Making Invisibility VISIBLE Exploring the basics of the Rochester Cloak with an Invisibility Kit by

Light Light travels in straight lines Cant curve around obstacles Usually bounces off them instead - reflection

How We See Light is created by a source Light rays reflect off of objects Reflected rays travel into our eyes We perceive as vision

Invisibility Cloak ideally Should not see device causing invisibility Should see “through” the invisible object so that you can see the background behind it.

History of invisibility Invisibility Cap in Greek mythology – worn by Perseus to slay Medusa Modern books and movies – Harry Potter and Romulan Spaceships on Star Trek Active Camera Cloaks – shows background projected on to a screen in front of an object.

Invisibility in Real Life Transformation Optics Uses human made materials called “metamaterials” that cause light to behave in ways it does not in nature. Expensive, often do not work in the visible range of the EM spectrum, large cumbersome set ups. Derive properties from engineered units, not their “natural” atoms.

Invisibility in Real Life 2 Paraxial Ray Optics Cloaking Uses conventional lenses and refraction to bend light around objects Rochester Cloak

Refraction 1 Light changes speed as it travels from one media to another (air to glass for example) Light always bends towards the denser Medium

Convex Lenses Lenses redirect light Convex lenses are thicker in the middle and thinner at the edges This shape causes rays to bend towards the center of the lens.

Focal Points All rays meet at a point called the FOCAL POINT The distance from the lens to the focal point is called the FOCAL LENGTH Thicker lenses have shorter focal lengths and thinner lenses have longer focal lengths.

Rochester Cloak First 3D visible light invisibility cloak Developed at the University of Rochester in NY Creates “regions of invisibility” that can completely hide objects. Uses refraction through a 4 lens system.

Rochester Cloak Setup Two pairs of convex lenses Pairs having focal lengths f1 and f2 Set up in straight line f2 lenses in middle, f1 on outside distance between f1 and f2 lenses on ends d1=d3=f1+f2 distance between f2 lenses in middle d2=2f2(f1+f2)(f1-f2)

How it works As light reflects off of the background, it refracts as it passes through the lenses creating “cloaked regions”. When an object is placed inside of a cloaked region, light from the background passes around object causing the background to be visible, rather than the object!

Rochester Cloak: Spacing Matters! Cloak depends on the spacing between the lenses Distance between first two lenses Parallel rays are refracted to focal point by lens 1 Resultant rays source at exactly f2 from lens 2 So… rays emerge parallel from lens 2 http://isites.harvard.edu/icb/icb.do?keyword=k16940&panel=icb.pagecontent879213%3Ar%241%3Fname%3Dindepth.html&pageid=icb.page93268&pageContentId=icb.pagecontent1572465&view=view.do&viewParam_name=indepth.html

Build your own cloak: Set up Place lenses in lens holders. Keep track of focal lengths. TIP: thicker lens has shorter focal length. Stick graph paper to wall at end of long surface. Place one f1 lens at zero mark. Place one f2 lens 200mm from first lens. TIP: measure from surface of the lenses, not centers! Place other f2 lens 200mm from second lens. Again, measure from the surface! Lastly, place remaining f1 lens 200mm from third lens.

Build your own cloak: testing Use a LASER pointer to check that lenses are aligned: Shine LASER through centre of first lens towards graph paper. Beam should emerge through all four lenses unchanged: no bigger or blurrier. Stand 2-3 meters from first lens. Crouch to be on eye-level with lenses. You should see the graph paper unmagnified through the lenses. Have someone move a pen or other long, thin object between lenses 2 and 3. Object should disappear towards top and bottom of lenses.

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