The refractive index, also known as index of refraction or refraction index, is a number which describes how fast light travels through the material.

In this article we will take a closer look at refractive indices, refraction, total internal reflection and the refractive index of plastics. We shall also take a closer look at using polymer optics instead of glass and why this might be a better fit for your project as well.

About refractive index

The refractive index determines how much the path of light is refracted or bent, when entering a specific material. This effect is described by Snell’s law of refraction. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection (more about this in a bit), their intensity (Fresnel’s equations) and Brewster’s angle.

Refraction and dispersion

Refractive index varies with wavelength. This is what causes white light to split into colors when refracted (an effect known as dispersion). This effect can be seen in prisms and rainbows, or as chromatic aberration in lenses. For most materials the refractive index changes with wavelength by a couple percent across the visible spectrum. Refractive indices are commonly reported using a single value for n, namely measured at 633 nm.

Check our datasheet for our most up-to-date capabilities and material properties.

From X-rays to radio waves

Refractive index applies within the electromagnetic spectrum, ranging from X-rays to radio waves. It can also be applied to other phenomena, such as sound. In this case the speed of sound is used instead of light, and instead of a vacuum a reference medium is used.

Refractive index and related quantities

Refraction

As soon as light moves from one medium to another it changes direction. In other words, it is refracted. If it moves from a medium with refractive index n1 tot one with refractive index n2, with an angle of incidence θ1, the refraction angle θ2 can be calculated using Snell’s law.

However, when light moves to a material with a higher refractive index, the angle of refraction will be smaller than the angle of incidence. As such the light will be refracted towards the normal of the surface. A higher refractive index will result in light moving closer to the normal direction. When light moves to a medium with a lower RI, the light will refract away from the normal and instead move towards the surface.

Total internal reflection (TIR)

When light cannot be transmitted it will undergo total internal reflection. This happens when light passes through a less optically dense material; material with a lower refractive index. In order for total internal reflection to occur, the angles of incidence θ1 must be larger than the critical angle.

Refractive index of plastic

Now onto the refractive index of plastics. Most plastics have a refractive index in the range of 1.3 to 1.6. However, there are also some high-refractive-index polymers which can range up to 1.76.

Our material

At Addoptics we produce custom polymer optics within 10 days, using a smart combination of 3D printing and vacuum casting. This enables you to have your optics in a cost-effective and timely fashion. Our optics manufacturing service can be used for both prototyping as well as small series production. Our process uses a proprietary material with a refractive index of 1,544 @ 650Nm.

Check our datasheet for our most up-to-date capabilities and material properties.

We only work with the industry-standard optical plastic or polymer, offering high-quality optical products. The material we use is polyurethane. Polyurethane can be cast in all possible shapes, ranging from complex products, such as three-dimensional products, and hollow products.

Why polymer optics?

Plastic optics have various advantages over glass. For instance, the costs are lower, they have a higher impact resistance, weigh less and there are more configuration possibilities for simplifying systems assembly. Light transmittance from polymer optics is similar to high-grade glass. Polymer optics that break do not splinter like glass, meaning they are less hazardous.

Polymer optics do have some disadvantages as well; they have a lower resistance to scratching and intolerance to severe temperature fluctuation. However, these disadvantages are far outweighed by the advantages of polymer optics.

Ready to start prototyping?

Are you in need of custom optics for your projects? Feel free to reach out to us, we’d love to discuss the possibilities of custom optics prototyping and manufacturing.

 

Sources