Now you see it; Now you don't

Hiding in plain sight is a common theme  in sci-fi or horror movies. Sci-fis show some respect for the rules of physics, or hypothesize  new ones ; but ghosts spurn  all laws and do as they please. And  Man? Man definitely knows ways and means to play with the laws, bend them, hoodwink them, and even  find loopholes. Light is an electromagnetic radiation(EMR) and hence one way to hide from light will be  to design a cloak using a special fabric with unique electrical  and magnetic characteristics (electrical permittivity and magnetic permeability to be precise).  Such a fabric will be able to bend and curve light  in every which way we want.  Not that we haven't been successful in this attempt; lo and behold! we have created metamaterials. These man-made materials, by virtue of their patterned structure can manipulate light very effectively. Thus metamaterial cloaking could  bestow invisibility on an object.  Yes cloaking indeed; precisely and literally covering up. The wrapper will gently and smoothly guide light that falls on it as if there is no solid object underneath.Though as of now a definite possibility, metamaterial cloaking  is not yet  a reality but  a field of intense research activity.  Inspired by the powerful and versatile attributes of the  binary system (0,1) of digital electronics, Giovampaola and Egheta have suggested the  concept of  Digital metamaterials  using just two elements of unique permittivity  as bits  to design a series of unique and versatile  metamaterial bytes  to suit any application. Still have miles to go in that direction. 

Peacock in the woods,
Painting by Thayer  Courtsey wikipedia

Bradypodion taeniabronchum
Courtsey : Wikipedia

We need to pause here and ask an important question: what exactly do we mean by being invisible? Please ignore the philosophical tone in the question and consider it  purely in the  practical context. Invisibility is neither abstract nor absolute in practical terms,  it is  always relative- relative to the ambiance, or the background. For example chameleons change color and merge with the ambiance and go undetected; Abralia, a variety of   mid-water squid, uses counter illumination to camouflage itself against the ocean glow. Likewise  Thayer with deft strokes of  brush  concealed his peacock in the woods.

Since light can't be fooled in a straight fight,  Schittny et al  decided to try   concealing objects in  in diffused  ambience such as mist, smoke, fog or  turbid liquids(1). In a murky medium photons gets tossed around, scatter and diffuse.  Hence its behavior is  best described by Fick's Law  of diffusion  and not by Maxwell's equations. The diffusivity of the ambience (D0), the object (D1) and  the wrapper(D2) assume significance.  Schittny et al caught on this point. How about negating the diffusivity  of the object to be hidden   with an equal but opposite diffusivity of a wrapper? 
  
In the July issue of Science Schittny et al demonstrate how an object can  be effectively hidden  in diffused light. They chose  hollow stainless steel cylinder and sphere as test objects to be concealed.  Since light doesn't  pass through steel these objects have zero diffusivity (that is D1=0). Based on the mathematical relationship that  linked  the dimensions of the object and  thickness of the  wrapper to D0 and D2 they could  formulate the conditions for perfect concealment (2).  Armed with these results they first coated the object with a very thin layer of white acrylic paint to ensure efficient diffusive reflection. For wrapper they chose polydimethylsiloxane (more popularly known as PDMS) doped with 10 micron size melamine resin particles.  To give a feel for the sizes involved : the cylinder (and the sphere) had a diameter of little more than 3 cm,  and in each case  the wrapper  thickness was less than 4mm. The objects were immersed in a rectangular transparent plastic  tank filled with deionized water containing  a small amount of white dispersion paint. The paint particles imparted the required turbidity to the medium. One side of the tank was illuminated by the glow from a computer screen. The monitor provided the flexibility of adjusting the incident light.  Images were captured using a digital camera positioned on the opposite side. Experiments were carried out with the cylinder as well as the sphere,  both  could be completely hidden from view.  The results are highly significant on two counts:  The omni-directionality of the phenomenon  and validity throughout the visible spectrum.

Schittney et al  conclude their paper with the suggestion " one could insert metal bars which are almost as thick as glass into bathroom frosted-glass window to prevent burglary. Usually these bars would be visible via the diffusive  shadow they cast. By adding thin diffusive cloaking shells around the metal bars, the window would again appear as a homogeneously bright milky glass".  

Tailpiece:
What does invisibility portend for Man? 

Almost 2400 years ago Plato cautioned  that invisibility will completely shatter   man's fragile morality mask;  he gave the parable of   Gyges of Lydia (The Republic  Plato -Book 2 : Story of Ring of Gyges). 

Nor did   Griffin  have a happy ending. For him invisiblity was a power tool to usher in reign of terror.( Invisible Man - 1897 novel by H.G Wells )

References:

1.Invisibility cloaking in diffusive light scattering medium Schittny et al Science 25th July 2014 Vol 345, pp 427-429
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2. Fick's second Law  transformed: One path to cloaking in mass diffusion  Guenneau  and Puvirajasinghe,   J. Royal Society Interface 10 20130106 (2013)