Chapter 11.pdf

A ray of light is electromagnetic radiation, whichis transmitted in waves. The particles in a mediumslow down the passage of the waves, which results inlight travelling more slowly through a block of glass,for example, than it travels through a vacuum.The effects of changes in the speed of light can beseen in Figure 11.32. The light ray strikes thePlexiglas at an angle. As the light enters the Plexiglas,it slows down and refracts (a). When the light leavesthe Plexiglas and enters the air, it speeds up andrefracts again (b). Notice that light does not refractinside the block. Light refracts only at the boundarywhen it is entering or leaving a medium. Thisphotograph also shows that refraction is a reversibleprocess, in that the angle of refraction entering the block is exactlyreversed as the light leaves the block. The light ray immediately speedsup again as it leaves the block.aFigure 11.32 Light refracts as it enters and then leaves this blockof Plexiglas.bThe Index of RefractionThe amount by which a transparent medium decreases the speedof light is indicated by a number called the index of refraction,also called the refractive index. The larger the refractive index,the more the medium decreases the speed of light.Light travels fastest in a vacuum. The refractive index of thespeed of light in a vacuum is assigned a value of 1.00. A value of1.00 can also be used for air, since the fourth decimal place doesnot affect calculations based on Table 11.5. Since water, glass,diamond, and other media all slow down light, they have highervalues than air.The refractive index of a medium, n, is determined bycomparing the speed of light in the medium, v, with the speed oflight in a vacuum, c. This leads to the following definition:speed of light in vacuumindex of refraction of material =speed of light in mediumcor n =vSince units cancel, a refractive index value does not haveany units.Table 11.5 Index of Refraction forSelected MediaMediavacuumIndex of Refraction1.00 (exactly)air 1.0003carbon dioxide gas 1.0005water 1.33alcohol 1.36Pyrex glass 1.47Plexiglas 1.49table salt 1.51flint glass 1.61sapphire 1.77cubic zirconia 2.16diamond 2.42gallium phosphide 3.50Ray diagrams model the behaviour of light in mirrors and lenses.437