Benham's Top (Doug Foster)

Author

Doug Foster, CSUN

Principle(s) Illustrated

1. Perception vs. "reality"

2. Refractory time and

persistence of vision

Standards

CA State Science Standards (old: 1998,2007)

7.7.c - Students will communicate the

logical connection among

hypotheses, science Fig. 1 Bigham's Top pattern

concepts, testsconducted, data

collected, and conclusions drawn

from scientific evidence.

7.6.e - Students know that white light is a mixture of many wavelengths (colors) and that

retinal cells react differently to different wavelengths.

Questioning Script

Teacher: "I have a disk here. What "colors do you see"?

Students: (Likely response) Black and white

Teacher: Now I'm going to spin the disk... if I can find something to spin it... Ah...a power

drill! (pull out as big and noisy drill as possible- but any technique, including

spinning by hand, will work). I'm going to spin the disk, and I want you to tell me

what colors you see...

Teacher: But ... before I start, what colors might you expect ?

Students: Gray

Teacher: What cells in the retina might be stimulated?

Students: Rods

Teacher: How about cones?

Students: ?

Teacher: what cells are stimulated by color?

Students: cones

Teacher: What is white light composed of?

Students: All frequencies

Teacher: So are cones stimulated by white light?

Prior knowledge & experience:

Students know that white light is composed of all wavelengths

Root question:

Describe what “colors” one might perceive when the disk is spun

Target response:

Blues, Reds faded.

Common Misconceptions:

Might expect gray

Photographs and Movies

Benham's Top Pattern HERE

Benham's Top in motion HERE

Explanation: White Light is composed of all frequencies, and consequently activates all three types of cones, red, green and blue. However, all three cell types slightly different response times (eg. the red cones might not stop (or start) sending nerve impulses at precisely the same instant as the blue cones and/or green cones. When the cones are alternately stimulated with

white light and darkness, the lack of synchronicity between Fig. 2. Cross section

the3 types of cones causes (for many people) the perception* of the eye

of color. showing

rods and

cones.

The top is named after an English scientist/toymaker named Charles Benham, who placed the image on spinning tops and sold them as toys in the late 1800s. Although the current version of the image is named after Benham, the phenomena has apparently been discovered independently a number of times (Bach, n.d.)

Note: The reaction varies between individuals, which is one reason TV engineers in the 1950's did not pursue research into using the effect to broadcast "color" images to black and white TVs. (see below).

*"The definition of perception in this context is " the process by which the brain interprets our observations."

References

Griffin, L. R. (1968) Color TV That Isn’t. Popular Electronics

http://www.earlytelevision.org/pdf/pop_elect_10-68.pdf

Butterfield Color Encoder from Human Color Perception J Sheppard, J. , Jr

http://www.earlytelevision.org/butterfield.html

Bach, M. (n.d.) Benham’s Top from “106 Visual Phenomena & Optical Illusions”

Great virtual Benham's Top with speed control HERE

Other excellent visual phenomena HERE