Conceptual Physics

32.10 - Gotchas

You calculated a material as having a refractive index of 0.76. Oops! This is not possible. The speed of light in a vacuum is greater than its

speed in any medium, meaning that the refractive index of the material must be greater than 1.00. Remember that when calculating the index

of refraction, you put the speed of light in a vacuum in the numerator, not the denominator.

Light strikes water in a placid lake at an angle of 30° from the horizontal, so its incident angle is 30°. No, the incident angle is 60°. It is

measured between the incident ray and the interface’s normal line. This line is perpendicular to the interface.

All wavelengths (colors) of light refract at the same angle when crossing the interface between different media. No, the angle of refraction

varies by wavelength.

32.11 - Summary

Refraction is the changing of a wave’s direction due to a change in its speed. It

occurs when a wave, like light, passes from one medium into another.

A material’s index of refraction is the ratio of the speed of light in a vacuum (c) to its

speed in the material. The index of refraction is represented by the letter n.

Snell’s law quantifies how much light bends when it crosses an interface. Light

bends towards a line normal to the interface between the media as it slows down

(passes into a medium with a higher index of refraction). When light passes through

multiple media its path can be calculated by applying Snell’s law at each interface.

Total internal reflection occurs when no light is refracted at an interface; it is all

reflected. This is only possible as light encounters an interface with a medium

having a smaller index of refraction, at a large enough incident angle. The critical

angle is the minimum angle of incidence at which total internal reflection occurs.

The index of refraction for a material varies somewhat by the wavelength of light

being refracted. Light composed of many wavelengths, such as white light, can be

separated into a spectrum by refraction. This effect is called dispersion. Prisms

exhibit dispersion.

Index of refraction

n = c/v

Snell’s law

ni sin și = nr sin șr

Critical angle

(^604) Copyright 2007 Kinetic Books Co. Chapter 32