Optical technology is ideal for inspiring young students. We have found that far field holograms are particularly successful at exciting students of all ages. To date, our activites in education have focused on demonstrations for local Maryland and Virginia K-12 students. We have begun working with instructors at the university level as well.
In the coming months we plan on putting together lesson plans for various age groups and will include them on this site.
We welcome your comments.
An explanation of the action of HoloSpex lenses is provided below:
Pre college explanation:
HoloSpex Glasses contain far field holograms that transform bright points of light into specific images.
When you look at the world without HoloSpex glasses, the lens in your eye forms an image of the world on the back of your eye (retina). Consider putting a piece of clear plastic or glass in front of your eye and looking around. If the plastic is good quality with few smudges or scratches everything should appear normal. Now consider purposely smudging the plastic with your breath. Now everything in the scene appears smudged. In particular, if you look at a bright point of light, it is no longer distinct. Instead you see an indistinct blob of light at each point of light. The smudge on the piece of glass is not permitting a good image to be formed on the back of your retina. Instead of traveling in a straight path through the plastic, the light is being bent by the smudge pattern.
If you remove the smudged plastic and place a far field hologram in front of the eye, once again the light is not allowed to image properly on the back of the eyeball. The hologram bends the light in a predesigned manner to form a specific image such as the text "NOEL" or some particular image. If you examine a HoloSpex lens under a high power magnifier or microscope, you will see an apparently random collection of opaque and clear square regions. When light passes through small openings it is "diffracted" or bent in a well understood way. The pattern of these small openings has been calculated by computer to bend the light into the specific desired pattern. These TYPES of holograms are often called computer generated holograms or diffractive optical elements.
A note on color:
Note that red light diffracts at greater angles than green and green diffracts more than blue. When you look at colored Christmas lights you may notice that the red holographic images are the largest. When you look at white lights, you will notice some color spreading with red being bent more than the other colors.
Other diffracting objects - Instead of holographic lenses look though a window screen at night at street lights. If the mesh is fine enough you will see a unique diffraction pattern surrounding street lights. The fine mesh in a stocking will diffract light even more.
College level or advanced high school physics level:
The lenses in HoloSpex glasses are far field computer generated holograms (CGH's).
Let's concentrate first on far field holograms: A far field hologram reconstructs the object in the far field. In theory, if you illuminate with a plane wave, the object will reconstruct an infinite distance away from the hologram. In practice, you need only go a sufficient distance away to see the far field pattern. An even better way is to place a lens after the hologram plane, then you find the far field exactly at the focal point of the lens. This is what is happening when you hold a far field hologram to the eye and look at a point source: The light is focused by the lens and cornea of your eye to a point at the retina where the object reconstructs.
Now we can attack CGH's. Here are two perspectives:
1) CGH's are a special form of a hologram. A hologram is a piece of film (or other medium) that tricks the eye into seeing an object. The hologram has a spatially varying amplitude and phase transmittance that mimics the absorption and phase delays that would be experienced had there been an actual object there. An optically formed hologram is made by recording a complicated fringe pattern made by a interfering a reference beam (often a plane wave) and a beam that has been bounced off the subject object. After the hologram has been properly developed, the net result is a mask with the appropriate absorption and phase delays across the hologram. (In practice, the phase is more important and often the hologram is bleached so that it appears clear).
Computer generated holography uses a computer to calculate the interference of the reference beam with an object beam. A data file is created that describes the complicated fringe pattern. The next step is to transfer this complicated pattern onto a piece of film. In the case of HoloSpex lenses we are limited to clear or opaque amplitude variations so we have to approximate the complicated fringe pattern by binarizing the fringe pattern. The final result is a piece of film that bends (or diffracts) light coming from a plane wave (or a distant point source) in such a manner as to mimic the complicated wavefront that would have come off of the object had it been there.
2) CGH's can be understood by first considering diffraction gratings: Introductory physics texts explain that simple gratings diffract light into various orders. In fact a true sinusoidal amplitude transmittance grating would only diffract light into two orders (three - if you count the undiffracted light as the zero order). A squarewave grating is more common and diffracts into the first orders as well as an infinite number of harmonics. This means that when we shine a laser beam through a squarewave grating at a wall, we see a central spot (undiffracted) and various equally spaced diffracted spots. If you choose a higher frequency grating (more line pairs per mm) the spots become more widely spaced. Now try to make a leap to a computer generated hologram that can be thought of as an information bearing grating that has been designed to diffract the light into many directions creating a multitude of spots that are spatially arranged to make some pattern like "SCIENCE IS FUN".