Blind Spot

Age

Middle School

Format

Hands-on

Materials

   Jelly Beans
   Cup for Jelly Beans
   Papers with 2 spots in appropriate locations (for blind spot demo)
   Paper towel tubes (they are decorated with lots of colored tape)
   Senses poster 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Don’t let anyone choke on the jelly beans.

Preparation

Put the jelly beans in the cups. Photo copies of the blind spot card should be made for each participant. Also it is useful to have a large version of the eye anatomy to use when presenting this demo.

Demonstration

Participants are given a card with an X and a Dot on it. To find their blind spot they are instructed to:

• Hold the card in their right hand with the X on the left side about a foot away from their face

• Close their left eye (or cover with left hand) and look at the X with their right eye or vice versa

• Slowly move the card in until the Dot disappears. You’ve found your blind spot!

At this point the image of the Dot is falling on the blind spot. You will find that the location of the card left to right is less important than the distance the card is from the eye. Participants should be reminded to stare at the X the entire time. It is natural for the eye to shift to the dot when it begins to disappear, however they must stare at the X in order to not see the dot. This may prove difficult for younger children. This test can be repeated with the right eye closed by turning the card so that the X is on the right. The eye anatomy figure describes the location of the blind spot, and explains why this part of the retina lacks light receptors.

What to Say

Did you know that there is a certain spot where your eyes cannot see things? Well to find where it is, everyone needs to take one of these papers. Hold it out in front of you and cover one eye.

Why It Is

In the back of the eye is an organ called the retina, which contains a layer fo very specialized cells which can sense visible light. When light particles (photons) strike one of these cells, the light is absorbed. Then a signal is passed to the vision center of the brain by way of nerved cell, and the signal is perceived as an image. The nerve cells which connect the retina to the brain are collected together at one spot in the back of the eye to form the optic nerve. This spot has no sensory cells, and thus light which falls on it is not detected. This is the location of the blind spot. Normally, we don’t notice we have a blind spot for several reasons. First, the blind spot is always in our peripheral vision, never is it going to interfere with what we are looking at directly. Also our eyes are constantly on the move, therefore nothing remains in the blind spot for very long. Lastly, our eyes compensate for each other in that what falls on the blind spot in one eye will be visible to the other. All of these factors come into play for this demonstration: you cover one eye, stare at the X (keeps your eye from moving), and the dot disappears from your peripheral vision.

• Sometimes kids, especially younger ones, have a hard time with this demo and can’t find it – either have them try it again, move the paper slower, or just don’t make them do it…*