It’s easy to forget about the incredible physics at play when dispensing a pair of spectacles. DR DANUTA SAMPSON dissects how the eye and lens interact with light.
Light is everywhere and it is beautiful. Light and light-based technologies impact almost every aspect of our lives, from entertainment and medicine through to communications, energy and culture. Recognising this significance, UNESCO established the International Day of Light celebrated annually on 16 May, commemorating the ground-breaking achievement of physicist Theodore Maiman’s successful operation of the first laser in 1960. Since 2018, this day has served as a global platform for individuals, universities, and industries to organise public events fostering awareness of light’s remarkable societal impact.
But what does this celebration have to do with optical dispensing? Well, quite a lot. Optical dispensing is all about improving how we use light to see and is underpinned by light technology and the underlying physics – the field of optics. In this short article, let me reflect on optics and its connection with optical dispensing.
“There has continued to be a strong connection between the science of light and the eye healthcare profession.”
Optics is the branch of physics that studies the behaviour and properties of light, with the term originating from the ancient Greek word, optikē, meaning appearance or look. In antiquity, optics focused on explaining how vision worked. Since then, there has continued to be a strong connection between the science of light and the eye healthcare profession.
Fundamentally, light interacts with matter through reflection, scattering, refraction, and absorption. When it encounters a surface, such as the air-cornea interface, it can remain in the original medium (air; through reflection) or travel into the second medium (cornea; changing both its speed and direction through refraction). The difference between the refractive indices (n) of media determines how much light is refracted/bent when it enters the second medium. The process can be mathematically described by Snell’s law.
Although the credit for the discovery of the law of refraction is given to Willebrord Snel van Royen (Snellius) in the 16th century, studies have shown that the law was discovered more than 600 years earlier by Abu Sad Al Alla Ibn Sahl, a brilliant physicist who significantly contributed to optics.
Scattering and absorption describe light interactions with matter that result in changes in light’s direction of travel and/or energy. In scattering from rough surfaces or from small fluctuations in a medium, light fragments into lots of smaller waves travelling in multiple random directions. This differs from reflection, which causes light to be redirected as a single wave in only one direction. In absorption, light energy is converted to another form of energy, commonly heat.
To see an object, light must interact with it. Let’s consider a green apple. All colours from the visible spectrum (that combine to make up ‘white light’) apart from the green are absorbed (extinguished/removed) by the apple. The green colour is reflected and travels to the eyes. When light passes through the cornea and crystalline lens, if it enters at an angle, the change in speed causes it to refract by an amount that varies with angle and refractive index difference according to Snell’s Law. If this bending is just the right amount for all angles of entry, all rays will converge on the retina to enable clear vision. This is known as emmetropia. There are two contributors to this bending – the curved cornea and the lens. A healthy cornea is responsible for approximately two thirds of the optical power of the eye (+40.00D) and the crystalline lens one third (+20.00D).
A mismatch between eyeball length and eye optics can lead to myopia (convergence in front of the retina) or hyperopia (convergence behind the retina), which causes blurry vision. Here again, optics plays an important role. For example, autorefractors quantify the eye’s optical imperfections. The personalised optical design of spectacles enables the correction of refractive errors, shifting all focusing points to the retina.
An important aspect of lens design is the choice of material, which involves trade-offs between factors such as refractive index (available range 1.49-1.89), Abbe number (that describes chromatic aberration; higher number–less aberration), impact resistance, weight, and cost. CR-39 (n=1.49) is a cost-effective material with a high Abbe number (59), often used to make lenses for low refractive error correction but is the thickest and heaviest. High-index glass (n=1.7; Abbe index=42) offers thinner profiles and clarity, enabling correction of high refractive errors, but is costlier due to complexity in manufacturing. More on lens design is coming in the next article.
In conclusion, optical dispensing, rooted in optics, spans the spectrum of patient care, from diagnosis to treatment. Advances in optical technology continuously reshape this field, demanding a deep understanding of optical principles that have not changed for millennia to deliver contemporary personalised solutions.
ABOUT THE AUTHOR: Dr Danuta Sampson is a translational researcher and senior lecturer jointly appointed between the Lions Eye Institute and the School of Optometry at the University of Western Australia. She is also involved in the global optics and photonics community through engagement with Optica and SPIE – the largest optics and photonics societies globally. She serves as light science ambassador (Optica), supports various committees (Optica) and designs STEM children’s books and educational games/materials (SPIE).
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