Hole in Hand and Other Eye Related Tricks

What To Do

Pre-Workshop Prep

1. Divvy up the cups and the nickels so that there are 5 nickels and one cup per two students.
2. Make enough copies of the blind spot templates (see file: Blind Spot Templates) so that there is one blind spot template per student. Cut out the templates (they are basically just small strips of paper. If you don’t have the templates, you can just take a piece of paper and cut small strips for each student.)

Activity 1 - Hole in Hand

(Around 10 minutes, including discussion)

1. Ask the students can anyone name all 5 senses that humans have? Prompt is touch a sense? How about taste? Any others?
2. Tell the students to roll up a sheet of paper into a tube. Tell them that it needs to be big enough so that you can put at least 2 fingers inside it. Have the students hold the tube to their right eye, so that they are looking straight through it. Now tell them to hold their left hand with the palm facing your face around halfway down the tube (demonstrate it yourself so that the students know what it’s supposed to look like). What do you think will happen when I keep both my eyes open? What do you see? If they don’t see a hole through their hand, tell them to move their left hand along the tube until they see it.
3. Have the students explain why this is happening. This is weird, huh? Why do you think this is happening?

Try to lead the students to the conclusion that we are now giving your brain two different images to look at: one through the tube and the other of your hand. Since your eyes are so used to combining both of the images that both of your eyes take in, that is exactly what your eyes are doing at this moment. But since the brightest image is coming from your right eye (the eye looking through the tube), your brain pays extra attention to it. So when your brain combines the images, there appears to be a hole through your hand. Cool huh? Try watching TV like this, or amaze your friends and family.

Activity 2 - Find your dominant eye

(Around 10 minutes, including discussion)

1. Tell the students that having two eyes can help with many different things when it comes to your vision.
2. Have the students keep both eyes open and make an “OK” sign with one hand (so that there’s a circle formed with the index finger and thumb). Tell the students to find a distant object, and look at it from inside the circle of your outstretched hand. Now tell the students to close their right eye. Did the image jump? If the image didn’t jump, tell the students to open the right eye and close the left.
3. Have the students explain why this is happening. Which eye is your dominant eye? Why do you think this is happening? Think back to the first activity.

The dominant eye is the eye that, when left open, makes the image jumps the least, ie: the image basically stays the same. For the discussion, try to lead the students to the conclusion that the 3-dimential world is seen only through two eyes. The images on each eye are different, and these differences provide strong clues to the locations of objects in three dimensions. The images jump because the image of the finger in one eye takes in a different part of the background than the image of the finger from the other eye. If we take these two images together with both of our eyes open, we see a nearby finger and a more distant background. This is essentially how 3-D movies are made. If you wanted to go more into that with the students, you could check out this link for more details. Let students know that the dominant eye has nothing to do with left- or right-handedness.

Activity 3 - Penny (or Nickel) Drop

(Around 15 minutes, including discussion)

1. Introduce this activity by telling the students that they will learn another way two eyes interact with each other.
2. Create or have the students form groups of 2. Hand out one cup and 5 nickels to each group.
3. One student will keep one eye closed and the other student will be on the other side of the desk with the cup in front of them. That student will hover the nickel around and over the cup. The student with one eye closed will tell the other student to drop the nickel in the cup when they think the nickel is directly over the cup. Have the students switch roles halfway through.
4. Have the students explain why it’s so hard to get the nickel in the cup.
5. Introduce the word “depth perception”, and explain to the students what it is. Depth perception is the visual ability to see the world in 3-D and determine the distance of an object.

Activity 4 - Make the Erasers Touch!

(Around 5 minutes)

1. Tell the students that you want to show them another activity involving depth perception.
2. Hand out two pencils to each student. Have the students keep one eye closed while trying to put the eraser ends together.

Activity 5 - Find Your Blind Spot

(Around 10 minutes)

1. Tell the students that your eyes also have a blind spot. In this activity, we will try to find your blind spot.
2. Hand out one laminated blind spot testers to each student. Students will then close their left eye, and look with their right eye at the sad face. Have the students try bringing the paper closer and farther away from their face until the smiley face on the tester disappears. Have them then close their right eye and look at the smiley face with their left eye, bringing the paper closer and farther away from their face until the sad face on the tester disappears.
3. Discuss why we have a blind spot. How do you think having a blind spot affects your daily vision?

Guide the students to the conclusion that our brain is actually “making images up”, in a sense, to make up for the blind spot. This is why you don’t see a black hole where your blind spot is. You don’t see the smiley face (or sad face) anymore, and your brain makes up the rest (you just see a continuous white space where the face is supposed to be).

Activity 6 - Make Your Own Blind Spot Template

(Around 10 minutes)

1. Tell the students that they will now be able to create their own blind spot bookmarks.
2. Hand out one blind spot template to each student, and have them draw two images, like how the blind spot testers were set up.