The trick behind the Disappearing Money experiment is the refraction of light. Images that we see are all light rays that reach our eyes. When these light rays travel through air, they experience little or no refraction. That's why you can still see the penny through the side of the empty glass.
When you poured water into the glass, it was as though the penny had disappeared, but it was really just some bending light rays. After traveling through the water and the side of the glass, none of the rays were able to reach your eyes. Refraction occurs because of the molecules in the substance that the light rays are passing through. Gas molecules are spread out. This is why little to no refraction occurs. However, when light rays pass through a substance such as water, the refraction is greater because the molecules are closer together.
So when the light rays are traveling from the money through the water, they are refracted and cannot make it to your eyes. In fact, the glass also refracts the light even more! The image ends up being projected near the top of the glass after the light refraction it has undergone. You would be able to see it... if the saucer were not strategically placed atop the glass.
I give student partners flashlights and mirrors. I direct students to explore shining the light on the mirror and locating the reflection. Then I direct students to try and trace the path of the light to notice the equal angles. Then I show students the reflection explanation worksheet (Reflection Explanation) and ask students if they noticed the equal angle of reflection when they used the flashlight and mirror. I lead a brief discussion about reflection and ask students to think of other times or experiences when they've seen light reflection.
In this video you can see a group of students working together to trace the path of light. Notice in this video that the students are not tracing the path of light, but rather a shadow. This was a common misconception.
In this video, the group of students are noticing the shape and angles of the light path reflecting off of the mirror.
For the last part of this explore and explain part of the lesson, I have students imagine they are sleeping in a dark room. I ask students to think about what happens when someone opens the door just a crack. (the room can actually get quite bright?) I ask students to discuss how the room can be so bright.
Next, I turn off the lights in the classroom. I have students gather near the door. I close the door, then open slightly. I instruct students to look at the shape of the light entering the room and instruct students to note the shape the light makes on the floor and see if the light seems to expand.
To end this lesson, I show this science video to help students make sense of diffraction.
As students make observations during this lesson, I informally assess their knowledge of the understand reflection. I am looking to see if students realize that the light hits the mirror and then is reflected off the mirror at the same angle it hits the mirror. While we have not yet studied angle measurement in math, this lesson will serve as a great way for me to use students prior knowledge and connect math and science later.