![]() ![]() The luminous intensity distribution on the right is from a glass lens with a typical refractive index of 1.5.Displayed on the left, a lens with a refractive index of 1.6.It could be made from a higher index of refraction glass or plastic, such as polycarbonate. The luminous intensity distribution of light traveling through a lens depends not only on lens shape but also on refractive index. ![]() This can be seen in the example below, where light travels through two identically shaped plano convex lenses with different refractive indices. Even a small change in the refractive index can affect the candela distribution of the transmitted light. When designing a lens that transmits light, it is necessary to consider the material’s refractive index. For normal incidence (Θ i = 0°), the amount of light reflected is found byįor most glasses with a refractive index of 1.5, reflection losses at the surface result in an approximate 4% decrease in light intensity. The reflection of light at the surface occurs due to an instantaneous change in refractive index between glass and its surrounding medium. Larger indices of refraction in glass result in greater differences between the angle of incidence and transmission of light. The angle of transmission can be calculated using Snell’s law: The index of refraction of the glass determines not only how much light is reflected and transmitted, but also its refracted angle in the glass. When a beam of light hits a glass surface, part of the beam is reflected and part is transmitted. For normal incidence, approximately 4 % of the light is reflected this value is determined by the refractive index of the glass. Light that is incident on a glass surface will be reflected at an angle equal to the angle of incidence and transmitted according to Snell’s law. The refractive index ( n) of a material is defined as the ratio of the speed of light in a vacuum to that of light in the material. This is why light travels fast in glass, faster in water, and fastest in a vacuum. Typically, higher electron densities in a material result in lower velocities. You’re probably familiar with the concept of “traveling at the speed of light”, but did you know that the speed of light can change? Light’s speed is reduced when it travels through a medium due to the interaction of photons with electrons. In this article, we review refractive index, transmission, absorption, and wavelength dependency and discuss how these properties impact product design. Still, familiarity with a few fundamental optical properties will help engineers pick the right material for their application. Today, most engineers use advanced software tools to simulate the properties of a material and their impact on optical performance. The optical properties of a material determine how it will interact with light. They ask “Can I use my existing lens design with the new glass material? Will the resulting light output have the same chromaticity, distribution, and intensity?” The answers to these questions are rooted in understanding the optical properties of materials. For example, they may be switching from an existing polycarbonate lens design to glass due to concerns about durability in harsh environments. ![]() We often hear from engineers who are evaluating the impact of a design change from one lens material to another. We will define common glass properties and explain their application and importance in component design. This is the second article in a three-part series that reviews the thermal, optical, and mechanical properties of glass. ![]()
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