Invisibility for Muggles
Recent advances have brought us closer than ever to mastering one of the holy grails of science: invisibility. With an Invisibility Kit and this hands-on activity uses a simple four lens set-up to get a close look at using light’s refractive property to create regions of invisibility. We can also explore the math that makes it work, and how invisibility is useful to Muggles.
Grades: 9 to 14
Duration: 1/2 Hour – 1 Hour
Supplies: Laser Classroom Invisibility Kit OR 2 double convex lenses with Focal Length 50mm 2 double convex lenses with Focal Length 150mm Optics Bench/ Meter Stick Laser Pointer
NGSS Connections PS4:A Wave Properties and PS4.B: ElectroMagnetic Radiation
- Waves can add or cancel one another as they cross, depending on their relative phase (i.e., relative position of peaks and troughs of the waves), but they emerge unaffected by each other. (HS-PS4-3)
- The path that light travels can be traced as straight lines, except at surfaces between different transparent materials (e.g. air and water, air and glass) where the light path bends (MS-PS4-2)
What is Invisibility
Start by remembering that the only thing we ever see is light. Light that comes from a source OR light that bounces off of an object and into our eyes.
With that in mind, the most basic and broad definition of something that is invisible is something which we humans can not see – because light from the object can not reach or be interpreted by our eyes and brain. Here are several reasons we may not see things:
- maybe it is too small (virus) to see
- maybe it is too far away (distant planet) to see
- maybe the light that bounces off of it and into our eyes is not from the part of the Electro Magnetic Spectrum that humans can detect (ultra violet or infra red, for example)
But none of those are really what we are talking about when we talk about making something that is usually visible INVISIBLE. What we really mean when we say we want to make something invisible is:
- We do not want light from that object to reach our eyes (this condition is met by all of the above 3 examples)
- We DO want light from what is around and directly behind the object to reach our eyes — in other words, we want the object to look like it has disappeared while leaving the surrounding environment intact and unobscured.
Harry Potter and Romulan Space Cloaks of Invisibility
What we would love to see is something like what is depicted in the movies – Harry Potter and Romulan Space Cloaks come to mind because they
- are light weight and portable
- provide full coverage
- allow you to “see through” the thing you are making invisible
THAT is not yet possible … BUT….
In 2014, Joseph Choi and colleagues developed the Rochester Cloak – a simple 4 lens set up that does things no other invisibility technology has been able to do.
- Uses a fundamental property of light – refraction and math that predicts how light travels and refracts through various lenses
- Creates “regions of invisibility” that can completely hide real objects in 3D
- Allows the back ground behind the hidden objects to be seen clearly without distortion
- Is inexpensive and easy to set up and demonstrate
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. See how part of Dr. Choi’s face is “missing’? That is not photoshop – it is literally invisible; you are looking right thorough his face to see the back ground!
To read the original published paper about the Rochester Cloak, go HERE
Invisibility Kit: The Math
The math for setting up a Rochester Cloak is relatively simple – the Linear Algebra and Optics Matrices required to derive this simple formula has already been done.
Start with two pairs of convex lenses with focal lengths f1 and f2
If you are using the kit, the two F1 focal length lenses have focal length of (150mm) and is the THINNER lens, and F2 focal length is (50mm) THICKER
Set up in straight line – use the meter stick F2 lenses in middle, F1 on outside (see image in this step)
To determine the distance between f1 and f2 lenses, use the formula t1=t3=F1+F2
To determine the distance between f2 lenses in middle use the formula t2=2F2(F1+F2)(F1-F2)
If you have lenses of different focal lengths, you can simply use the formula above to determine the distances between the lenses and it will work.
Build your own Invisibility Cloak with the Invisibility Kit for Muggles!
- 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 (150mm) lens at zero mark.
- Place one f2 (50mm) 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.
Test and Observe Invisibility with your Invisibility Kit
1.Use a LASER pointer to check that lenses are aligned:
1.Shine LASER pointer through centre of first lens towards graph paper.
2.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.
Invisibility Discussion and Inquiry
Where do things disappear?
Why? How is the light moving and bending to allow you to observe what you do?