Refraction of Light and water Gel Balls Vanderbilt Student Volunteers for Science Fall 2012 Training Presentation

I. Introduction What are some properties of light? Light travels in straight lines. Show students the red laser. Laser Light—to show students that light travels in a straight line. Obtain the bottle of flour from the kit. Shine the laser light towards a wall away from the students. Compress bottle steadily and lightly so that the flour drops through the path of the laser light in a constant stream. Students should be able to see the laser light reflected upon the particles of flour. Today’s lesson is going to focus on refraction. Divide the class into 8 groups.

II. Refraction Give each group of students a jar containing water and a straw lying at an angle in the jar. Tell students to rotate the jar while looking at the straw and ask them what they observe. Tell students to unscrew the lid and to hold the straw vertically in the center of the jar so that it is half in/out of the water. Have them look at the straw through the side of the jar and slowly move the straw to either side of the jar (do not move the jar or your head). Explain that the bending of light (refraction) occurs when light waves pass from one medium to another.

III. The Appearing Coin Place the Styrofoam cup with the penny on the desk. Select one student in each group to pour the water while the other students in the group stand and observe the coin. Tell them to stand in a position where they can easily see the coin. Now have the students slowly back up until the coin has just disappeared from sight. Tell the designated student to slowly pour water into the cup. Tell students to raise their hands as soon as they can see the coin again.

III. The Appearing Coin (Cont.) Explain that refraction causes this effect. When water is added, the light is bent so that the coin becomes visible (light travels through one medium to another).

IV. Disappearing crystals Give each pair a water gel ball and a glass marble (in a 1oz cup) and a jar of water. Ask students to describe the water gel ball and the marble. Tell students to put the marble into the jar of water. Ask students if they can see the marble? (yes) Explain – refractive index of water and glass are different so that the light waves are bent when they enter the marble.

IV. Disappearing Crystals (Cont.)  Now put the water gel ball into the water. Can you see it? (No) Explain – these gel balls are made up almost entirely of water (99%). So the refractive index of water and the ball is the same. It is difficult to see the spheres in water because light rays are not bent when they travel between two substances with the same indices of refraction. Ask students what they could do to make the water gel ball appear visible? What happens when you put a laser beam thru the water plus water gel ball? (the laser shows a path through the gel). What happens when you put food coloring into the water? Add a drop of blue food coloring and show the students that you can now see the water gel ball.

Review Review basic concepts with students and collect all materials Make sure that all lasers have been returned