Fun and Amazing Grow Lenses!

Growth and care of grow lenses

Warning: The grow lens could become a choking hazard for children under the age of 5 years!

Your tiny grow lens spheres are composed of carbon-based polymers (similar to gelatin or agar agar gel) and can absorb tremendous amounts of water increasing their volume 1000x!  To grow them, just toss them into a glass jar filled with water (distilled is best but not necessary) and leave them overnight.  By morning, they should be the size of a large marble!  For different colored spheres, add food-coloring dyes to the water either before or after growth. Handle the grow lenses with some care, as they are fragile and will break if bounced hard or stressed. They can be kept for a few days out of water and for longer periods if stored in water.

Science Fun

Invisibility Cloak! 

Amaze your friends by showing them a jar with fully-grown grow lenses immersed in water. They will have a hard time seeing them. As a “magic” trick, have one grow lens already hidden in a jar of water, crush a second grow lens, drop the crushed pieces in the water, pretend to stir the pieces and then pull out the whole grow lens.  They will think that you reconstituted the crushed lens!

Why are the grow lenses invisible in water but not in air? We see an object when light is reflected off the object’s surface and into our eyes.

We can also perceive a transparent object if light passing through the object is bent (refracted).

The amount of reflection and refraction of light depends on the difference in the speed of light in the object as compared with the speed of light outside of the object.  A ratio called the index of refraction of the material is used to describe the speed of light in the material.  The index of refraction, n, is defined as the ratio

n = (speed of light in empty space)/(speed of light in material)

where the speed of light in empty space is 300,000 km/s.

The index of refraction in empty space is then

n_space = 300,000/300,000 = 1

Since nothing can exceed the speed of light in empty space, the index of refraction is greater than or equal to 1.

The index of refraction of air is nearly the same as for empty space:

n_air ≈ 1

The index of refraction of water, where the speed of light is only 225,000 km/s, is approximately

n_water = 300,000/225,000 ≈ 1.33.

When the grow lens is viewed in air, there is a large change in the speed of light as it passes from air into- or out of- the lens, so light is both reflected and refracted as in Figs. 1 (a) and (b).

The grow lens is mostly made of water. As long as the grow lens is immersed in water, there is little change in the speed of light at the boundary between water and lens. Thus, there will be neither reflection nor refraction at the surface as in Figs. 2 (a) and (b).  The lens will be virtually invisible!

Grow lens magnifying glass

A magnifying glass is a lens that uses curved glass surfaces to refract (bend) light and form an image. The proper method of using a magnifying glass is to place it very close to the printed letters you are trying to read. The images of the letters are then enlarged and also correct side up for reading. If a magnifying glass is used incorrectly to view an object far away, the object appear upside down. Try placing the grow lens so that it almost touches the print on a page. The image appears as shown in Fig. 3.

If the object is too far away from a magnifying glass, its image appears inverted. Observe the printed letters when the grow lens is moved a few centimeters away as in Fig. 4 below.

For fun and understanding, try the geometrical optic simulator found here. Click the Virtual Image button and slide the object closer and farther from the converging lens to observe the rays as they form images.

Converging rays to find the focal length

Never look at the sun either directly or through the grow lens as it can cause permanent damage to your eyes!

You are probably aware that a magnifying glass can converge the sun’s rays to start a fire! Light from a distant object arrives as nearly parallel rays. A quality lens will focus all of the rays striking the lens at a single point called the focal point (see Fig. 5). The distance between the center of the lens and the focal point is called the focal length.

The spherical grow lens can also converge rays but not quite as perfectly as a magnifying lens will. Allow sunlight to pass through the grow lens and strike a sheet of paper. If you don’t have sunshine available, use a flashlight some distance away as the source of light. Adjust the distance between the lens and sheet of paper until a bright spot appears. The focal length is the distance from the center of the grow lens to the sheet of paper. Try using your hand instead of the paper and you can feel the heat of the concentrated light rays.

Award-winning science fair project

If interested in using grow lenses for a science fair project, ask the question: “How does the size of a grow lens affect the focal length?” You can adjust the sizes of grow lenses by removing them from the water at earlier times. If needed, more lenses are available from Arbor Scientific .

Grow lens as a model eye

The human eye operates in a manner not too different from the grow lens. Light enters the eye from a distant object and is focused primarily by the cornea. Since the cornea and the air have very different indices of refraction, most of the focusing of light rays occurs as light passes into the cornea. Unlike the grow lens, the human eye uses an interior lens to further converge and adjust the focus of the light on the retina where sensors convert the light image into a stimulus sent by neurons to the brain. Since the grow lens has the equivalent of the cornea at the surface where light enters but contains no lens inside, the grow lens focuses light beyond the lens rather than directly on the back surface where a retina would be located in the eye.

Fig. 6 shows how light from outside a dark window can be focused at the focal point by a grow lens. The image is inverted with sky on the bottom of the image and grass on top.

Compare with the ray trace of light rays entering the eye and forming an image directly on the retina in Fig. 7:

Ray Optics

Use a laser pointer with a grow lens to see how rays are deflected as they pass through the lens. This works best in dim light and with grow lenses that have been dyes with food coloring when grown.

In Fig. 8, we show light passing through several lenses placed side by side to make something like an optical fiber:

Never look at light from a laser pointer either directly or reflected from or refracted through the grow lens as it can cause permanent damage to your eyes!

Osmosis

When you are finished playing with your grow lens or break one, try shaking salt on top and watch the lens melt over time. The higher concentration of salt on the outside of the grow lens uses osmosis to draw water out of the lens to lower the concentration of salt on the exterior. As water is drawn out of the lens it collapses.  (See Fig. 9)