There are very few people who would not stop by to see, if ever they pass by a Lighthouse. A Lighthouse is one of those few things that has always intrigued us, and caught imagination. Standing by the sea or the ocean, the Lighthouse has stood tall through the test of centuries to help men navigate through the many vagaries of sea life and reach destination safely.
Though the history of Lighthouse can be traced back to the Roman Empire, or beyond, the modern Lighthouse optics is credited to Augustin Jean Fresnel in the 18th Century. Prior to Fresnel, Lighthouse optics consisted of huge lenses that collected the light from an equally big lamp and threw it parallel and collimated many hundred miles over the sea for the ship to navigate. Fresnel understood that much of the refraction done by these huge lenses could be replaced by a thinner lens with a stepped design, each of these steps would bend the light by a certain degree to make the lights emerging from the lamp of the Lighthouse parallel. Thus Fresnel lenses allowed the construction of Lighthouse optics with short focal length and large aperture, without the mass and volume of material that went into prior medieval Lighthouse optics.
Of course, Fresnel lenses were constructed into many types, for the benefit of different types of Lighthouse and their objectives. Thus a Fresnel Lighthouse optics could be of Zero Order ( to throw light to infiniti for ships to see from hundreds of miles away in the ocean ) , First Order to Sixth Orders ( all focusing at a particular distance from the Lighthouse, depending on the need. A sixth order lens would throw light at a shorter distance, specifically placed near harbor.
The modern day diffractive multifocal lenses are a concept from Fresnel Lenses. As you are aware, these lenses are stepped or saw tooth designed to create multiple focal points. The Fresnel lenses applied in diffractive multifocal lenses is to collect parallel rays and focus it on the retina. In the Lighthouse, it is however the other way round , where the light is collected from the lamp, and thrown parallel to the sea. The basic optics however remains the same.
The Fresnel optics together with Thomas Young’s Diffractive Interference of Light has helped create the modern diffractive multifocal lenses. Young’s interpretation of the wave theory of light showed the world how light could be played with, to create constructive interference and thus “orders” or focal points. Ophthalmic scientists are heavily indebted to Fresnel and Young, for their contribution to optics and paving the way for modern diffractive multifocal lenses.