This lesson addresses the question: What is light?
The following video is an animation the famous double slit experiment, which demonstrates that light travels as waves. The video goes by quickly; play it several times drawing student’s attention to the single wave approaching and striking the “wall” with two openings (bottom of the picture). The single wave passing through these two openings becomes two synchronous sets of waves that interact with each other as they proceed onward. Where the two sets of waves cross, peaks interact to make higher peaks; troughs interact to make lower troughs. Hitting the screen (top of picture) note lighter bands where peaks hit, darker bands where troughs hit.
The text describes a very easy way for students to conduct this “wave interference pattern” experiment and see this effect for themselves. It makes a lasting impression. Don’t omit doing it.
Another phenomenon that is interpreted by light traveling as waves is the effect of polarized sunglasses. Demonstrate how the amount of light passing through a polarizing filter changes as the filter is rotated. As described in the following video, this effect is explained in terms of light traveling as waves and the filter only allowing passage of waves with a particular orientation:
Kids have fun experimenting with polarizing filter material as shown here: