Becoming Invisible: From Camouflage to Cloaks

Our weekend science fun was inspired by a book, Becoming Invisible: From Camouflage to Cloaks by Carla Mooney. Most children’s books about camouflage talk about how animals try to blend into their environment. This book is very different. It is about how modern engineers and scientists are trying to make the fictional invisibility cloak from the Harry Potter books into a reality. For a review of the book and links to more science books, see our sister blog, Wrapped in Foil.

Camouflage and invisibility are two very different things. Camouflage is the use of color or other aspects of appearance to help an object blend with its environment.

â€œOptical camouflageâ€ is a form of camouflage which uses projectors to display scenes of the moving background onto special reflective cloaks. From the right angle, it is impossible to tell where the cloaked person (or object) is standing because he or she seems to be part of the background images. If the viewer isnâ€™t in line with the projectors, however, the illusion doesnâ€™t work.

Invisibility, on the other hand, is changing the way light reflects or refracts when it hits an object and thus preventing the light from reaching our eyes. We literally can not see an object that is invisible.

Becoming Invisible: From Camouflage to Cloaks tells how scientists have been able to bend electromagnetic waves that are near relatives of visible light (see Exploring Light and its Relatives Part I and Part 2) with special man-made materials called metamaterials. Using metamaterials made of metal and fiberglass, scientists have been able to develop “cloaks” that bend microwaves and infrared light around an object, hiding it from detection. Very cool!

Activity 1. Hiding with color – camouflage for younger children

Read one of the books suggested below to introduce the idea of camouflage.

Gather:

Fabric swatches of different colors and designs
Painter’s blue tape (doesn’t leave a residue when it is removed, but it is more expensive) or masking tape
Suitable area to play hide and seek, indoors or out

You might want to sort the children into teams if you are working with a large group. You will need enough swatches of fabric so that each child or team can hide a few different swatches. Make loops of tape with sticky-side out and then apply a few to the back of each swatch. Now select a child/team to be the hider. They will hide the swatch in plain sight in the play area by taping the swatch to items while the other children, who will be the seekers, close their eyes or wait in another area. When the hiders have applied their swatch, have the seekers come look for it. Once they have found it, change roles. The goal is to find a background object that matches the color close enough that the fabric is difficult to see, and thus takes longer to find.

Activity 2. Hiding with color- camouflage for older children

Gather:

Paper
Art supplies such as markers, crayons, colored pencils, etc.
Painter’s blue tape (doesn’t leave a residue when it is removed, but it is more expensive) or masking tape (optional)
Suitable area to play hide and seek (optional)

For older children, have them create their own camouflage patterns for different settings. You could also have the children do the hiding activity (1), but substitute their own designs on paper for the fabric swatches. Or figure out an experiment to test the effectiveness of various camouflage designs.

Activity 3. Making glass “disappear”

We don’t need high tech metamaterials to make an object invisible. We can hide a piece of PyrexÂ® glass by immersing it in a material that has a similar index of refraction, WessonÂ® oil.

Gather:

small PyrexÂ® glass bowl
A larger glass bowl
WessonÂ® oil or baby oil, or a mix of the two

Place the smaller bowl inside the larger bowl. Fill them both with WessonÂ® oil, baby oil, or a mix of the two. Once covered with oil, the smaller PyrexÂ® bowl should disappear from view.

See a similar activity using a PyrexÂ® stirring rod and explanation from Exploratorium

Activity 4. Make jelly marbles disappear in water

Obtain some jelly marbles from science supply stores, for example from Steve Spangler. Soak the jelly marbles overnight in water. Then fill a clear glass container with water. When you drop in the soaked jelly marbles, they will disappear.

Jelly marbles are polymers that absorb water. When they are swollen with water, they have the same index of refraction and disappear when you place them in water.

This video shows examples of both of the index of refraction activities: