Benham's Top (Doug Foster)

Fig. 1 Benham's Disk. Access image HERE

Possible Script

Teacher: I have a disk that I am going to spin. But before I do, I'd like you to tell me what colors you see on it's face.

Teacher: black and white.

Teacher: Are you sure?

Students: Yes

Teacher: How do we define "black"

Students: (wait for it) Black is the absence of color.

Teacher: Good. What is "white light" composed of?

Student: (wait for it) All colors.

Teacher: OK. When I rotate the disk, the rotating image will be projected onto the retina of your eye.

Teacher: The white portions of the disk are reflecting which frequencies of visible light?

Student: All frequencies

Teacher: The black portions of the disk are reflecting which frequencies of visible light?

Student: None. Black pigment aborbs all visible light.

Teacher: If white light is composed of all wavelengths of visible light, it must stimulate the color

receptors in our eye. Which cells of the retina cause the perception of color? (if additional

hint/scaffolding needed... "rods or cones?")

Students: cones

Teacher:: What are the three types of cone cells?

Students: Red, blue and green

Teacher: Keep that answer in mind. OK...

I have here a magic drill. It has the

ability to impart magical qualities upon

ordinary objects (Note: drill is for

drama... turning the disk by hand also

works). (Teacher turns on drill. Best

to view from directly in front- teacher

should slowly rotate from one side of

the room to the other with the spinning

Benham Disk in hand). Fig. 2 Light receptors of the eye. (Source)

Teacher: Write down the color(s) you observe.

Teacher: What did you see?

Students: Common answers are pale blues, reds/oranges.

Teacher: Consult with the person across from you. Offer an explanation.

Teacher: How have we defined "perception?"

Teacher: "The process by which our brain interprets our observations."

Teacher: How have we defined "reality."

Students: "The quality or state of being actual or true."

Teacher: Are the colors real, perceived, both real and perceived, or some combination?

(Discussion can become philosophical.)

Possible summary:

Teacher: Lawyers are well aware that different witnesses may honestly perceive the same event

differently. These variations in perception are sometimes used to further their argument (whatever

it may be).

The scientific method helps to ensure that what we perceive is actually real. (And even that

statement boarders on circular reasoning)

Explanation: The cause of the perceived colors produced by Benham's top is not completely understood (Benham's Disk, n.d.). When the "white" image falls on the retina, all of the cones are stimulated. One leading theory suggests the perceived color is caused by variations in the response time and/or refractory periods of the three types of cones ("Response time" can be modeled by dividing the class into approximately 3 groups. Tell the class that each group represents a type of cone. Move to a light switch, and instruct the whole class to slap the desk when the light goes off, AND when it is turned on. The scattered slaps are analogous to varied response times. "Refractory periods" may also have a role, and can be modeled by switching the light on and off so quickly that students cannot keep pace. The period of the on/off cycle is shorter than the period required to raise and lower one's hand ie. refractory period). A momentary flash of white light for an instant results in some cones being active while others remain quiescent.

The Benham Top was named after an English toymaker and amateur scientist named Charles Benham. Benham discovered the phenomenon in1894, and shortly after began selling spinning tops with the pattern as toys. The effect had actually been discovered independently a number of times, the earliest instance dating back to at least 1826 (Bach, 2002). In the 1950s engineers exploited the effect to transmit "color" to black and white TVs, but variations in viewer perceptions discouraged them from pursuing the technology ("Early Color Television", 2013)

Terms:

response time- (technology) the time it takes a system or functional unit to respond to an input. (Wikipedia)

refractory period- the amount of time it takes for an excitable membrane to prepare for a second stimulus. (Wikipedia)

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) (I would not have known the answer before seeing the demo).

Common Misconceptions:

Might expect gray

Photographs and Movies