Page 78 - Engineering Plastics Handbook

52 Introduction

When the radius of curvature of a convex surface such as a light pipe
or lens is too small, the critical angle is compromised.
Internal reflection at the interface with air or water keeps the light
rays within the light pipe. However, just as power loss is important to
determine power cable design, minimal light ray loss during light trans-
mission through a light pipe is important for efficient light pipe design.
Refractive index n is the ratio of the velocity of light in a vacuum to
the velocity of light in a medium, as shown in Table 3.7. It is the degree
to which a transparent material bends light.
The higher the refractive index of a given medium, the slower the light
rays travel through the medium. Slower light rays travel closer to the
normal line in Fig. 3.2, and the angle of refraction is smaller.
The ratio of the refractive index of two clear materials such as plastics
and air at their interface is related to the angle of refraction (bending)
according to Snell’s law. Snell’s law equates the sine of the angle of inci-
dence to the sine of the angle of refraction [9]. “Incidence angle is the
angle formed by a ray arriving at a surface and the perpendicular to that
surface at the point of arrival” [10].
When light travels through a clear plastic to the interface with air, it
refracts unless the angle of incidence is greater than the critical angle.
At the critical angle the refractive angle is 90° from the line normal to
the interface surface. See Fig. 3.2.
1. The normal line is a perpendicular line (y axis) to the interface sur-
face (boundary) at the point where the incident light rays pass
through the interface surface. It is the reference line for the angle of
incidence and angle of refraction.
2. Light rays refract toward the normal line when the speed of the light
rays is slower than that of the incident light rays. “Slower is closer.”

TABLE 3.7 Refractive Index Ratios

Medium Refractive index
Acrylic 1.49–1.51
Polystyrene 1.59
Polycarbonate 1.58–1.60
Polysulfone 1.63
Flint glass 1.57–1.75
Crown glass 1.50–1.62
o
Water (20 C) 1.333
Air 1.0003
Vacuum 1.00000