1. Activate prior knowledge

Pour water into the clear drinking glass. Place a pencil into the glass of water so that half of it is submerged. Have students walk past the glass, bending over to look at it from the side. When all have seen it, discuss what they observed. Ask:

• Is the pencil really bent? (No, it's an optical illusion)
• What do you think makes the pencil appear to bend under water? (Adjust the expected answer to the students' ages. The water acts as a prism; the light reflecting off the submerged half of the pencil is refracted, while the half sticking out of the water is not. Clearly explain what it means for light to be refracted.)
• What are some other odd things you have noticed light doing? (If necessary, prompt students to think about light making a rainbow effect on the ground when seen through angled glass or prisms; how objects sometimes look larger when viewed through water, and so on.)

Explain that the behavior of light often affects the way we see in odd and interesting ways.

2. Prepare groups for the activity

Watch the clip “How the Human Brain Sees Light” from the film Mysteries of the Unseen World. Explain that the class is now going to create a model that will show more clearly how light behaves when it enters their eyeballs.

NOTE: Instructions are written for small group presentations. Adjust accordingly if you prefer to perform this as a large-group demonstration.

Separate students into groups of three to four students each. Assemble each group around a table or counter. Distribute the cardboard pieces as well as the clear, round drinking glasses or glass bowls (whichever you chose to use for the experiment).

3. Guide groups in preparing for the experiment

Explain the procedure, having a different group member perform each of the following tasks.

• Fill each drinking glass with water. OR fill the glass bowl with water.
• Poke a hole into the center of the BLACK cardboard with a sharp pencil. The hole should be small—less than the diameter of the pencil itself—and located so that it can be lined up with the center of the glass when the cardboard is held up beside it.
• Have one group member hold the black cardboard steady against the glass or bowl.
• Have another group member hold the white cardboard one or two inches away from the glass or bowl on the opposite side. This student should be prepared to move the cardboard during the experiment.

4. Use the candle to complete the model

[Safety tip] An adult must take charge of each candle and the matches.

Guide students in continuing the investigation as follows:

• Turn off the lights and lower window shades to darken the room as much as possible.
• Move from one group to another to conduct the next part of the experiment. (NOTE: If more than one adult is available, this part of the activity will take less time.)
• Instruct groups not to discuss what they see aloud, so all groups can observe the experiment for themselves.
• At each table, light the candle and hold or set it in front of the black cardboard so the light reflects through the hole, through the glass or bowl, and onto the white cardboard on the other side.

NOTE: The student holding the white cardboard may need to move it around until the candle flame is visible there.

• Make sure every student has the chance to see the image of the candle on the white cardboard and make note of its appearance. One member of the group can move a pencil or other non-flammable object over the flame (not touching it), so it is obvious that the top of the flame shows up on the bottom of the image on the white cardboard. (Warn students not to mention what they see aloud.)
• Blow out the candle and move to the next group to repeat the procedure until everyone has observed the results.

5. Discuss what was observed

Use the guided questions listed below or your own discussion starters to debrief the experiment. Tell students that you are going to talk about the parts of the eye that the model represented: the pupil, eyeball, and retina. Remind students that in the video they learned that an object can be seen when light reflected from its surface enters the eye.

Ask and discuss the following questions:

• What was our light source in this case? (The candle)
• What do you think the small hole in the black cardboard represented? (The pupil, a small hole in the front of the eyeball that lets light into the eye)
• What did the glass (or bowl) filled with water represent? (The eyeball, which is shaped roughly like a sphere)
• What does the white cardboard represent, and what is its job? (The retina. It receives the image from the light source)
• What was odd about the image you saw on the model "retina"? (It was upside down and backwards)
• Think back to when we looked at the pencil in the glass of water. What do you think caused the image in our model to flip? (The curvature of the glass or bowl, which imitates the curvature of the eyeball, bends the light in the same way the water bent the image of the pencil. Older students should include understanding of the terms reflection and refraction)
• According to the video, why don't we "perceive" everything we look at upside down and backwards, then? (The brain interprets the image and flips it back for us)

6. Wrap up by reviewing parts of the eye and how humans see

Distribute the worksheet "Model of the Naked Eye" and have students complete the work independently. When they have completed the worksheet, lead students in a discussion about their responses on the worksheet and their experiences with the experiment. Collect the worksheets to assess students’ understanding.