nm and 700.0 nm, for which the index of refraction of the glass is 1.4831 and 1.4754, respectively. What is the angle between the two refracted beams?

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Textbook Question

A beam of light is emitted 7.7 cm beneath the surface of a liquid and strikes the surface 7.2 cm from the point directly above the source. If total internal reflection occurs, what can you say about the index of refraction of the liquid?

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