Simple invisibility cloak from off-the-shelf lenses

The cloaking device is made up of four lenses of different focal lengths. Here, the object being cloaked is the horizontal bar placed between the first and the second lens. The rising sun in the logo of the National Institute of Physics was not blocked by the bar. The bar is 1.2 cm thick while the lenses are 5 cm in diameter.

Invisibility cloaks are common in folklore and fiction. The notion of hiding or rendering an object transparent strikes the fancy of many, especially writers. In J.K.Rowling’s book “The Tales of Beedle theBard,” one of the three brothers escaped Death by putting on an invisibility cloak that was given by Death himself. The cloak was passed down through generations until it reached the hands of Harry Potter who eventually defeated Voldemort with the help of the same cloak.

Even scientists are intrigued by the concept of invisibility. For many years, they have proposed, developed and mounted different optical systems to achieve the effect of invisibility cloaks. The simplest of these designs relies on geometric optics, which regards light as rays when it travels.

The Rochester cloak is the most popular cloak that uses geometric optics. It comprises four centrally-aligned lenses arranged at a specific distance from each other. The area between two lenses is the cloaking region,which means that when an object is placed there, it will not be visible when viewed from the last lens. Unfortunately, the Rochester cloak works only when the focal lengths of the first and the fourth lenses, and of the second and the third lenses are the same. This limitation discourages the use of lenses that are readily available.

At the National Institute of Physics of the University of the Philippines, a team led by Dr. Nathaniel Hermosa created a Rochester cloak that did not impose conditions on the focal lengths. Using analytical ray tracing method, they recalculated the separation distances of lenses with arbitrary focal lengths. The experiments demonstrated the possibility of producing cloaking regions even when lenses have varied focal lengths because they can freely change the focal lengths. They have also found a way to reduce the length of the cloaking system based on the focal lengths.

With this discovery, students can learn analytical ray tracing in a way that is more exciting than the usual image formation calculations. Exhibits and in-class demonstration of optics phenomena can be made more stimulating using accessible materials. But its benefits are not limited to optics education; it has applications in many areas of optical science and engineering.The study of invisibility cloaks can improve existing technologies. For example, in physics, invisibility cloaks serve to guide light around an object. Knowing how to direct light through an obstacle without disturbing the light can help improve telecommunication technology, medical imaging or even machine vision.

The paper on the research has been provisionally accepted for publication in the European Journal of Physics.

(This article originally appeared in the February-April 2019 issue of QS WOWNEWS.)