Virtual Library ID:1395 Course:  SPH 3U-3

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Disappearing Finger

Have students;

1. Stare straight ahead at a distant object like the word “Refraction” on the blackboard behind you. Ask them to close their right eye or have them put their hand over their right eye.

2. Hold the index finger of their right hand in a horizontal position, at arms length away from their side.

3. While still staring straight ahead, slowly rotate their right arm forward until they can just see their index finger with the peripheral vision of their left eye. Wiggling the finger helps as peripheral vision is mostly due to rod cells. These cells are more sensitive to light intensity and as a result more sensitive to movement. See segue below and footnote below

4. Rotate their left eye, not their head, to look at their index finger. The finger disappears behind their nose.

Use the diagrams below to explain refraction

Light, that just passes by your nose, strikes the eyes lens and refracts into the eye. If you wear eyeglasses the effect is even more pronounced. Try removing your glasses when the finger is first visible.

When you rotate your eye, the light travelling from your finger toward your eye is blocked by your nose

Segue I once talked with an old hunter who talked about “opening his eyes widely” while in the bush. This probably meant he concentrated on his peripheral vision. He also mentioned that the most common mistake of most young people who wanted to see animals, was that they move too much. This proved true for me while walking along a trail. Far ahead a fox jumped out onto the trail. I froze. It looked around, nose up and down, obviously hunting. At times it even looked directly at me. After a few minutes, I moved slightly to get a camera. The fox immediately looked in my direction and disappeared into the brush.

Reference

I heard this effect described by Jearl Walker on the CBC program QUIRKS & QUARKS date unknown

From Peripheral Vision (http://en.wikipedia.org/wiki/Peripheral_vision

Rod cells are photoreceptor cells in the retina that function in less intense light. They are concentrated at the outer edges of the retina.

Rods require less light to function than cone cells, and therefore are the primary source of visual information at night. A rod cell can respond to a single photon of light. Cone cells, on the other hand, require tens to hundreds of photons to become activated. Additionally, multiple rod cells converge on a single interneuron, amplifying the signal. This convergence is at a cost to visual acuity, however, since the pooled information from multiple cells is less distinct than if the visual system received information from each rod cell individually.

The convergence of rod cells also tends to make peripheral vision very sensitive to movement.

This feature of rod cells causes Peripheral vision to be good at detecting motion, and is relatively strong at night or in the dark, when the lack of colour cues and lighting makes cone cells far less useful. This makes it useful for avoiding predators, who tend to hunt at night and may attack suddenly from ambush.