What does a smaller refractive index mean?

The refractive index (also known as the index of refraction) is defined as the quotient of the speed of light as it passes through two media. It is a dimensionless number that depends on the temperature and wavelength of the beam of light.

“Refractive index describes how fast a light beam travels through media.”

In this article we will take a closer look at what refractive index is, what total internal reflection has to do with it and the index of refraction of our custom polymer optics.

Refractive index explained

The refractive index of a material, expressed in a number, shows how much a path of light is bent or refracted when it enters that material. The index of refraction also determines the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection.

Refractive index formula

The index of refraction formula is as follows: n = c / v

• n is the index of refraction
• c is the speed of light in a vacuum (or air)
• v is the speed of light in the media (e.g. water, olive oil, etc.)

Snell’s Law

Snell’s Law relates the refraction and incidence angles to the refraction indices of the involved media. Snell’s Law determines that the product of the sine of the angle formed between the ray of light, the normal straight line and the refractive index of the media must be constant.

Refractive index varies per wavelength

The refractive indices of a material vary per wavelength. For many materials the refractive index changes for each wavelength by a couple of percentages. Refractive indices are most often reported using a single value of n, namely measure at 633 nm.

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

Total internal reflection

In the case that light cannot be transmitted it will undergo total internal reflection. This happens when light passes from a less optically dense material, e.g. a material with a lower Refractive Index. Total internal reflection can only occur when the angles of incidence are larger than the critical angle.

Refraction

Light changes direction when it moves from one medium into another, as a result the light is refracted. This effect can be observed when watching white light split into colors when refracted. The same effect occurs in prims and rainbows. When light moves to a material with a higher refractive index the angle of refraction will be smaller than the angle of incidence. This results in the light being refracted towards the normal of the surface. When light moves through a medium with a lower Refractive Index the light will instead refract away from the normal and thus move towards the surface.

Refractive indices of glass and plastics

Glass

The refractive index of glass varies per composition and wavelength. Ordinary crown glass, when illuminated by white light, has an index of refraction of 1.52, whereas medium flint glass has an RI of 1.63 and acrylic has an 1.49 index of refraction.

Plastic

Common clear plastics have a refractive index in the range of 1.3 to 1.6. Nonetheless there are also some high refractive index polymers with an index of refraction up to 1.76.

Custom optics and Addoptics

At Addoptics we produce custom optics within a matter of days. We use our own prototyping and manufacturing technology for optics prototyping and series production. With our unique manufacturing service you can receive your optics both cost-effective and fast.

Our process uses a proprietary material with a refractive index of 1,544 @ 650Nm. Check out our datasheet for our most up-to-date material properties and current capabilities.

Why use custom polymer optics?

There are various advantages that plastic optics have over glass optics. For starters, the production costs are lower. Plastic optics have a higher impact resistance, they do not splinter like glass. Polymer optics weigh less. The light transmittance itself is similar to that of high-grade glass.

Of course there are some downsides as well; polymer optics are more prone to damage caused by scratches or dents and polymer optics tend to have an intolerance for severe temperate fluctuation. However, in our opinion these downsides are outweighed by the benefits of using polymer optics.

In need of custom plastic optics?

Are your ready to scale your prototyping and production with high-quality custom optics? Choose Addoptics as your optics supplier and we will deliver high-grade, quality optics in the shortest lead times possible. We offer affordable, competitively priced custom optics with a significant discount when ordering multiples.

Feel free to reach out to us to discuss the possibilities of custom optics prototyping and manufacturing.

Sources

Refractive Index (Index of Refraction) is a value calculated from the ratio of the speed of light in a vacuum to that in a second medium of greater density. The refractive index variable is most commonly symbolized by the letter n or n' in descriptive text and mathematical equations.

As presented in the figure above, a wavefront incident upon a plane surface separating two media is refracted upon entering the second medium if the incident wave is oblique to the surface. The incident angle (θ(1)) is related to the refraction angle (θ(2)) by the simple relationship known as Snell's law:

n1 × sin(θ1) = n2 × sin(θ2)

Where n represents the refractive indices of material 1 and material 2 and θ are the angles of light traveling through these materials with respect to the normal. There are several important points that can be drawn from this equation. When n(1) is greater than n(2), the angle of refraction is always larger than the angle of incidence. Alternatively when n(2) is greater than n(1) the angle of refraction is always smaller than the angle of incidence. When the two refractive indices are equal (n(1) = n(2)), then the light is passed through without refraction.

In optical microscopy, refractive index is an important variable in calculating numerical aperture, which is a measure of the light-gathering and resolving power of an objective. In most instances, the imaging medium for microscopy is air, but high-magnification objectives often employ oil or a similar liquid between the objective front lens and the specimen to improve resolution. The numerical aperture equation is given by:

NA (numerical aperture) = n × sin(θ)

where n is the refractive index of the imaging medium and θ is the angular aperture of the objective. It is obvious from the equation that increasing the refractive index by replacing the imaging medium from air (refractive index = 1.000) with a low-dispersion oil (refractive index = 1.515) dramatically increases the numerical aperture.

Snell's law was originally defined by the relationship between the incident angles and the ratio of the velocities of light in the two media. The refractive index or index of refraction is the ratio between the velocity of light (c) in free space (for all practical purposes, either air or a vacuum) and its velocity η in a particular medium:

n = c/η

As the refractive index of a material increases, the greater the extent to which a light beam is deflected (or refracted) upon entering or leaving the material. The refractive index of a medium is dependent (to some extent) upon the frequency of light passing through, with the highest frequencies having the highest values of n. For example, in ordinary glass the refractive index for violet light is about one percent greater than that for red light. A consequence of this phenomenon is that each wavelength experiences a slightly different degree of refraction when a heterogeneous light beam containing more than one frequency enters or leaves the medium. This effect is termed dispersion and is responsible for chromatic aberration in microscope objectives.