Lesson Title: Light and Crystals
To use hydro-gel crystals in a hands-on activity to study light refraction through two media. Also, to stimulate critical thinking.
Lesson Support Materials
Clear plastic cups
Index of Refraction Chart (Optional)
Refraction is the bending of light waves caused by a change in their speed. The amount of bending that occurs depends on the speed of light in both materials. The greater the difference between the speeds of light in two media, the more the light is bent as it passes at an angle from one medium to another. This meeting place of two different media is called the interface between the media. All refraction of light (and reflection) occurs at the interface. When light is passing into a material that will slow it down, the light is refracted (bent) toward an imaginary line drawn perpendicularly through the surface of the medium at the point where the wave strikes the surface. This imaginary line is called the normal. The angle between the normal and the ray of light entering the medium is called the angle of incidence. The angle between the normal and the light ray after it enters the medium is called the angle of refraction. See figure 1.
The change of direction of light as it passes from one medium to another is associated with a change in velocity and wavelength. When visible light in air enters a medium such as glass, the velocity of light decreases to 75% of its velocity in air and in other materials the decrease can be even more substantial. For example, in linseed oil, the velocity decreases to 66% of its velocity in air. Figure 2 displays in bar chart format the velocity of light in different media. The 100% value is the velocity of light in vacuum. For air, the velocity is 99.7% of the speed in vacuum. For some pigments such as titanium (Ti) white, the velocity decreases to 40%. The Index of Refraction is the grouping of material based on their refraction of light.
Figure 2. Bar chart of the velocity of visible light in different media. The value of 100% refers to the velocity of light in a vacuum.
A day before the expect date of the activity fill a jar with distilled water. If distilled water is unavailable, water from the tap can be used. Place one disappearing crystal into the jar. The crystal will begin to grow immediately. Place the lid on the jar and wait from an hour to all that day for the crystal to complete its growth. If bubbles form inside the crystal, try placing the jar in the refrigerator for a few hours to dissolve the excess gas. When the crystal has grown to a point where it appears to be invisible, take it out and tie a string around it. Place the crystal back into the water.
Begin the activity by asking a student to place his or her finger at an angle halfway into a glass of water and look at it through the side of the glass. Ask the youngster does he notice anything bizarre. The student's finger will appear to be bent or even split into two pieces. Ask what is happening to the light as it passes through the water. The students will probably say that the light is bending as it passes through the glass and water, if they do not you may wish to bring this point to their attention. Ask the class had they ever noticed how there legs look bent when they dangle them in the water from the edge of a pool and that fish seem to change position as they are observed from different positions in an aquarium.
Tell the class that the bending of light is called refraction. Explain to the class that light is made up of tiny particles called photons that travel as waves. When light waves pass through different transparent substances it appears to bend or refract because the substances changes the speed of light. The amount of bending that occurs depends on the speed of light in both materials. The greater the difference between the speeds of light in two substances or media, the more the light is bent as it passes at an angle from one medium to another. For example air to water.
When the discussion is completed have each student fill a small clear plastic cup with distilled water. Give each youngster a small disappearing crystal. Have them place the crystal in the cup and cover tightly with plastic wrap secured with a rubber band. They should place their cups in a secure place allowing them to sit undisturbed over night.
At the next class meeting tell the students to get their cups and observe their crystals. To their surprise the crystals will seemed to have disappeared! This fact will generate great excitement and speculation on the part of the class.
In the meantime display the large clear cup or jar containing the crystal in which you have tied a string around submerged in the water. To the class it would appear that this is simply a noose hanging in the water. Ask the students are there any things in the container. Accept all answers. Raise the string and to the amazement of the class a shiny crystal will appear! Lower the crystal back into the water and it will disappear!
At this point the students will speculate that they too have crystals in their cups that appear to be invisible underwater but will appear when removed. Have the students remove their crystals and place them on colored paper. Distribute pieces of string to the youngsters and challenge them to duplicate your crystal demonstration.
When the excitement has subsided ask the class why is it possible to see the crystals in the air or on the colored paper, but not in the water. The students might infer that the crystals look and even feel a lot like water. Inform the class that these crystals are made up almost entirely of water. Ask the class if two substances refract (bend) the light at the same angle would they both appear to be the same. At this point it will become clear to the class that maybe the crystals and the water refract the light the same and therefore the crystals cannot be seen in the water.
If available show the class a large index of refraction chart or draw a chart on the chalkboard. Inform the class that substances can be grouped by their ability to bend light. This grouping is called the index of refraction. The crystals and the water are said to have the same index of refraction. When the crystals are lifted out of the water and into the air, they become instantly visible. We can see the crystal's shape and size because the air's index of refection is very different then that of the water.
Give each student in the class some disappearing crystals to take home to grow and to share with their families and friends what they have learned.
Have the students write essays relating their disappearing crystals to the refraction of light. Students can also, make up questions that arose from this activity. An oral assessment is to have students give brief explanations demonstrating their knowledge about refraction.
Enrichment and Reinforcement Activities
Students can develop a qualitative "index of refraction" like table, by placing disappearing crystals into colored water, cooking oil, rubbing alcohol, even carbonated soda. They could record the results as to the degree of "visibility" in the liquids to that of their distilled water control. The data could also be displayed as a bar graph of "crystal visibility" vs. liquid medium.
Another unique and fascinating activity is to use the disappearing crystals as a plant growth medium. Because the disappearing crystals are a hydro-gel they can store water. Have the students place two or three bean plant seeds into a cup of disappearing crystals. The plants will germinate in a few days. The students will be captivated as they observe the plant roots as they grow and develop. The disappearing crystals need to be watered only once every 2 to 3 weeks! This activity lends itself to an examination of the relationship between light and photosynthesis.