Lizzie

 Hager-­‐Barnard,  The  NISE  Network

Building  Your  Own  “Invisibility  Cloak”  Using  Lenses  

Introduction  

Researchers  at  the  University  of  Rochester  have  recently  developed  a  new  cloaking  device  
that  uses  lenses.  To  learn  more  about  this  “Rochester  Cloak”,  check  out  this  press  release.  

 
Figure  1.  Photograph  of  the  “Rochester  Cloak”  (University  of  Rochester)  

This  cloaking  device  could  be  a  great  addition  to  any  International  Year  of  Light  event!  It’s  
also  a  great  addition  to  the  NISE  Network’s  other  materials  related  to  invisibility  (http://
www.nisenet.org/search?query=invisibility).    
Lizzie  Hager-­‐Barnard,  The  NISE  Network

Choosing  the  lenses    

The  press  release  discusses  the  steps  involved  in  building  the  “Rochester  Cloak”.  Here  we  
will  review  the  steps  and  offer  some  tips.  

 
Figure  2.  How  to  set  up  the  “Rochester  cloak”  (University  of  Rochester)  
   

As  described  in  the  press  release,  you  need  to  buy  two  pairs  of  lenses.  Each  pair  of  lenses  
should  have  a  different  focal  length.  To  decide  what  focal  lengths  to  buy,  you  need  to  Rigure  
out  how  you  want  to  set  things  up.  In  creating  our  demo,  we  decided  to  buy  lenses  that  
could  be  mounted  onto  a  meter  stick.  This  created  the  restriction  that  everything  had  to  Rit  
on  a  meter  stick.  

After  looking  for  possible  lenses,  we  decided  to  buy  one  pair  of  150mm  focal  length  lenses  
and  one  pair  of  50mm  focal  length  lenses.  Following  the  math  in  the  press  release:  
    t1  =  150mm  +  50mm  =  200mm  
t2  =  (2)(50mm)(150mm+50mm)/(150mm-­‐50mm)  =  200mm  

So  in  our  case,  it  turns  out  that  t1  =  t2  =  200mm.  The  total  distance  between  the  Rirst  and  last  
senses  is  (200mm  +  200mm  +  200mm)  =  600mm,  so  everything  will  Rit  on  a  meter  stick  
(1000mm).  

Many  companies  sell  optics  bench  kits  and  lenses,  but  there  are  often  not  many  choices  for  
lens  focal  lengths.  We  bought  our  materials  from  Home  Training  Tools  because  they  offered  
the  lenses  we  needed  (see  Figure  3).  
Lizzie  Hager-­‐Barnard,  The  NISE  Network

 
Figure  3.    These  are  the  materials  we  ordered.  

   
Lizzie  Hager-­‐Barnard,  The  NISE  Network

Building  the  “Rochester  Cloak”    

Note:  The  instructions  don’t  appear  to  indicate  which  lenses  should  go  on  the  outside  and  
which  should  go  on  the  inside.  But  the  equation  for  t2  indicates  that  f1  >  f2,  since  this  is  the  
case  that  produces  a  positive  value  for  t2.    

Once  you  have  the  positions  of  the  lenses,  you’re  ready  to  set  everything  up.  Use  the  
diagram  in  Figure  2  and  calculate  the  values  for  t1  and  t2  for  the  lenses  you  have  (see  Figure  
4  for  an  example).  Try  to  align  everything  as  carefully  as  possible.  If  your  meter  stick  is  too  
skinny  and  the  lens  holders  are  moving  around  too  much,  try  using  two  meter  sticks.  

 
Figure  4.  Example  of  a  “Rochester  Cloak”  (schematic  and  photo)  
Lizzie  Hager-­‐Barnard,  The  NISE  Network

Does  it  work?    Can  you  cloak  things?  

After  everything  is  set  up,  try  it  out.  Are  there  places  where  you  can  “hide”/“cloak”  objects?  
If  not,  try  looking  at  Figures  5  and  6  to  get  an  idea  of  the  optimal  cloaking  positions.  If  you  
are  using  the  same  objectives  that  we  chose,  try  a  position  150mm  from  the  Rirst  lens.  It’s  
also  good  to  keep  in  mind  that  objects  aligned  with  the  centerline  can’t  be  cloaked.  For  
more  about  this,  go  to  time  2:00  in  the  video  from  the  press  release.  

 
Figure  5.  SimpliSied  path  of  light  rays  through  “Rochester  Cloak”  

 
Figure  6.    SimpliSied  picture  of  cloaked  areas  of  a  “Rochester  Cloak”  
Lizzie  Hager-­‐Barnard,  The  NISE  Network

               
Figure  7.  Photos  showing  the  ability  of  a  “Rochester  Cloak”  to  cloak  objects  

       
Figure  8.  At  certain  positions  objects  can  be  cloaked,  while  at  other  positions  they  
can’t.  Here  are  a  couple  examples  where  the  object  doesn’t  get  cloaked.  For  more  
about  this,  start  the  video  in  the  press  release  at  2:00.